Cover Page Introduction to Programming Using Java Version 4.1, June 2004 (With minor changes from Version 4.0 of July 2002) Author: David J. Eck Department of Mathematics and Computer Science Hobart and William Smith Colleges Geneva, New York 14456 Email: eck@hws.edu WWW: http://math.hws.edu/eck/ This PDF file or printout contains parts of a free textbook that covers introductory programming with Java. The entire text is available on the World-Wide Web, for use on-line and for downloading, at this Web address: http://math.hws.edu/javanotes/ The PDF file and printouts that are made from it do not show Java applets that are embedded throughout the text. Also not included are Java source code examples from Appendix 3 and solutions to the quizzes and programming exercises. This version of the textbook requires Java 1.3 or higher. © 2002 and 2004, David J. Eck. This is a free textbook. There are no restrictions on using or redistributing a complete, unmodified copy of this material. There are some restrictions on modified copies. To be precise: Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.1 or any later version published by the Free Software Foundation; with no invariant sections, front cover text, or back cover text. http://math.hws.edu/eck/cs124/javanotes4/pdf-front-page.html [6/11/2004 11:05:08 AM] Java Programming, Main Index Please Note: This is the Fourth Edition of this textbook. Many applets in this version require Java 1.3 or higher. The Third Edition, which only requires Java 1.1, is still available at: http://math.hws.edu/eck/cs124/javanotes3/ Introduction to Programming Using Java Version 4.1, June 2004 (With minor changes from Version 4.0 of July 2002) Author: David J. Eck (eck@hws.edu) WELCOME TO Introduction to Programming Using Java, the fourth edition of a free, on-line textbook on introductory programming, which uses Java as the language of instruction. Previous versions have been used as a textbook for an introductory programming class at Hobart and William Smith Colleges. See http://math.hws.edu/eck/cs124/ for information about this course. This on-line book contains Java applets, many of which require Java 1.3 or higher. To see these applets, you will need a Web browser that uses a recent version of Java. To learn more, please read the preface. Links for downloading copies of this text can be found at the bottom of this page. Search this Text: Although this book does not have a conventional index, you can search it for terms that interest you. Note that this feature searches the book at its on-line site, so you must be working on-line to use it. Search Introduction to Programming Using Java for pages... Containing all of these words: Search Short Table of Contents: ● Full Table of Contents ● Preface ● Chapter 1: Overview: The Mental Landscape ● Chapter 2: Programming in the Small I: Names and Things ● Chapter 3: Programming in the Small II: Control ● Chapter 4: Programming in the Large I: Subroutines ● Chapter 5: Programming in the Large II: Objects and Classes http://math.hws.edu/eck/cs124/javanotes4/index.html (1 of 3) [6/11/2004 11:06:27 AM] Java Programming, Main Index ● Chapter 6: Applets, HTML, and GUI's ● Chapter 7: Advanced GUI Programming ● Chapter 8: Arrays ● Chapter 9: Correctness and Robustness ● Chapter 10: Advanced Input/Output ● Chapter 11: Linked Data Structures and Recursion ● Chapter 12: Generic Programming and Collection Classes ● Appendix 1: Other Features of Java ● Appendix 2: Some Notes on Java Programming Environments ● Appendix 3: Source Code for All Examples in this Book ● News and Errata © 2002 and 2004, David J. Eck. This is a free textbook. There are no restrictions on using or redistributing or posting on the web a complete, unmodified copy of this material. There are some restrictions on modified copies. To be precise: Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.1 or any later version published by the Free Software Foundation; with no invariant sections, front cover text, or back cover text. The most recent version of this book is always available, at no charge, for downloading and for on-line use at the Web address http://math.hws.edu/javanotes/. The previous edition, which covered Java 1.1, can be found at http://math.hws.edu/eck/cs124/javanotes3/. Downloading Links Use one of the following links to download a compressed archive of this textbook: ● Windows: http://math.hws.edu/eck/cs124/downloads/javanotes4.zip (1.8 Megabytes), with text files in Windows/DOS format. This archive can be used directly in Windows XP. On any versin of Windows, this archive can be extracted with WinZip, or with the free program, Aladdin Stuffit Expander for Windows, available from http://www.stuffit.com/expander/. ● Linux/UNIX and MacOS X: http://math.hws.edu/eck/cs124/downloads/javanotes4.tar.bz2 (1.0 Megabytes), with text files in Linux/UNIX format. If you have the bzip2 program, you should be able to extract this archive with the commands "bunzip2 javanotes4.tar.bz2" followed by "tar xf javanotes4.tar". On Macintosh, this archive will probably be extracted automatically when you download it, or it can be extracted using Aladdin Stuffit Expander for Macintosh, available from http://www.stuffit.com/expander/. ● Linux/UNIX: http://math.hws.edu/eck/cs124/downloads/javanotes4.tar.Z (2.0 Megabytes), with text files in Linux/UNIX format. If you can't use the previous archive, try this one. You can extract this archive on most UNIX systems with the commands "uncompress javanotes4.tar.Z" followed by "tar xf javanotes4.tar". (Note: javanotes4.tar.Z contains Version 4.0, not Version 4.1.) I know from experience that a lot of people will want to print all or part of the text. The following PDF file is provided to make this a little easier. This is nothing fancy -- just the Web pages captured in a single file. To use this file, you need Adobe Acrobat Reader Version 4 or later. (When you click on this link, the file might open in your Web browser; to download it, right-click the link and choose "Save Link As" or similar command.) http://math.hws.edu/eck/cs124/javanotes4/index.html (2 of 3) [6/11/2004 11:06:27 AM] Java Programming, Main Index ● http://math.hws.edu/eck/cs124/downloads/javanotes4.pdf (2.1 Megabytes; over 550 pages) David Eck (eck@hws.edu) Version 4.0, July 2002 Version 4.1, with minor changes, June 2004 http://math.hws.edu/eck/cs124/javanotes4/index.html (3 of 3) [6/11/2004 11:06:27 AM] Java Programming: Contents Introduction to Programming Using Java, Fourth Edition Table of Contents THIS IS THE FULL TABLE OF CONTENTS for version 4.1 of an on-line introductory programming textbook. For more information about the text, please see its front page. The text is available on-line at http://math.hws.edu/javanotes/. Preface Chapter 1: Overview: The Mental Landscape ● Section 1: The Fetch-and-Execute Cycle: Machine Language ● Section 2: Asynchronous Events: Polling Loops and Interrupts ● Section 3: The Java Virtual Machine ● Section 4: Fundamental Building Blocks of Programs ● Section 5: Objects and Object-oriented Programming ● Section 6: The Modern User Interface ● Section 7: The Internet and World-Wide Web ● Quiz on this Chapter Chapter 2: Programming in the Small I: Names and Things ● Section 1: The Basic Java Application ● Section 2: Variables and the Primitive Types ● Section 3: Strings, Objects, and Subroutines ● Section 4: Text Input and Output ● Section 5: Details of Expressions ● Programming Exercises ● Quiz on this Chapter Chapter 3: Programming in the Small II: Control ● Section 1: Blocks, Loops, and Branches ● Section 2: Algorithm Development ● Section 3: The while and do..while Statements ● Section 4: The for Statement ● Section 5: The if Statement ● Section 6: The switch Statement http://math.hws.edu/eck/cs124/javanotes4/contents.html (1 of 4) [6/11/2004 11:06:35 AM] Java Programming: Contents ● Section 7: Introduction to Applets and Graphics ● Programming Exercises ● Quiz on this Chapter Chapter 4: Programming in the Large I: Subroutines ● Section 1: Black Boxes ● Section 2: Static Subroutines and Static Variables ● Section 3: Parameters ● Section 4: Return Values ● Section 5: Toolboxes, API's, and Packages ● Section 6: More on Program Design ● Section 7: The Truth about Declarations ● Programming Exercises ● Quiz on this Chapter Chapter 5: Programming in the Large II: Objects and Classes ● Section 1: Objects, Instance Variables, and Instance Methods ● Section 2: Constructors and Object Initialization ● Section 3: Programming with Objects ● Section 4: Inheritance, Polymorphism, and Abstract Classes ● Section 5: this and super ● Section 6: Interfaces, Nested Classes and Other Details ● Programming Exercises ● Quiz on this Chapter Chapter 6: Applets, HTML, and GUI's ● Section 1: The Basic Java Applet and JApplet ● Section 2: HTML Basics and the Web ● Section 3: Graphics and Painting ● Section 4: Mouse Events ● Section 5: Keyboard Events ● Section 6: Introduction to Layouts and Components ● Programming Exercises ● Quiz on this Chapter Chapter 7: Advanced GUI Programming ● Section 1: More about Graphics ● Section 2: More about Layouts and Components ● Section 3: Basic Components and Their Events http://math.hws.edu/eck/cs124/javanotes4/contents.html (2 of 4) [6/11/2004 11:06:35 AM] Java Programming: Contents ● Section 4: Programming with Components ● Section 5: Menus and Menubars ● Section 6: Timers, Animation, and Threads ● Section 7: Frames and Applications ● Programming Exercises ● Quiz on this Chapter Chapter 8: Arrays ● Section 1: Creating and Using Arrays ● Section 2: Programming with Arrays ● Section 3: Dynamic Arrays, ArrayLists, and Vectors ● Section 4: Searching and Sorting ● Section 5: Multi-Dimensional Arrays ● Programming Exercises ● Quiz on this Chapter Chapter 9: Correctness and Robustness ● Section 1: Introduction to Correctness and Robustness ● Section 2: Writing Correct Programs ● Section 3: Exceptions and the try...catch Statement ● Section 4: Programming with Exceptions ● Programming Exercises ● Quiz on this Chapter Chapter 10: Advanced Input/Output ● Section 1: Streams, Readers, and Writers ● Section 2: Files ● Section 3: Programming with Files ● Section 4: Networking ● Section 5: Programming Networked Applications ● Programming Exercises ● Quiz on this Chapter Chapter 11: Linked Data Structures and Recursion ● Section 1: Recursion ● Section 2: Linking Objects ● Section 3: Stacks and Queues ● Section 4: Binary Trees ● Section 5: A Simple Recursive-descent Parser http://math.hws.edu/eck/cs124/javanotes4/contents.html (3 of 4) [6/11/2004 11:06:35 AM] Java Programming: Contents ● Programming Exercises ● Quiz on this Chapter Chapter 12: Generic Programming and Collection Classes ● Section 1: Generic Programming ● Section 2: List and Set Classes ● Section 3: Map Classes ● Section 4: Programming with Collection Classes ● Programming Exercises ● Quiz on this Chapter Appendix 1: Other Features of Java Appendix 2: Some Notes on Java Programming Environments Appendix 3: Source code for all examples in the text News and Errata David J. Eck (eck@hws.edu), July 2002 and June 2004 http://math.hws.edu/eck/cs124/javanotes4/contents.html (4 of 4) [6/11/2004 11:06:35 AM] Java Programming: Preface to the Fourth Edition Introduction to Programming Using Java, Fourth Edition (Version 4.1) Preface "INTRODUCTION TO PROGRAMMING USING JAVA" is a free, on-line textbook. It is suitable for use in an introductory programming course and for people who are trying to learn programming on their own. There is probably enough material for a full year College programming course. There are no prerequisites beyond a general familiarity with the ideas of computers and programs. In style, this is a textbook rather than a tutorial. That is, it concentrates on explaining concepts rather than giving step-by-step how-to-do-it guides. I have tried to use a conversational writing style that might be closer to classroom lecture than to a typical textbook. You'll find programming exercises at the end of most chapters, and you will find a detailed solution for each exercise, with the sort of discussion that I would give if I presented the solution in class. (I strongly advise that you read the exercise solutions if you want to get the most out of this book.) This is certainly not a Java reference book, and it is not even close to a comprehensive survey of all the features of Java. It is not written as a quick introduction to Java for people who already know another programming language. Instead, it is directed mainly towards people who are learning programming for the first time, and it is as much about general programming concepts as it is about Java in particular. I believe that Introduction to Programming using Java is fully competitive with the conventionally published, printed programming textbooks that are available on the market. (Well, all right, I'll confess that I think it's better.) This version of the book covers "Java 2", the version of Java that was introduced with version 1.2 of the Java Software Development Kit. It was written using version 1.3 of the development kit and should work with later versions as well. The current version, which as of June 2004 is J2SE SDK 1.4.2, can be downloaded from Sun Microsystem's Java page at http://java.sun.com/. ("J2SE SDK 1.4.2 stands for Java 2 Standard Edition Software Development Kit Version 1.4.2, and it is often referred to by its older acronym, JDK, which stands for Java Development Kit. Sun has not exactly been sensible about the way it names things.) The previous versions of this book used Java 1.1. For a long time, I was reluctant to move to Java 2 because it has been much less widely supported and because most of the new features don't represent new programming concepts. I have finally decided to make the change for several reasons. First of all, there is one genuinely new concept in Java 2: generic programming. I have added a chapter on this topic. Second, Java 2 is, after some initial roughness, working well and can be used with a variety of Web browsers. Third, there is the upcoming switch from C++ to Java in the High School Advanced Placement Test in Computer Programming. I have taken the opportunity of a new version to make the book more compatible with the requirements of that test. There is more information below on the changes that I have made in the new version. There are applets embedded in the pages of this book. If you want to see the applets running in your Web browser, you will need one that supports Java 2. Unfortunately, Microsoft's version of Java, which is used by default in Internet Explorer on Windows, does not support it. However, if you install the Java Software Development Kit, the installer will give you a chance to use Sun's version of Java with Internet Explorer. If you do that, Internet Explorer will be able to run all Java applets, not just old ones. (This might not work in all versions of Windows.) As far as I know, all Web browsers for MacOS X use Apple's version of Java, which supports Java 2, and should work fine. MacOS 9 and earlier, however, will never run Java 2. On Linux, the new Mozilla 1.0 Web browser runs Java 2 reasonably well, but in my experience, it still crashes regularly when it is used with Java. (I have a lot of experience since I use Mozilla on Linux as my primary browser.) Every version of Java seems to get bigger. If you are unable to run Java 2, remember that Version 3 of this book only requires Java 1.1, and it covers most of the same basic programming concepts. http://math.hws.edu/eck/cs124/javanotes4/preface.html (1 of 4) [6/11/2004 11:06:43 AM] Java Programming: Preface to the Fourth Edition There are several approaches to teaching Java. One approach uses applets and graphical user interface programming from the very beginning. Some people believe that object oriented programming should also be emphasized from the very beginning. This is not the approach that I take. The approach that I favor starts with the more basic building blocks of programming and builds from there. I cover procedural programming in Chapters 2, 3, and 4. Object-oriented programming is introduced in Chapter 5. Chapters 6 and 7 cover the closely related topic of event-oriented programming and graphical user interfaces. Arrays are covered in Chapter 8, with more coverage of data structures in Chapters 11 and 12. Chapter 10 covers files and input/output streams. Chapter 9 covers exception handling, which is a prerequisite for using files in Java, and uses the opportunity for a more general discussion of the problem of writing correct and robust programs. The current edition of Introduction to Programming using Java will always be available at the following Web address: http://math.hws.edu/javanotes/ All editions are still available and are permanently archived at the following Web addresses: First edition: http://math.hws.edu/eck/cs124/javanotes1/ Second edition: http://math.hws.edu/eck/cs124/javanotes2/ Third edition: http://math.hws.edu/eck/cs124/javanotes3/ Fourth edition: http://math.hws.edu/eck/cs124/javanotes4/ Changes from Version 4.0 Version 4.1 of the text incorporates corrections for a small number of errors that have been found since the publication of Version 4.0. I have also taken the opportunity to add a few comments on new features of Java that have been added since the release of Version 4.0. Since that release, Java 1.4 has been introduced and Java 1.5 is in the final stages of testing before its official release. (Note that these new topics are not covered in any detail; for more information, see the documentation at java.sun.com.) Java 1.4 introduced assertions into the Java language, and I have added a short discussion of assertions in Java to Section 9.4. This is the only change that I cover from Java 1.4. As for Java 1.5, I have added short notes about: a new formatted printing capability (Section 2.4); an enumerated type facility (Section 4.7); templates for type-safe generic programming (Section 12.1); a new for-loop syntax for use with Collections (Section 12.1); and automatic conversion between primitive and wrapper types (Section 12.1). Changes from the Third Edition The big change in the fourth edition, of course, is the switch from Java 1 to Java 2. Java 2 did not change the fundamentals of the Java language, but it did introduce many new features. The two that affect this book are the "Swing" graphical user interface library and the framework for generic programming. Java 1 used something called the AWT for GUI programming. Swing has more features and is more sophisticated. I have rewritten all the GUI programming examples to use Swing, and I have extensively rewritten the parts of the book that cover GUI programming. Because Swing is so complex, there are many parts of it that I do not cover, but I do cover enough to write real GUI programs. The other important new feature is a set of "Collection" and "Map" classes that represent generic, reusable data structures. Generic programming has become an important topic, and I have added a new chapter to cover this material. http://math.hws.edu/eck/cs124/javanotes4/preface.html (2 of 4) [6/11/2004 11:06:43 AM] Java Programming: Preface to the Fourth Edition Java is scheduled to be used as the programming language for the High School Advanced Placement test in computer science starting in the academic year 2003-2004. I've heard from several high school teachers who have used the previous version of this book in their classes, and of several more who are considering using it. Although the previous version already covered most of the AP material, I have made a few changes to improve the coverage. For example, I moved preconditions and postconditions for subroutines from Chapter 9 to Chapter 4, I changed examples that used the Vector class to use the ArrayList class instead, and I briefly introduce the class java.util.Random. There are two versions of the AP test. A course that covered Chapters 1 though 5 and Chapter 8 would include essentially all the material required for the "A" test. Sections 5.5 and 5.6 could be omitted. For the "AB" test, all of Chapter 5 and topics from Chapters 9 through 12 should be added. The AP exam does not require any GUI programming, so Chapters 6 and 7 could be omitted entirely. But a Java course with no graphical programming would be missing out on a lot of the fun. Here is more detailed chapter-by-chapter description of the changes: ● Chapters 1, 2, and 3 are almost unchanged, except for mentions of Swing in Sections 1.6 and 3.7. ● Chapter 4: I've added some material to Section 4.6 on preconditions and postconditions for subroutines and included a mention of the javax package in Section 4.5. ● Chapter 5: I added some material on java.util.Random and ArrayList and rewrote an example, ShapeDraw, to use ArrayList instead of Vector. I moved material on nested classes from Chapter 7 to Chapter 5. This material plus Section 5.5 was then split into two sections, 5.5 and 5.6. Some of the material from the old 5.5 has moved into Section 5.4. ● Chapter 6 and 7 were throughly rewritten to use Swing, although the set of concepts that is covered is actually not much changed. Besides the more significant changes, I dropped most of the coverage of threads in favor of timers, since timers are used instead of threads for animation in Swing. I also dropped material on using double-buffering for animation, since double buffering is automatic in Swing. I dropped Section 6.7, which was a short description of event-handling in Java 1.0. ● Chapters 8 through 11: Mostly unchanged, although all graphical examples and exercises have been rewritten to use Swing. Also, I added ArrayLists to Section 8.3, and I rewrote the material on networking in Sections 10.4 and 10.5. Some of the material on threads that I dropped from Chapter 7 is now in Section 10.5. ● Chapter 12, on generic programming, is all new. ● Appendices: I removed the old Appendix 1, which was a description of C++ for Java programmers, and replaced it with a short description of some of the advanced features of Java that are not covered in the text. I rewrote Appendix 2, which is about Java programming environments. And of course, many of the source code files in Appendix 3 have been rewritten to use Java 2. Usage Restrictions Introduction to Programming using Java is free, but it is not in the public domain. As of Version 4.0, it is published under the terms of the GNU Free Documentation License. This license allows redistribution and modification under certain terms. For example, you can: ● Post an unmodified copy of this textbook on your own Web site. ● Give away or sell printed, unmodified copies of this book, as long as they meet the requirements of the license. ● Post on the web or otherwise distribute modified copies, provided that the modifications are clearly noted in accordance with the license. While it is not actually required by the license, I do appreciate hearing from people who are using or distributing my work. http://math.hws.edu/eck/cs124/javanotes4/preface.html (3 of 4) [6/11/2004 11:06:43 AM] Java Programming: Preface to the Fourth Edition Professor David J. Eck Department of Mathematics and Computer Science Hobart and William Smith Colleges Geneva, New York 14456, USA Email: eck@hws.edu WWW: http://math.hws.edu/eck/ Version 4.0, July, 2002 Version 4.1, with minor changes, June 2004 [ Main Index ] http://math.hws.edu/eck/cs124/javanotes4/preface.html (4 of 4) [6/11/2004 11:06:43 AM] Java Programming: Chapter 1 Chapter 1 Overview: The Mental Landscape WHEN YOU BEGIN a journey, it's a good idea to have a mental map of the terrain you'll be passing through. The same is true for an intellectual journey, such as learning to write computer programs. In this case, you'll need to know the basics of what computers are and how they work. You'll want to have some idea of what a computer program is and how one is created. Since you will be writing programs in the Java programming language, you'll want to know something about that language in particular and about the modern, networked computing environment for which Java is designed. As you read this chapter, don't worry if you can't understand everything in detail. (In fact, it would be impossible for you to learn all the details from the brief expositions in this chapter.) Concentrate on learning enough about the big ideas to orient yourself, in preparation for the rest of the course. Most of what is covered in this chapter will be covered in much greater detail later in the course. Contents of Chapter 1: ● Section 1: The Fetch-and-Execute Cycle: Machine Language ● Section 2: Asynchronous Events: Polling Loops and Interrupts ● Section 3: The Java Virtual Machine ● Section 4: Fundamental Building Blocks of Programs ● Section 5: Objects and Object-oriented Programming ● Section 6: The Modern User Interface ● Section 7: The Internet and World-Wide Web ● Quiz on this Chapter [ First Section | Next Chapter | Main Index ] http://math.hws.edu/eck/cs124/javanotes4/c1/ [6/11/2004 11:07:45 AM] Java Programming: Section 1.1 Section 1.1 The Fetch and Execute Cycle: Machine Language A COMPUTER IS A COMPLEX SYSTEM consisting of many different components. But at the heart -- or the brain, if you want -- of the computer is a single component that does the actual computing. This is the Central Processing Unit, or CPU. In a modern desktop computer, the CPU is a single "chip" on the order of one square inch in size. The job of the CPU is to execute programs. A program is simply a list of unambiguous instructions meant to be followed mechanically by a computer. A computer is built to carry out instructions that are written in a very simple type of language called machine language. Each type of computer has its own machine language, and it can directly execute a program only if it is expressed in that language. (It can execute programs written in other languages if they are first translated into machine language.) When the CPU executes a program, that program is stored in the computer's main memory (also called the RAM or random access memory). In addition to the program, memory can also hold data that is being used or processed by the program. Main memory consists of a sequence of locations. These locations are numbered, and the sequence number of a location is called its address. An address provides a way of picking out one particular piece of information from among the millions stored in memory. When the CPU needs to access the program instruction or data in a particular location, it sends the address of that information as a signal to the memory; the memory responds by sending back the data contained in the specified location. The CPU can also store information in memory by specifying the information to be stored and the address of the location where it is to be stored. On the level of machine language, the operation of the CPU is fairly straightforward (although it is very complicated in detail). The CPU executes a program that is stored as a sequence of machine language instructions in main memory. It does this by repeatedly reading, or fetching, an instruction from memory and then carrying out, or executing, that instruction. This process -- fetch an instruction, execute it, fetch another instruction, execute it, and so on forever -- is called the fetch-and-execute cycle. With one exception, which will be covered in the next section, this is all that the CPU ever does. The details of the fetch-and-execute cycle are not terribly important, but there are a few basic things you should know. The CPU contains a few internal registers, which are small memory units capable of holding a single number or machine language instruction. The CPU uses one of these registers -- the program counter, or PC -- to keep track of where it is in the program it is executing. The PC stores the address of the next instruction that the CPU should execute. At the beginning of each fetch-and-execute cycle, the CPU checks the PC to see which instruction it should fetch. During the course of the fetch-and-execute cycle, the number in the PC is updated to indicate the instruction that is to be executed in the next cycle. (Usually, but not always, this is just the instruction that sequentially follows the current instruction in the program.) A computer executes machine language programs mechanically -- that is without understanding them or thinking about them -- simply because of the way it is physically put together. This is not an easy concept. A computer is a machine built of millions of tiny switches called transistors, which have the property that they can be wired together in such a way that an output from one switch can turn another switch on or off. As a computer computes, these switches turn each other on or off in a pattern determined both by the way they are wired together and by the program that the computer is executing. Machine language instructions are expressed as binary numbers. A binary number is made up of just two possible digits, zero and one. So, a machine language instruction is just a sequence of zeros and ones. Each particular sequence encodes some particular instruction. The data that the computer manipulates is also encoded as binary numbers. A computer can work directly with binary numbers because switches can readily represent such numbers: Turn the switch on to represent a one; turn it off to represent a zero. http://math.hws.edu/eck/cs124/javanotes4/c1/s1.html (1 of 2) [6/11/2004 11:07:46 AM] Java Programming: Section 1.1 Machine language instructions are stored in memory as patterns of switches turned on or off. When a machine language instruction is loaded into the CPU, all that happens is that certain switches are turned on or off in the pattern that encodes that particular instruction. The CPU is built to respond to this pattern by executing the instruction it encodes; it does this simply because of the way all the other switches in the CPU are wired together. So, you should understand this much about how computers work: Main memory holds machine language programs and data. These are encoded as binary numbers. The CPU fetches machine language instructions from memory one after another and executes them. It does this mechanically, without thinking about or understanding what it does -- and therefore the program it executes must be perfect, complete in all details, and unambiguous because the CPU can do nothing but execute it exactly as written. Here is a schematic view of this first-stage understanding of the computer: This figure is taken from The Most Complex Machine: A Survey of Computers and Computing, a textbook that serves as an introductory overview of the whole field of computer science. If you would like to know more about the basic operation of computers, please see Chapters 1 to 3 of that text. [ Next Section | Chapter Index | Main Index ] http://math.hws.edu/eck/cs124/javanotes4/c1/s1.html (2 of 2) [6/11/2004 11:07:46 AM] Java Programming: Section 1.2 Section 1.2 Asynchronous Events: Polling Loops and Interrupts THE CPU SPENDS ALMOST ALL ITS TIME fetching instructions from memory and executing them. However, the CPU and main memory are only two out of many components in a real computer system. A complete system contains other devices such as: ● A hard disk for storing programs and data files. (Note that main memory holds only a comparatively small amount of information, and holds it only as long as the power is turned on. A hard disk is necessary for permanent storage of larger amounts of information, but programs have to be loaded from disk into main memory before they can actually be executed.) ● A keyboard and mouse for user input. ● A monitor and printer which can be used to display the computer's output. ● A modem that allows the computer to communicate with other computers over telephone lines. ● A network interface that allows the computer to communicate with other computers that are connected to it on a network. ● A scanner that converts images into coded binary numbers that can be stored and manipulated on the computer. The list of devices is entirely open ended, and computer systems are built so that they can easily be expanded by adding new devices. Somehow the CPU has to communicate with and control all these devices. The CPU can only do this by executing machine language instructions (which is all it can do, period). The way this works is that for each device in a system, there is a device driver, which consists of software that the CPU executes when it has to deal with the device. Installing a new device on a system generally has two steps: plugging the device physically into the computer, and installing the device driver software. Without the device driver, the actual physical device would be useless, since the CPU would not be able to communicate with it. A computer system consisting of many devices is typically organized by connecting those devices to one or more busses. A bus is a set of wires that carry various sorts of information between the devices connected to those wires. The wires carry data, addresses, and control signals. An address directs the data to a particular device and perhaps to a particular register or location within that device. Control signals can be used, for example, by one device to alert another that data is available for it on the data bus. A fairly simple computer system might be organized like this: http://math.hws.edu/eck/cs124/javanotes4/c1/s2.html (1 of 3) [6/11/2004 11:07:46 AM] Java Programming: Section 1.2 (This illustration is taken from The Most Complex Machine.) Now, devices such as keyboard, mouse, and network interface can produce input that needs to be processed by the CPU. How does the CPU know that the data is there? One simple idea, which turns out to be not very satisfactory, is for the CPU to keep checking for incoming data over and over. Whenever it finds data, it processes it. This method is called polling, since the CPU polls the input devices continually to see whether they have any input data to report. Unfortunately, although polling is very simple, it is also very inefficient. The CPU can waste an awful lot of time just waiting for input. To avoid this inefficiency, interrupts are often used instead of polling. An interrupt is a signal sent by another device to the CPU. The CPU responds to an interrupt signal by putting aside whatever it is doing in order to respond to the interrupt. Once it has handled the interrupt, it returns to what it was doing before the interrupt occurred. For example, when you press a key on your computer keyboard, a keyboard interrupt is sent to the CPU. The CPU responds to this signal by interrupting what it is doing, reading the key that you pressed, processing it, and then returning to the task it was performing before you pressed the key. Again, you should understand that this is purely mechanical process: A device signals an interrupt simply by turning on a wire. The CPU is built so that when that wire is turned on, it saves enough information about what it is currently doing so that it can return to the same state later. This information consists of the contents of important internal registers such as the program counter. Then the CPU jumps to some predetermined memory location and begins executing the instructions stored there. Those instructions make up an interrupt handler that does the processing necessary to respond to the interrupt. (This interrupt handler is part of the device driver software for the device that signalled the interrupt.) At the end of the interrupt handler is an instruction that tells the CPU to jump back to what it was doing; it does that by restoring its previously saved state. Interrupts allow the CPU to deal with asynchronous events. In the regular fetch-and-execute cycle, things happen in a predetermined order; everything that happens is "synchronized" with everything else. Interrupts make it possible for the CPU to deal efficiently with events that happen "asynchronously", that is, at unpredictable times. As another example of how interrupts are used, consider what happens when the CPU needs to access data that is stored on the hard disk. The CPU can only access data directly if it is in main memory. Data on the disk has to be copied into memory before it can be accessed. Unfortunately, on the scale of speed at which the CPU operates, the disk drive is extremely slow. When the CPU needs data from the disk, it sends a signal to the disk drive telling it to locate the data and get it ready. (This signal is sent synchronously, under the control of a regular program.) Then, instead of just waiting the long and unpredictalble amount of time the disk drive will take to do this, the CPU goes on with some other task. When the disk drive has the data http://math.hws.edu/eck/cs124/javanotes4/c1/s2.html (2 of 3) [6/11/2004 11:07:46 AM] Java Programming: Section 1.2 ready, it sends an interrupt signal to the CPU. The interrupt handler can then read the requested data. Now, you might have noticed that all this only makes sense if the CPU actually has several tasks to perform. If it has nothing better to do, it might as well spend its time polling for input or waiting for disk drive operations to complete. All modern computers use multitasking to perform several tasks at once. Some computers can be used by several people at once. Since the CPU is so fast, it can quickly switch its attention from one user to another, devoting a fraction of a second to each user in turn. This application of multitasking is called timesharing. But even modern personal computers with a single user use multitasking. For example, the user might be typing a paper while a clock is continuously displaying the time and a file is being downloaded over the network. Each of the individual tasks that the CPU is working on is called a thread. (Or a process; there are technical differences between threads and processes, but they are not important here.) At any given time, only one thread can actually be executed by a CPU. The CPU will continue running the same thread until one of several things happens: ● The thread might voluntarily yield control, to give other threads a chance to run. ● The thread might have to wait for some asynchronous event to occur. For example, the thread might request some data from the disk drive, or it might wait for the user to press a key. While it is waiting, the thread is said to be blocked, and other threads have a chance to run. When the event occurs, an interrupt will "wake up" the thread so that it can continue running. ● The thread might use up its allotted slice of time and be suspended to allow other threads to run. Not all computers can "forcibly" suspend a thread in this way; those that can are said to use preemptive multitasking. To do preemptive multitasking, a computer needs a special timer device that generates an interrupt at regular intervals, such as 100 times per second. When a timer interrupt occurs, the CPU has a chance to switch from one thread to another, whether the thread that is currently running likes it or not. Ordinary users, and indeed ordinary programmers, have no need to deal with interrupts and interrupt handlers. They can concentrate on the different tasks or threads that they want the computer to perform; the details of how the computer manages to get all those tasks done are not important to them. In fact, most users, and many programmers, can ignore threads and multitasking altogether. However, threads have become increasingly important as computers have become more powerful and as they have begun to make more use of multitasking. Indeed, threads are built into the Java programming language as a fundamental programming concept. Just as important in Java and in modern programming in general is the basic concept of asynchronous events. While programmers don't actually deal with interrupts directly, they do often find themselves writing event handlers, which, like interrupt handlers, are called asynchronously when specified events occur. Such "event-driven programming" has a very different feel from the more traditional straight-though, synchronous programming. We will begin with the more traditional type of programming, which is still used for programming individual tasks, but we will return to threads and events later in the text. By the way, the software that does all the interrupt handling and the communication with the user and with hardware devices is called the operating system. The operating system is the basic, essential software without which a computer would not be able to function. Other programs, such as word processors and World Wide Web browsers, are dependent upon the operating system. Common operating systems include UNIX, Linux, DOS, Windows 98, Windows 2000 and the Macintosh OS. [ Next Section | Previous Section | Chapter Index | Main Index ] http://math.hws.edu/eck/cs124/javanotes4/c1/s2.html (3 of 3) [6/11/2004 11:07:46 AM] Java Programming: Section 1.3 Section 1.3 The Java Virtual Machine MACHINE LANGUAGE CONSISTS of very simple instructions that can be executed directly by the CPU of a computer. Almost all programs, though, are written in high-level programming languages such as Java, Pascal, or C++. A program written in a high-level language cannot be run directly on any computer. First, it has to be translated into machine language. This translation can be done by a program called a compiler. A compiler takes a high-level-language program and translates it into an executable machine-language program. Once the translation is done, the machine-language program can be run any number of times, but of course it can only be run on one type of computer (since each type of computer has its own individual machine language). If the program is to run on another type of computer it has to be re-translated, using a different compiler, into the appropriate machine language. There is an alternative to compiling a high-level language program. Instead of using a compiler, which translates the program all at once, you can use an interpreter, which translates it instruction-by-instruction, as necessary. An interpreter is a program that acts much like a CPU, with a kind of fetch-and-execute cycle. In order to execute a program, the interpreter runs in a loop in which it repeatedly reads one instruction from the program, decides what is necessary to carry out that instruction, and then performs the appropriate machine-language commands to do so. One use of interpreters is to execute high-level language programs. For example, the programming language Lisp is usually executed by an interpreter rather than a compiler. However, interpreters have another purpose: they can let you use a machine-language program meant for one type of computer on a completely different type of computer. For example, there is a program called "Virtual PC" that runs on Macintosh computers. Virtual PC is an interpreter that executes machine-language programs written for IBM-PC-clone computers. If you run Virtual PC on your Macintosh, you can run any PC program, including programs written for Windows. (Unfortunately, a PC program will run much more slowly than it would on an actual IBM clone. The problem is that Virtual PC executes several Macintosh machine-language instructions for each PC machine-language instruction in the program it is interpreting. Compiled programs are inherently faster than interpreted programs.) The designers of Java chose to use a combination of compilation and interpretation. Programs written in Java are compiled into machine language, but it is a machine language for a computer that doesn't really exist. This so-called "virtual" computer is known as the Java virtual machine. The machine language for the Java virtual machine is called Java bytecode. There is no reason why Java bytecode could not be used as the machine language of a real computer, rather than a virtual computer. In fact, Sun Microsystems -- the originators of Java -- have developed CPU's that run Java bytecode as their machine language. However, one of the main selling points of Java is that it can actually be used on any computer. All that the computer needs is an interpreter for Java bytecode. Such an interpreter simulates the Java virtual machine in the same way that Virtual PC simulates a PC computer. Of course, a different Jave bytecode interpreter is needed for each type of computer, but once a computer has a Java bytecode interpreter, it can run any Java bytecode program. And the same Java bytecode program can be run on any computer that has such an interpreter. This is one of the essential features of Java: the same compiled program can be run on many different types of computers. http://math.hws.edu/eck/cs124/javanotes4/c1/s3.html (1 of 2) [6/11/2004 11:07:47 AM] Java Programming: Section 1.3 Why, you might wonder, use the intermediate Java bytecode at all? Why not just distribute the original Java program and let each person compile it into the machine language of whatever computer they want to run it on? There are many reasons. First of all, a compiler has to understand Java, a complex high-level language. The compiler is itself a complex program. A Java bytecode interpreter, on the other hand, is a fairly small, simple program. This makes it easy to write a bytecode interpreter for a new type of computer; once that is done, that computer can run any compiled Java program. It would be much harder to write a Java compiler for the same computer. Furthermore, many Java programs are meant to be downloaded over a network. This leads to obvious security concerns: you don't want to download and run a program that will damage your computer or your files. The bytecode interpreter acts as a buffer between you and the program you download. You are really running the interpreter, which runs the downloaded program indirectly. The interpreter can protect you from potentially dangerous actions on the part of that program. I should note that there is no necessary connection between Java and Java bytecode. A program written in Java could certainly be compiled into the machine language of a real computer. And programs written in other languages could be compiled into Java bytecode. However, it is the combination of Java and Java bytecode that is platform-independent, secure, and network-compatible while allowing you to program in a modern high-level object-oriented language. I should also note that the really hard part of platform-independence is providing a "Graphical User Interface" -- with windows, buttons, etc. -- that will work on all the platforms that support Java. You'll see more about this problem in Section 6. [ Next Section | Previous Section | Chapter Index | Main Index ] http://math.hws.edu/eck/cs124/javanotes4/c1/s3.html (2 of 2) [6/11/2004 11:07:47 AM] Java Programming: Section 1.4 Section 1.4 Fundamental Building Blocks of Programs THERE ARE TWO BASIC ASPECTS of programming: data and instructions. To work with data, you need to understand variables and types; to work with instructions, you need to understand control structures and subroutines. You'll spend a large part of the course becoming familiar with these concepts. A variable is just a memory location (or several locations treated as a unit) that has been given a name so that it can be easily referred to and used in a program. The programmer only has to worry about the name; it is the compiler's responsibility to keep track of the memory location. The programmer does need to keep in mind that the name refers to a kind of "box" in memory that can hold data, even if the programmer doesn't have to know where in memory that box is located. In Java and most other languages, a variable has a type that indicates what sort of data it can hold. One type of variable might hold integers -- whole numbers such as 3, -7, and 0 -- while another holds floating point numbers -- numbers with decimal points such as 3.14, -2.7, or 17.0. (Yes, the computer does make a distinction between the integer 17 and the floating-point number 17.0; they actually look quite different inside the computer.) There could also be types for individual characters ('A', ';', etc.), strings ("Hello", "A string can include many characters", etc.), and less common types such as dates, colors, sounds, or any other type of data that a program might need to store. Programming languages always have commands for getting data into and out of variables and for doing computations with data. For example, the following "assignment statement," which might appear in a Java program, tells the computer to take the number stored in the variable named "principal", multiply that number by 0.07, and then store the result in the variable named "interest": interest = principal * 0.07; There are also "input commands" for getting data from the user or from files on the computer's disks and "output commands" for sending data in the other direction. These basic commands -- for moving data from place to place and for performing computations -- are the building blocks for all programs. These building blocks are combined into complex programs using control structures and subroutines. A program is a sequence of instructions. In the ordinary "flow of control," the computer executes the instructions in the sequence in which they appear, one after the other. However, this is obviously very limited: the computer would soon run out of instructions to execute. Control structures are special instructions that can change the flow of control. There are two basic types of control structure: loops, which allow a sequence of instructions to be repeated over and over, and branches, which allow the computer to decide between two or more different courses of action by testing conditions that occur as the program is running. For example, it might be that if the value of the variable "principal" is greater than 10000, then the "interest" should be computed by multiplying the principal by 0.05; if not, then the interest should be computed by multiplying the principal by 0.04. A program needs some way of expressing this type of decision. In Java, it could be expressed using the following "if statement": if (principal > 10000) interest = principal * 0.05; else interest = principal * 0.04; (Don't worry about the details for now. Just remember that the computer can test a condition and decide http://math.hws.edu/eck/cs124/javanotes4/c1/s4.html (1 of 2) [6/11/2004 11:07:47 AM] Java Programming: Section 1.4 what to do next on the basis of that test.) Loops are used when the same task has to be performed more than once. For example, if you want to print out a mailing label for each name on a mailing list, you might say, "Get the first name and address and print the label; get the second name and address and print the label; get the third name and address and print the label..." But this quickly becomes ridiculous -- and might not work at all if you don't know in advance how many names there are. What you would like to say is something like "While there are more names to process, get the next name and address, and print the label." A loop can be used in a program to express such repetition. Large programs are so complex that it would be almost impossible to write them if there were not some way to break them up into manageable "chunks." Subroutines provide one way to do this. A subroutine consists of the instructions for performing some task, grouped together as a unit and given a name. That name can then be used as a substitute for the whole set of instructions. For example, suppose that one of the tasks that your program needs to perform is to draw a house on the screen. You can take the necessary instructions, make them into a subroutine, and give that subroutine some appropriate name -- say, "drawHouse()". Then anyplace in your program where you need to draw a house, you can do so with the single command: drawHouse(); This will have the same effect as repeating all the house-drawing instructions in each place. The advantage here is not just that you save typing. Organizing your program into subroutines also helps you organize your thinking and your program design effort. While writing the house-drawing subroutine, you can concentrate on the problem of drawing a house without worrying for the moment about the rest of the program. And once the subroutine is written, you can forget about the details of drawing houses -- that problem is solved, since you have a subroutine to do it for you. A subroutine becomes just like a built-in part of the language which you can use without thinking about the details of what goes on "inside" the subroutine. Variables, types, loops, branches, and subroutines are the basis of what might be called "traditional programming." However, as programs become larger, additional structure is needed to help deal with their complexity. One of the most effective tools that has been found is object-oriented programming, which is discussed in the next section. [ Next Section | Previous Section | Chapter Index | Main Index ] http://math.hws.edu/eck/cs124/javanotes4/c1/s4.html (2 of 2) [6/11/2004 11:07:47 AM] Java Programming: Section 1.5 Section 1.5 Objects and Object-oriented Programming PROGRAMS MUST BE DESIGNED. No one can just sit down at the computer and compose a program of any complexity. The discipline called software engineering is concerned with the construction of correct, working, well-written programs. The software engineer tends to use accepted and proven methods for analyzing the problem to be solved and for designing a program to solve that problem. During the 1970s and into the 80s, the primary software engineering methodology was structured programming. The structured programming approach to program design was based on the following advice: To solve a large problem, break the problem into several pieces and work on each piece separately; to solve each piece, treat it as a new problem which can itself be broken down into smaller problems; eventually, you will work your way down to problems that can be solved directly, without further decomposition. This approach is called top-down programming. There is nothing wrong with top-down programming. It is a valuable and often-used approach to problem-solving. However, it is incomplete. For one thing, it deals almost entirely with producing the instructions necessary to solve a problem. But as time went on, people realized that the design of the data structures for a program was as least as important as the design of subroutines and control structures. Top-down programming doesn't give adequate consideration to the data that the program manipulates. Another problem with strict top-down programming is that it makes it difficult to reuse work done for other projects. By starting with a particular problem and subdividing it into convenient pieces, top-down programming tends to produce a design that is unique to that problem. It is unlikely that you will be able to take a large chunk of programming from another program and fit it into your project, at least not without extensive modification. Producing high-quality programs is difficult and expensive, so programmers and the people who employ them are always eager to reuse past work. So, in practice, top-down design is often combined with bottom-up design. In bottom-up design, the approach is to start "at the bottom," with problems that you already know how to solve (and for which you might already have a reusable software component at hand). From there, you can work upwards towards a solution to the overall problem. The reusable components should be as "modular" as possible. A module is a component of a larger system that interacts with the rest of the system in a simple, well-defined, straightforward manner. The idea is that a module can be "plugged into" a system. The details of what goes on inside the module are not important to the system as a whole, as long as the module fulfills its assigned role correctly. This is called information hiding, and it is one of the most important principles of software engineering. One common format for software modules is to contain some data, along with some subroutines for manipulating that data. For example, a mailing-list module might contain a list of names and addresses along with a subroutine for adding a new name, a subroutine for printing mailing labels, and so forth. In such modules, the data itself is often hidden inside the module; a program that uses the module can then manipulate the data only indirectly, by calling the subroutines provided by the module. This protects the data, since it can only be manipulated in known, well-defined ways. And it makes it easier for programs to use the module, since they don't have to worry about the details of how the data is represented. Information about the representation of the data is hidden. Modules that could support this kind of information-hiding became common in programming languages in the early 1980s. Since then, a more advanced form of the same idea has more or less taken over software engineering. This latest approach is called object-oriented programming, often abbreviated as OOP. The central concept of object-oriented programming is the object, which is a kind of module containing http://math.hws.edu/eck/cs124/javanotes4/c1/s5.html (1 of 3) [6/11/2004 11:07:48 AM] Java Programming: Section 1.5 data and subroutines. The point-of-view in OOP is that an object is a kind of self-sufficient entity that has an internal state (the data it contains) and that can respond to messages (calls to its subroutines). A mailing list object, for example, has a state consisting of a list of names and addresses. If you send it a message telling it to add a name, it will respond by modifying its state to reflect the change. If you send it a message telling it to print itself, it will respond by printing out its list of names and addresses. The OOP approach to software engineering is to start by identifying the objects involved in a problem and the messages that those objects should respond to. The program that results is a collection of objects, each with its own data and its own set of responsibilities. The objects interact by sending messages to each other. There is not much "top-down" in such a program, and people used to more traditional programs can have a hard time getting used to OOP. However, people who use OOP would claim that object-oriented programs tend to be better models of the way the world itself works, and that they are therefore easier to write, easier to understand, and more likely to be correct. You should think of objects as "knowing" how to respond to certain messages. Different objects might respond to the same message in different ways. For example, a "print" message would produce very different results, depending on the object it is sent to. This property of objects -- that different objects can respond to the same message in different ways -- is called polymorphism. It is common for objects to bear a kind of "family relationship" to one another. Objects that contain the same type of data and that respond to the same messages in the same way belong to the same class. (In actual programming, the class is primary; that is, a class is created and then one or more objects are created using that class as a template.) But objects can be similar without being in exactly the same class. For example, consider a drawing program that lets the user draw lines, rectangles, ovals, polygons, and curves on the screen. In the program, each visible object on the screen could be represented by a software object in the program. There would be five classes of objects in the program, one for each type of visible object that can be drawn. All the lines would belong to one class, all the rectangles to another class, and so on. These classes are obviously related; all of them represent "drawable objects." They would, for example, all presumably be able to respond to a "draw yourself" message. Another level of grouping, based on the data needed to represent each type of object, is less obvious, but would be very useful in a program: We can group polygons and curves together as "multipoint objects," while lines, rectangles, and ovals are "two-point objects." (A line is determined by its endpoints, a rectangle by two of its corners, and an oval by two corners of the rectangle that contains it.) We could diagram these relationships as follows: DrawableObject, MultipointObject, and TwoPointObject would be classes in the program. MultipointObject and TwoPointObject would be subclasses of DrawableObject. The class Line would be a subclass of TwoPointObject and (indirectly) of DrawableObject. A subclass of a class is said to inherit the properties of that class. The subclass can add to its inheritance and it can even "override" part of that inheritance (by defining a different response to some method). Nevertheless, lines, rectangles, and so on are drawable objects, and the class DrawableObject expresses this relationship. Inheritance is a powerful means for organizing a program. It is also related to the problem of reusing http://math.hws.edu/eck/cs124/javanotes4/c1/s5.html (2 of 3) [6/11/2004 11:07:48 AM] Java Programming: Section 1.5 software components. A class is the ultimate reusable component. Not only can it be reused directly if it fits exactly into a program you are trying to write, but if it just almost fits, you can still reuse it by defining a subclass and making only the small changes necessary to adapt it exactly to your needs. So, OOP is meant to be both a superior program-development tool and a partial solution to the software reuse problem. Objects, classes, and object-oriented programming will be important themes throughout the rest of this text. [ Next Section | Previous Section | Chapter Index | Main Index ] http://math.hws.edu/eck/cs124/javanotes4/c1/s5.html (3 of 3) [6/11/2004 11:07:48 AM] Java Programming: Section 1.6 Section 1.6 The Modern User Interface WHEN COMPUTERS WERE FIRST INTRODUCED, ordinary people -- including most programmers -- couldn't get near them. They were locked up in rooms with white-coated attendants who would take your programs and data, feed them to the computer, and return the computer's response some time later. When timesharing -- where the computer switches its attention rapidly from one person to another -- was invented in the 1960s, it became possible for several people to interact directly with the computer at the same time. On a timesharing system, users sit at "terminals" where they type commands to the computer, and the computer types back its response. Early personal computers also used typed commands and responses, except that there was only one person involved at a time. This type of interaction between a user and a computer is called a command-line interface. Today, of course, most people interact with computers in a completely different way. They use a Graphical User Interface, or GUI. The computer draws interface components on the screen. The components include things like windows, scroll bars, menus, buttons, and icons. Usually, a mouse is used to manipulate such components. Assuming that you are reading these notes on a computer, you are no doubt already familiar with the basics of graphical user interfaces! A lot of GUI interface components have become fairly standard. That is, they have similar appearance and behavior on many different computer platforms including Macintosh, Windows, and various UNIX window systems. Java programs, which are supposed to run on many different platforms without modification to the program, can use all the standard GUI components. They might vary in appearance from platform to platform, but their functionality should be identical on any computer on which the program runs. Below is a very simple Java program -- actually an "applet," since it is running right here in the middle of a page -- that shows a few standard GUI interface components. There are four components that you can interact with: a button, a checkbox, a text field, and a pop-up menu. These components are labeled. There are a few other components in the applet. The labels themselves are components (even though you can't interact with them). The right half of the applet is a text area component, which can display multiple lines of text. In fact, in Java terminology, the whole applet is itself considered to be a "component." Try clicking on the button and on the checkbox, and try selecting an item from the pop-up menu. You will see a message in the text area about each action that you perform. You can type in the text field, but you might have to click on it first to activate it. When you press return while typing in the text field, you will see a message in the text area: (Applet "GUIDemo" would be displayed here if Java were available.) Now, Java actually has two complete sets of GUI components. One of these, the AWT or Abstract Windowing Toolkit, was available in the original version of Java. The other, which is known as Swing, is included in Java version 1.2 or later. Here is a version of the applet that uses Swing instead of the AWT. If you see the above applet but just see a blank area here, it means that you are using a Web browser that uses an older version of Java: (Applet "GUIDemo2" would be displayed here if Java were available.) As you interact with the GUI components in these applets, you generate "events." For example, clicking a push button generates an event. Each time an event is generated, a message is sent to the applet telling it that the event has occurred, and the applet responds according to its program. In fact, the program consists mainly of "event handlers" that tell the applet how to respond to various types of events. In this example, the applets have been programmed to respond to each event by displaying a message in the text area. http://math.hws.edu/eck/cs124/javanotes4/c1/s6.html (1 of 2) [6/11/2004 11:07:48 AM] Java Programming: Section 1.6 The use of the term "message" here is deliberate. Messages, as you saw in the previous section, are sent to objects. In fact, Java GUI components are implemented as objects. Java includes many predefined classes that represent various types of GUI components. Some of these classes are subclasses of others. Here is a diagram showing some of the GUI classes in the AWT and their relationships: Don't worry about the details for now, but try to get some feel about how object-oriented programming and inheritance are used here. Note that all the GUI classes are subclasses, directly or indirectly, of a class called Component. Two of the direct subclasses of Component themselves have subclasses. The classes TextArea and TextField, which have certain behaviors in common, are grouped together as subclasses of TextComponent. The class named Container refers to components that can contain other components. The Applet class is, indirectly, a subclass of Container since applets can contain components such as buttons and text fields. Just from this brief discussion, perhaps you can see how GUI programming can make effective use of object-oriented design. In fact, GUI's, with their "visible objects," are probably a major factor contributing to the popularity of OOP. Programming with GUI components and events is one of the most interesting aspects of Java. However, we will spend several chapters on the basics before returning to this topic in Chapter 6. [ Next Section | Previous Section | Chapter Index | Main Index ] http://math.hws.edu/eck/cs124/javanotes4/c1/s6.html (2 of 2) [6/11/2004 11:07:48 AM] Java Programming: Section 1.7 Section 1.7 The Internet and the World-Wide Web COMPUTERS CAN BE CONNECTED together on networks. A computer on a network can communicate with other computers on the same network by exchanging data and files or by sending and receiving messages. Computers on a network can even work together on a large computation. Today, millions of computers throughout the world are connected to a single huge network called the Internet. New computers are being connected to the Internet every day. In fact, a computer can join the Internet temporarily by using a modem to establish a connection through telephone lines. There are elaborate protocols for communication over the Internet. A protocol is simply a detailed specification of how communication is to proceed. For two computers to communicate at all, they must both be using the same protocols. The most basic protocols on the Internet are the Internet Protocol (IP), which specifies how data is to be physically transmitted from one computer to another, and the Transmission Control Protocol (TCP), which ensures that data sent using IP is received in its entirety and without error. These two protocols, which are referred to collectively as TCP/IP, provide a foundation for communication. Other protocols use TCP/IP to send specific types of information such as files and electronic mail. All communication over the Internet is in the form of packets. A packet consists of some data being sent from one computer to another, along with addressing information that indicates where on the Internet that data is supposed to go. Think of a packet as an envelope with an address on the outside and a message on the inside. (The message is the data.) The packet also includes a "return address," that is, the address of the sender. A packet can hold only a limited amount of data; longer messages must be divided among several packets, which are then sent individually over the net and reassembled at their destination. Every computer on the Internet has an IP address, a number that identifies it uniquely among all the computers on the net. The IP address is used for addressing packets. A computer can only send data to another computer on the Internet if it knows that computer's IP address. Since people prefer to use names rather than numbers, many computers are also identified by names, called domain names. For example, the main computer at Hobart and William Smith Colleges has the domain name hws3.hws.edu. (Domain names are just for convenience; your computer still needs to know IP addresses before it can communicate. There are computers on the Internet whose job it is to translate domain names to IP addresses. When you use a domain name, your computer sends a message to a domain name server to find out the corresponding IP address. Then, your computer uses the IP address, rather than the domain name, to communicate with the other computer.) The Internet provides a number of services to the computers connected to it (and, of course, to the users of those computers). These services use TCP/IP to send various types of data over the net. Among the most popular services are remote login, electronic mail, FTP, and the World-Wide Web. Remote login allows a person using one computer to log on to another computer. (Of course, that person needs to know a user name and password for an account on the other computer.) There are several different protocols for remote login, including the traditional telnet and the more secure ssh (secure shell). Telnet and ssh provide only a command-line interface. Essentially, the first computer acts as a terminal for the second. Remote login is often used by people who are away from home to access their computer accounts back home -- and they can do so from any computer on the Internet, anywhere in the world. Electronic mail, or email, provides person-to-person communication over the Internet. An email message is sent by a particular user of one computer to a particular user of another computer. Each person is identified by a unique email address, which consists of the domain name of the computer where they receive their mail together with their user name or personal name. The email address has the form http://math.hws.edu/eck/cs124/javanotes4/c1/s7.html (1 of 2) [6/11/2004 11:07:49 AM] Java Programming: Section 1.7 "username@domain.name". For example, my own email address is: eck@hws.edu. Email is actually transferred from one computer to another using a protocol called SMTP (Simple Mail Transfer Protocol). Email might still be the most common and important use of the Internet, although it has certainly been challenged in popularity by the World-Wide Web. FTP (File Transport Protocol) is designed to copy files from one computer to another. As with remote login, an FTP user needs a user name and password to get access to a computer. However, many computers have been set up with special accounts that can be accessed through FTP with the user name "anonymous" and any password. This so-called anonymous FTP can be used to make files on one computer publically available to anyone with Internet access. The World-Wide Web (WWW) is based on pages which can contain information of many different kinds as well as links to other pages. These pages are viewed with a Web browser program such as Netscape or Internet Explorer. Many people seem to think that the World-Wide Web is the Internet, but it's really just a graphical user interface to the Internet. The pages that you view with a Web browser are just files that are stored on computers connected to the Internet. When you tell your Web browser to load a page, it contacts the computer on which the page is stored and transfers it to your computer using a protocol known as HTTP (HyperText Transfer Protocol). Any computer on the Internet can publish pages on the World-Wide Web. When you use a Web browser, you have access to a huge sea of interlinked information that can be navigated with no special computer expertise. The Web is the most exciting part of the Internet and is driving the Internet to a truly phenomenal rate of growth. If it fulfills its promise, the Web might become a universal and fundamental part of everyday life. I should note that a typical Web browser can use other protocols besides HTTP. For example, it can also use FTP to transfer files. The traditional user interface for FTP was a command-line interface, so among all the other things it does, a Web browser provides a modern graphical user interface for FTP. This allows people to use FTP without even knowing that there is such a thing! (This fact should help you understand the difference between a program and a protocol. FTP is not a program. It is a protocol, that is, a set of standards for a certain type of communication between computers. To use FTP, you need a program that implements those standards. Different FTP programs can present you with very different user interfaces. Similarly, different Web browser programs can present very different interfaces to the user, but they must all use HTTP to get information from the Web.) Now just what, you might be thinking, does all this have to do with Java? In fact, Java is intimately associated with the Internet and the World-Wide Web. As you have seen in the previous section, special Java programs called applets are meant to be transmitted over the Internet and displayed on Web pages. A Web server transmits a Java applet just as it would transmit any other type of information. A Web browser that understands Java -- that is, that includes an interpreter for the Java virtual machine -- can then run the applet right on the Web page. Since applets are programs, they can do almost anything, including complex interaction with the user. With Java, a Web page becomes more than just a passive display of information. It becomes anything that programmers can imagine and implement. Its association with the Web is not Java's only advantage. But many good programming languages have been invented only to be soon forgotten. Java has had the good luck to ride on the coattails of the Web's immense and increasing popularity. End of Chapter 1 [ Next Chapter | Previous Section | Chapter Index | Main Index ] http://math.hws.edu/eck/cs124/javanotes4/c1/s7.html (2 of 2) [6/11/2004 11:07:49 AM] Java Programming: Chapter 1 Quiz Quiz Questions For Chapter 1 THIS PAGE CONTAINS A SAMPLE quiz on material from Chapter 1 of this on-line Java textbook. You should be able to answer these questions after studying that chapter. Sample answers to all the quiz questions can be found here. Question 1: One of the components of a computer is its CPU. What is a CPU and what role does it play in a computer? Question 2: Explain what is meant by an "asynchronous event." Give some examples. Question 3: What is the difference between a "compiler" and an "interpreter"? Question 4: Explain the difference between high-level languages and machine language. Question 5: If you have the source code for a Java program, and you want to run that program, you will need both a compiler and an interpreter. What does the Java compiler do, and what does the Java interpreter do? Question 6: What is a subroutine? Question 7: Java is an object-oriented programming language. What is an object? Question 8: What is a variable? (There are four different ideas associated with variables in Java. Try to mention all four aspects in your answer. Hint: One of the aspects is the variable's name.) Question 9: Java is a "platform-independent language." What does this mean? Question 10: What is the "Internet"? Give some examples of how it is used. (What kind of services does it provide?) [ Answers | Chapter Index | Main Index ] http://math.hws.edu/eck/cs124/javanotes4/c1/quiz.html [6/11/2004 11:07:49 AM] Java Programming: Chapter 2 Index Chapter 2 Programming in the Small I Names and Things ON A BASIC LEVEL (the level of machine language), a computer can perform only very simple operations. A computer performs complex tasks by stringing together large numbers of such operations. Such tasks must be "scripted" in complete and perfect detail by programs. Creating complex programs will never be really easy, but the difficulty can be handled to some extent by giving the program a clear overall structure. The design of the overall structure of a program is what I call "programming in the large." Programming in the small, which is sometimes called coding, would then refer to filling in the details of that design. The details are the explicit, step-by-step instructions for performing fairly small-scale tasks. When you do coding, you are working fairly "close to the machine," with some of the same concepts that you might use in machine language: memory locations, arithmetic operations, loops and decisions. In a high-level language such as Java, you get to work with these concepts on a level several steps above machine language. However, you still have to worry about getting all the details exactly right. This chapter and the next examine the facilities for programming in the small in the Java programming language. Don't be misled by the term "programming in the small" into thinking that this material is easy or unimportant. This material is an essential foundation for all types of programming. If you don't understand it, you can't write programs, no matter how good you get at designing their large-scale structure. Contents of Chapter 2: ● Section 1: The Basic Java Application ● Section 2: Variables and the Primitive Types ● Section 3: Strings, Objects, and Subroutines ● Section 4: Text Input and Output ● Section 5: Details of Expressions ● Programming Exercises ● Quiz on this Chapter [ First Section | Next Chapter | Previous Chapter | Main Index ] http://math.hws.edu/eck/cs124/javanotes4/c2/ [6/11/2004 11:08:11 AM] Java Programming: Section 2.1 Section 2.1 The Basic Java Application A PROGRAM IS A SEQUENCE OF INSTRUCTIONS that a computer can execute to perform some task. A simple enough idea, but for the computer to make any use of the instructions, they must be written in a form that the computer can use. This means that programs have to be written in programming languages. Programming languages differ from ordinary human languages in being completely unambiguous and very strict about what is and is not allowed in a program. The rules that determine what is allowed are called the syntax of the language. Syntax rules specify the basic vocabulary of the language and how programs can be constructed using things like loops, branches, and subroutines. A syntactically correct program is one that can be successfully compiled or interpreted; programs that have syntax errors will be rejected (hopefully with a useful error message that will help you fix the problem). So, to be a successful programmer, you have to develop a detailed knowledge of the syntax of the programming language that you are using. However, syntax is only part of the story. It's not enough to write a program that will run. You want a program that will run and produce the correct result! That is, the meaning of the program has to be right. The meaning of a program is referred to as its semantics. A semantically correct program is one that does what you want it to. When I introduce a new language feature in these notes, I will explain both the syntax and the semantics of that feature. You should memorize the syntax; that's the easy part. Then you should try to get a feeling for the semantics by following the examples given, making sure that you understand how they work, and maybe writing short programs of your own to test your understanding. Of course, even when you've become familiar with all the individual features of the language, that doesn't make you a programmer. You still have to learn how to construct complex programs to solve particular problems. For that, you'll need both experience and taste. You'll find hints about software development throughout this textbook. We begin our exploration of Java with the problem that has become traditional for such beginnings: to write a program that displays the message "Hello World!". This might seem like a trivial problem, but getting a computer to do this is really a big first step in learning a new programming language (especially if it's your first programming language). It means that you understand the basic process of: 1. getting the program text into the computer, 2. compiling the program, and 3. running the compiled program. The first time through, each of these steps will probably take you a few tries to get right. I can't tell you the details here of how you do each of these steps; it depends on the particular computer and Java programming environment that you are using. (See Appendix 2 for information on some common Java programming environments.) But in general, you will type the program using some sort of text editor and save the program in a file. Then, you will use some command to try to compile the file. You'll either get a message that the program contains syntax errors, or you'll get a compiled version of the program. In the case of Java, the program is compiled into Java bytecode, not into machine language. Finally, you can run the compiled program by giving some appropriate command. For Java, you will actually use an interpreter to execute the Java bytecode. Your programming environment might automate some of the steps for you, but you can be sure that the same three steps are being done in the background. Here is a Java program to display the message "Hello World!". Don't expect to understand what's going on here just yet -- some of it you won't really understand until a few chapters from now: public class HelloWorld { http://math.hws.edu/eck/cs124/javanotes4/c2/s1.html (1 of 3) [6/11/2004 11:08:12 AM] Java Programming: Section 2.1 // A program to display the message // "Hello World!" on standard output public static void main(String[] args) { System.out.println("Hello World!"); } } // end of class HelloWorld The command that actually displays the message is: System.out.println("Hello World!"); This command is an example of a subroutine call statement. It uses a "built-in subroutine" named System.out.println to do the actual work. Recall that a subroutine consists of the instructions for performing some task, chunked together and given a name. That name can be used to "call" the subroutine whenever that task needs to be performed. A built-in subroutine is one that is already defined as part of the language and therefore automatically available for use in any program. When you run this program, the message "Hello World!" (without the quotes) will be displayed on standard output. Unfortunately, I can't say exactly what that means! Java is meant to run on many different platforms, and standard output will mean different things on different platforms. However, you can expect the message to show up in some convenient place. (If you use a command-line interface, like that in Sun Microsystem's Java Development Kit, you type in a command to tell the computer to run the program. The computer will type the output from the program, Hello World!, on the next line.) You must be curious about all the other stuff in the above program. Part of it consists of comments. Comments in a program are entirely ignored by the computer; they are there for human readers only. This doesn't mean that they are unimportant. Programs are meant to be read by people as well as by computers, and without comments, a program can be very difficult to understand. Java has two types of comments. The first type, used in the above program, begins with // and extends to the end of a line. The computer ignores the // and everything that follows it on the same line. Java has another style of comment that can extend over many lines. That type of comment begins with /* and ends with */. Everything else in the program is required by the rules of Java syntax. All programming in Java is done inside "classes." The first line in the above program says that this is a class named HelloWorld. "HelloWorld," the name of the class, also serves as the name of the program. Not every class is a program. In order to define a program, a class must include a subroutine called main, with a definition that takes the form: public static void main(String[] args) { statements } When you tell the Java interpreter to run the program, the interpreter calls the main() subroutine, and the statements that it contains are executed. These statements make up the script that tells the computer exactly what to do when the program is executed. The main() routine can call subroutines that are defined in the same class or even in other classes, but it is the main() routine that determines how and in what order the other subroutines are used. The word "public" in the first line of main() means that this routine can be called from outside the program. This is essential because the main() routine is called by the Java interpreter. The remainder of the first line of the routine is harder to explain at the moment; for now, just think of it as part of the required syntax. The definition of the subroutine -- that is, the instructions that say what it does -- consists of the sequence of "statements" enclosed between braces, { and }. Here, I've used statements as a placeholder for the actual statements that make up the program. Throughout this textbook, I will always use a similar format: anything that you see in this style of text (which is green if your browser supports colored text) is a http://math.hws.edu/eck/cs124/javanotes4/c2/s1.html (2 of 3) [6/11/2004 11:08:12 AM] Java Programming: Section 2.1 placeholder that describes something you need to type when you write an actual program. As noted above, a subroutine can't exist by itself. It has to be part of a "class". A program is defined by a public class that takes the form: public class program-name { optional-variable-declarations-and-subroutines public static void main(String[] args) { statements } optional-variable-declarations-and-subroutines } The name on the first line is the name of the program, as well as the name of the class. If the name of the class is HelloWorld, then the class should be saved in a file called HelloWorld.java. When this file is compiled, another file named HelloWorld.class will be produced. This class file, HelloWorld.class, contains the Java bytecode that is executed by a Java interpreter. HelloWorld.java is called the source code for the program. To execute the program, you only need the compiled class file, not the source code. Also note that according to the above syntax specification, a program can contain other subroutines besides main(), as well as things called "variable declarations." You'll learn more about these later (starting with variables, in the next section). By the way, recall that one of the neat features of Java is that it can be used to write applets that can run on pages in a Web browser. Applets are very different things from stand-alone programs such as the HelloWorld program, and they are not written in the same way. For one thing, an applet doesn't have a main() routine. Applets will be covered in Chapter 6 and Chapter 7. In the meantime, you will see applets in this text that simulate stand-alone programs. The applets you see are not really the same as the stand-alone programs that they simulate, since they run right on a Web page, but they will have the same behavior as the programs I describe. Here, just for fun, is an applet simulating the HelloWorld program. To run the program, click on the button: (Applet "ConsoleApplet" would be displayed here if Java were available.) [ Next Section | Previous Chapter | Chapter Index | Main Index ] http://math.hws.edu/eck/cs124/javanotes4/c2/s1.html (3 of 3) [6/11/2004 11:08:12 AM] Java Programming: Section 2.2 Section 2.2 Variables and the Primitive Types N AMES ARE FUNDAMENTAL TO PROGRAMMING. In programs, names are used to refer to many different sorts of things. In order to use those things, a programmer must understand the rules for giving names to things and the rules for using the names to work with those things. That is, the programmer must understand the syntax and the semantics of names. According to the syntax rules of Java, a name is a sequences of one or more characters. It must begin with a letter and must consist entirely of letters, digits, and the underscore character '_'. For example, here are some legal names: N n rate x15 quite_a_long_name HelloWorld Uppercase and lowercase letters are considered to be different, so that HelloWorld, helloworld, HELLOWORLD, and hElloWorLD are all distinct names. Certain names are reserved for special uses in Java, and cannot be used by the programmer for other purposes. These reserved words include: class, public, static, if, else, while, and several dozen other words. Java is actually pretty liberal about what counts as a letter or a digit. Java uses the Unicode character set, which includes thousands of characters from many different languages and different alphabets, and many of these characters count as letters or digits. However, I will be sticking to what can be typed on a regular English keyboard. Finally, I'll note that often things are referred to by "compound names" which consist of several ordinary names separated by periods. You've already seen an example: System.out.println. The idea here is that things in Java can contain other things. A compound name is a kind of path to an item through one or more levels of containment. The name System.out.println indicates that something called "System" contains something called "out" which in turn contains something called "println". I'll use the term identifier to refer to any name -- single or compound -- that can be used to refer to something in Java. (Note that the reserved words are not identifiers, since they can't be used as names for things.) Programs manipulate data that are stored in memory. In machine language, data can only be referred to by giving the numerical address of the location in memory where it is stored. In a high-level language such as Java, names are used instead of numbers to refer to data. It is the job of the computer to keep track of where in memory the data is actually stored; the programmer only has to remember the name. A name used in this way -- to refer to data stored in memory -- is called a variable. Variables are actually rather subtle. Properly speaking, a variable is not a name for the data itself but for a location in memory that can hold data. You should think of a variable as a container or box where you can store data that you will need to use later. The variable refers directly to the box and only indirectly to the data in the box. Since the data in the box can change, a variable can refer to different data values at different times during the execution of the program, but it always refers to the same box. Confusion can arise, especially for beginning programmers, because when a variable is used in a program in certain ways, it refers to the container, but when it is used in other ways, it refers to the data in the container. You'll see examples of both cases below. (In this way, a variable is something like the title, "The President of the United States." This title can refer to different people at different time, but it always refers to the same office. If I say "the President went fishing," I mean that George W. Bush went fishing. But if I say "Hillary Clinton wants to be President" I mean that she wants to fill the office, not that she wants to be George Bush.) In Java, the only way to get data into a variable -- that is, into the box that the variable names -- is with an assignment statement. An assignment statement takes the form: variable = expression; where expression represents anything that refers to or computes a data value. When the computer comes to an assignment statement in the course of executing a program, it evaluates the expression and puts the resulting data value into the variable. For example, consider the simple assignment statement http://math.hws.edu/eck/cs124/javanotes4/c2/s2.html (1 of 5) [6/11/2004 11:08:13 AM] Java Programming: Section 2.2 rate = 0.07; The variable in this assignment statement is rate, and the expression is the number 0.07. The computer executes this assignment statement by putting the number 0.07 in the variable rate, replacing whatever was there before. Now, consider the following more complicated assignment statement, which might come later in the same program: interest = rate * principal; Here, the value of the expression "rate * principal" is being assigned to the variable interest. In the expression, the * is a "multiplication operator" that tells the computer to multiply rate times principal. The names rate and principal are themselves variables, and it is really the values stored in those variables that are to be multiplied. We see that when a variable is used in an expression, it is the value stored in the variable that matters; in this case, the variable seems to refer to the data in the box, rather than to the box itself. When the computer executes this assignment statement, it takes the value of rate, multiplies it by the value of principal, and stores the answer in the box referred to by interest. (Note, by the way, that an assignment statement is a command that is executed by the computer at a certain time. It is not a statement of fact. For example, suppose a program includes the statement "rate = 0.07;". If the statement "interest = rate * principal;" is executed later in the program, can we say that the principal is multiplied by 0.07? No! The value of rate might have been changed in the meantime by another statement. The meaning of an assignment statement is completely different from the meaning of an equation in mathematics, even though both use the symbol "=".) A variable in Java is designed to hold only one particular type of data; it can legally hold that type of data and no other. The compiler will consider it to be a syntax error if you try to violate this rule. We say that Java is a strongly typed language because it enforces this rule. There are eight so-called primitive types built into Java. The primitive types are named byte, short, int, long, float, double, char, and boolean. The first four types hold integers (whole numbers such as 17, -38477, and 0). The four integer types are distinguished by the ranges of integers they can hold. The float and double types hold real numbers (such as 3.6 and -145.99). Again, the two real types are distinguished by their range and accuracy. A variable of type char holds a single character from the Unicode character set. And a variable of type boolean holds one of the two logical values true or false. Any data value stored in the computer's memory must be represented as a binary number, that is as a string of zeros and ones. A single zero or one is called a bit. A string of eight bits is called a byte. Memory is usually measured in terms of bytes. Not surprisingly, the byte data type refers to a single byte of memory. A variable of type byte holds a string of eight bits, which can represent any of the integers between -128 and 127, inclusive. (There are 256 integers in that range; eight bits can represent 256 -- two raised to the power eight -- different values.) As for the other integer types, ● short corresponds to two bytes (16 bits). Variables of type short have values in the range -32768 to 32767. ● int corresponds to four bytes (32 bits). Variables of type int have values in the range -2147483648 to 2147483647. ● long corresponds to eight bytes (64 bits). Variables of type long have values in the range -9223372036854775808 to 9223372036854775807. You don't have to remember these numbers, but they do give you some idea of the size of integers that you can work with. Usually, you should just stick to the int data type, which is good enough for most purposes. The float data type is represented in four bytes of memory, using a standard method for encoding real numbers. The maximum value for a float is about 10 raised to the power 38. A float can have about 7 significant digits. (So that 32.3989231134 and 32.3989234399 would both have to be rounded off to about 32.398923 in order to be stored in a variable of type float.) A double takes up 8 bytes, can range up to about 10 to the power 308, and has about 15 significant digits. Ordinarily, you should stick to the double type for real values. A variable of type char occupies two bytes in memory. The value of a char variable is a single character such as A, *, x, or a space character. The value can also be a special character such a tab or a carriage return or one of the many Unicode characters that come from different languages. When a character is typed into a program, it must be http://math.hws.edu/eck/cs124/javanotes4/c2/s2.html (2 of 5) [6/11/2004 11:08:13 AM] Java Programming: Section 2.2 surrounded by single quotes; for example: 'A', '*', or 'x'. Without the quotes, A would be an identifier and * would be a multiplication operator. The quotes are not part of the value and are not stored in the variable; they are just a convention for naming a particular character constant in a program. A name for a constant value is called a literal. A literal is what you have to type in a program to represent a value. 'A' and '*' are literals of type char, representing the character values A and *. Certain special characters have special literals that use a backslash, \, as an "escape character". In particular, a tab is represented as '\t', a carriage return as '\r', a linefeed as '\n', the single quote character as '\'', and the backslash itself as '\\'. Note that even though you type two characters between the quotes in '\t', the value represented by this literal is a single tab character. Numeric literals are a little more complicated than you might expect. Of course, there are the obvious literals such as 317 and 17.42. But there are other possibilities for expressing numbers in a Java program. First of all, real numbers can be represented in an exponential form such as 1.3e12 or 12.3737e-108. The "e12" and "e-108" represent powers of 10, so that 1.3e12 means 1.3 times 1012 and 12.3737e-108 means 12.3737 times 10-108. This format is used for very large and very small numbers. Any numerical literal that contains a decimal point or exponential is a literal of type double. To make a literal of type float, you have to append an "F" or "f" to the end of the number. For example, "1.2F" stands for 1.2 considered as a value of type float. (Occasionally, you need to know this because the rules of Java say that you can't assign a value of type double to a variable of type float, so you might be confronted with a ridiculous-seeming error message if you try to do something like "float x = 1.2;". You have to say "float x = 1.2F;". This is one reason why I advise sticking to type double for real numbers.) Even for integer literals, there are some complications. Ordinary integers such as 177777 and -32 are literals of type byte, short, or int, depending on their size. You can make a literal of type long by adding "L" as a suffix. For example: 17L or 728476874368L. As another complication, Java allows octal (base-8) and hexadecimal (base-16) literals. (I don't want to cover base-8 and base-16 in these notes, but in case you run into them in other people's programs, it's worth knowing that a zero at the beginning of an integer makes it an octal literal, as in 045 or 077. A hexadecimal literal begins with 0x or 0X, as in 0x45 or 0xFF7A. By the way, the octal literal 045 represents the number 37, not the number 45.) For the type boolean, there are precisely two literals: true and false. These literals are typed just as I've written them here, without quotes, but they represent values, not variables. Boolean values occur most often as the values of conditional expressions. For example, rate > 0.05 is a boolean-valued expression that evaluates to true if the value of the variable rate is greater than 0.05, and to false if the value of rate is not greater than 0.05. As you'll see in Chapter 3, boolean-valued expressions are used extensively in control structures. Of course, boolean values can also be assigned to variables of type boolean. Java has other types in addition to the primitive types, but all the other types represent objects rather than "primitive" data values. For the most part, we are not concerned with objects for the time being. However, there is one predefined object type that is very important: the type String. A String is a sequence of characters. You've already seen a string literal: "Hello World!". The double quotes are part of the literal; they have to be typed in the program. However, they are not part of the actual string value, which consists of just the characters between the quotes. Within a string, special characters can be represented using the backslash notation. Within this context, the double quote is itself a special character. For example, to represent the string value I said, "Are you listening!" with a linefeed at the end, you would have to type the literal: "I said, \"Are you listening!\"\n" Because strings are objects, their behavior in programs is peculiar in some respects (to someone who is not used to objects). I'll have more to say about them in the next section. A variable can be used in a program only if it has first been declared. A variable declaration statement is used to declare one or more variables and to give them names. When the computer executes a variable declaration, it sets aside memory for the variable and associates the variable's name with that memory. A simple variable declaration http://math.hws.edu/eck/cs124/javanotes4/c2/s2.html (3 of 5) [6/11/2004 11:08:13 AM] Java Programming: Section 2.2 takes the form: type-name variable-name-or-names; The variable-name-or-names can be a single variable name or a list of variable names separated by commas. (We'll see later that variable declaration statements can actually be somewhat more complicated than this.) Good programming style is to declare only one variable in a declaration statement, unless the variables are closely related in some way. For example: int numberOfStudents; String name; double x, y; boolean isFinished; char firstInitial, middleInitial, lastInitial; In this chapter, we will only use variables declared inside the main() subroutine of a program. Variables declared inside a subroutine are called local variables for that subroutine. They exist only inside the subroutine, while it is running, and are completely inaccessible from outside. Variable declarations can occur anywhere inside the subroutine, as long as each variable is declared before it is used in any expression. Some people like to declare all the variables at the beginning of the subroutine. Others like to wait to declare a variable until it is needed. My preference: Declare important variables at the beginning of the subroutine, and use a comment to explain the purpose of each variable. Declare "utility variables" which are not important to the overall logic of the subroutine at the point in the subroutine where they are first used. Here is a simple program using some variables and assignment statements: public class Interest { /* This class implements a simple program that will compute the amount of interest that is earned on $17,000 invested at an interest rate of 0.07 for one year. The interest and the value of the investment after one year are printed to standard output. */ public static void main(String[] args) { /* Declare the variables. */ double principal; // The value of the investment. double rate; // The annual interest rate. double interest; // Interest earned in one year. /* Do the computations. */ principal = 17000; rate = 0.07; interest = principal * rate; // Compute the interest. principal = principal + interest; // Compute value of investment after one year, with interest. // (Note: The new value replaces the old value of principal.) /* Output the results. */ System.out.print("The interest earned is $"); System.out.println(interest); System.out.print("The value of the investment after one year is $"); System.out.println(principal); } // end of main() http://math.hws.edu/eck/cs124/javanotes4/c2/s2.html (4 of 5) [6/11/2004 11:08:13 AM] Java Programming: Section 2.2 } // end of class Interest And here is an applet that simulates this program: (Applet "Interest1Console" would be displayed here if Java were available.) This program uses several subroutine call statements to display information to the user of the program. Two different subroutines are used: System.out.print and System.out.println. The difference between these is that System.out.println adds a carriage return after the end of the information that it displays, while System.out.print does not. Thus, the value of interest, which is displayed by the subroutine call "System.out.println(interest);", follows on the same line after the string displayed by the previous statement. Note that the value to be displayed by System.out.print or System.out.println is provided in parentheses after the subroutine name. This value is called a parameter to the subroutine. A parameter provides a subroutine with information it needs to perform its task. In a subroutine call statement, any parameters are listed in parentheses after the subroutine name. Not all subroutines have parameters. If there are no parameters in a subroutine call statement, the subroutine name must be followed by an empty pair of parentheses. [ Next Section | Previous Section | Chapter Index | Main Index ] http://math.hws.edu/eck/cs124/javanotes4/c2/s2.html (5 of 5) [6/11/2004 11:08:13 AM] Java Programming: Section 2.3 Section 2.3 Strings, Objects, and Subroutines THE PREVIOUS SECTION introduced the eight primitive data types and the type String. There is a fundamental difference between the primitive types and the String type: Values of type String are objects. While we will not study objects in detail until Chapter 5, it will be useful for you to know a little about them and about a closely related topic: classes. This is not just because strings are useful but because objects and classes are essential to understanding another important programming concept, subroutines. Recall that a subroutine is a set of program instructions that have been chunked together and given a name. In Chapter 4, you'll learn how to write your own subroutines, but you can get a lot done in a program just by calling subroutines that have already been written for you. In Java, every subroutine is contained in a class or in an object. Some classes that are standard parts of the Java language contain predefined subroutines that you can use. A value of type String, which is an object, contains subroutines that can be used to manipulate that string. You can call all these subroutines without understanding how they were written or how they work. Indeed, that's the whole point of subroutines: A subroutine is a "black box" which can be used without knowing what goes on inside. Classes in Java have two very different functions. First of all, a class can group together variables and subroutines that are contained in that class. These variables and subroutines are called static members of the class. You've seen one example: In a class that defines a program, the main() routine is a static member of the class. The parts of a class definition that define static members are marked with the reserved word "static", just like the main() routine of a program. However, classes have a second function. They are used to describe objects. In this role, the class of an object specifies what subroutines and variables are contained in that object. The class is a type -- in the technical sense of a specification of a certain type of data value -- and the object is a value of that type. For example, String is actually the name of a class that is included as a standard part of the Java language. It is also a type, and actual strings such as "Hello World" are values of type String. So, every subroutine is contained either in a class or in an object. Classes contain subroutines called static member subroutines. Classes also describe objects and the subroutines that are contained in those objects. This dual use can be confusing, and in practice most classes are designed to perform primarily or exclusively in only one of the two possible roles. For example, although the String class does contain a few rarely-used static member subroutines, it exists mainly to specify a large number of subroutines that are contained in objects of type String. Another standard class, named Math, exists entirely to group together a number of static member subroutines that compute various common mathematical functions. To begin to get a handle on all of this complexity, let's look at the subroutine System.out.print as an example. As you have seen earlier in this chapter, this subroutine is used to display information to the user. For example, System.out.print("Hello World") displays the message, Hello World. System is one of Java's standard classes. One of the static member variables in this class is named out. Since this variable is contained in the class System, its full name -- which you have to use to refer to it in your programs -- is System.out. The variable System.out refers to an object, and that object in turn contains a subroutine named print. The compound identifier System.out.print refers to the subroutine print in the object out in the class System. (As an aside, I will note that the object referred to by System.out is an object of the class PrintStream. PrintStream is another class that is a standard part of Java. Any object of type PrintStream is a destination to which information can be printed; any object of type PrintStream has a print subroutine that can be used to send information to that destination. The object System.out is just one possible destination, and System.out.print is the subroutine that sends information to that destination. Other objects of type PrintStream might send information to other destinations such as files or across a network to http://math.hws.edu/eck/cs124/javanotes4/c2/s3.html (1 of 5) [6/11/2004 11:08:14 AM] Java Programming: Section 2.3 other computers. This is object-oriented programming: Many different things which have something in common -- they can all be used as destinations for information -- can all be used in the same way -- through a print subroutine. The PrintStream class expresses the commonalities among all these objects.) Since class names and variable names are used in similar ways, it might be hard to tell which is which. All the built-in, predefined names in Java follow the rule that class names begin with an upper case letter while variable names begin with a lower case letter. While this is not a formal syntax rule, I recommend that you follow it in your own programming. Subroutine names should also begin with lower case letters. There is no possibility of confusing a variable with a subroutine, since a subroutine name in a program is always followed by a left parenthesis. Classes can contain static member subroutines, as well as static member variables. For example, the System class contains a subroutine named exit. In a program, of course, this subroutine must be referred to as System.exit. Calling this subroutine will terminate the program. You could use it if you had some reason to terminate the program before the end of the main routine. (For historical reasons, this subroutine takes an integer as a parameter, so the subroutine call statement might look like "System.exit(0);" or "System.exit(1);". The parameter tells the computer why the program is being terminated. A parameter value of 0 indicates that the program is ending normally. Any other value indicates that the program is being terminated because an error has been detected.) Every subroutine performs some specific task. For some subroutines, that task is to compute or retrieve some data value. Subroutines of this type are called functions. We say that a function returns a value. The returned value must then be used somehow in the program. You are familiar with the mathematical function that computes the square root of a number. Java has a corresponding function called Math.sqrt. This function is a static member subroutine of the class named Math. If x is any numerical value, then Math.sqrt(x) computes and returns the square root of that value. Since Math.sqrt(x) represents a value, it doesn't make sense to put it on a line by itself in a subroutine call statement such as Math.sqrt(x); // This doesn't make sense! What, after all, would the computer do with the value computed by the function in this case? You have to tell the computer to do something with the value. You might tell the computer to display it: System.out.print( Math.sqrt(x) ); // Display the square root of x. or you might use an assignment statement to tell the computer to store that value in a variable: lengthOfSide = Math.sqrt(x); The function call Math.sqrt(x) represents a value of type double, and it can be used anyplace where a numerical value of type double could be used. The Math class contains many static member functions. Here is a list of some of the more important of them: ● Math.abs(x), which computes the absolute value of x. ● The usual trigonometric functions, Math.sin(x), Math.cos(x), and Math.tan(x). (For all the trigonometric functions, angles are measured in radians, not degrees.) ● The inverse trigonometric functions arcsin, arccos, and arctan, which are written as: Math.asin(x), Math.acos(x), and Math.atan(x). ● The exponential function Math.exp(x) for computing the number e raised to the power x, and the natural logarithm function Math.log(x) for computing the logarithm of x in the base e. ● Math.pow(x,y) for computing x raised to the power y. ● Math.floor(x), which rounds x down to the nearest integer value that is less than or equal to x. (For example, Math.floor(3.76) is 3.0.) ● Math.random(), which returns a randomly chosen double in the range 0.0 <= http://math.hws.edu/eck/cs124/javanotes4/c2/s3.html (2 of 5) [6/11/2004 11:08:14 AM] Java Programming: Section 2.3 Math.random() < 1.0. (The computer actually calculates so-called "pseudorandom" numbers, which are not truly random but are random enough for most purposes.) For these functions, the type of the parameter -- the value inside parentheses -- can be of any numeric type. For most of the functions, the value returned by the function is of type double no matter what the type of the parameter. However, for Math.abs(x), the value returned will be the same type as x. If x is of type int, then so is Math.abs(x). (So, for example, while Math.sqrt(9) is the double value 3.0, Math.abs(9) is the int value 9.) Note that Math.random() does not have any parameter. You still need the parentheses, even though there's nothing between them. The parentheses let the computer know that this is a subroutine rather than a variable. Another example of a subroutine that has no parameters is the function System.currentTimeMillis(), from the System class. When this function is executed, it retrieves the current time, expressed as the number of milliseconds that have passed since a standardized base time (the start of the year 1970 in Greenwich Mean Time, if you care). One millisecond is one thousandth second. The value of System.currentTimeMillis() is of type long. This function can be used to measure the time that it takes the computer to perform a task. Just record the time at which the task is begun and the time at which it is finished and take the difference. Here is a sample program that performs a few mathematical tasks and reports the time that it takes for the program to run. On some computers, the time reported might be zero, because it is too small to measure in milliseconds. Even if it's not zero, you can be sure that most of the time reported by the computer was spent doing output or working on tasks other than the program, since the calculations performed in this program occupy only a tiny fraction of a second of a computer's time. public class TimedComputation { /* This program performs some mathematical computations and displays the results. It then reports the number of seconds that the computer spent on this task. */ public static void main(String[] args) { long startTime; // Starting time of program, in milliseconds. long endTime; // Time when computations are done, in milliseconds. double time; // Time difference, in seconds. startTime = System.currentTimeMillis(); double width, height, hypotenuse; // sides of a triangle width = 42.0; height = 17.0; hypotenuse = Math.sqrt( width*width + height*height ); System.out.print("A triangle with sides 42 and 17 has hypotenuse "); System.out.println(hypotenuse); System.out.println("\nMathematically, sin(x)*sin(x) + " + "cos(x)*cos(x) - 1 should be 0."); System.out.println("Let's check this for x = 1:"); System.out.print(" sin(1)*sin(1) + cos(1)*cos(1) - 1 is "); System.out.println( Math.sin(1)*Math.sin(1) + Math.cos(1)*Math.cos(1) - 1 ); System.out.println("(There can be round-off errors when" + " computing with real numbers!)"); System.out.print("\nHere is a random number: "); http://math.hws.edu/eck/cs124/javanotes4/c2/s3.html (3 of 5) [6/11/2004 11:08:14 AM] Java Programming: Section 2.3 System.out.println( Math.random() ); endTime = System.currentTimeMillis(); time = (endTime - startTime) / 1000.0; System.out.print("\nRun time in seconds was: "); System.out.println(time); } // end main() } // end class TimedComputation Here is a simulated version of this program. If you run it several times, you should see a different random number in the output each time. (Applet "TimedComputationConsole" would be displayed here if Java were available.) A value of type String is an object. That object contains data, namely the sequence of characters that make up the string. It also contains subroutines. All of these subroutines are in fact functions. For example, length is a subroutine that computes the length of a string. Suppose that str is a variable that refers to a String. For example, str might have been declared and assigned a value as follows: String str; str = "Seize the day!"; Then str.length() is a function call that represents the number of characters in the string. The value of str.length() is an int. Note that this function has no parameter; the string whose length is being computed is str. The length subroutine is defined by the class String, and it can be used with any value of type String. It can even be used with String literals, which are, after all, just constant values of type String. For example, you could have a program count the characters in "Hello World" for you by saying System.out.print("The number of characters in "); System.out.println("the string \"Hello World\" is "); System.out.println( "Hello World".length() ); The String class defines a lot of functions. Here are some that you might find useful. Assume that s1 and s2 refer to values of type String: ● s1.equals(s2) is a function that returns a boolean value. It returns true if s1 consists of exactly the same sequence of characters as s2, and returns false otherwise. ● s1.equalsIgnoreCase(s2) is another boolean-valued function that checks whether s1 is the same string as s2, but this function considers upper and lower case letters to be equivalent. Thus, if s1 is "cat", then s1.equals("Cat") is false, while s1.equalsIgnoreCase("Cat") is true. ● s1.length(), as mentioned above, is an integer-valued function that gives the number of characters in s1. ● s1.charAt(N), where N is an integer, returns a value of type char. It returns the N-th character in the string. Positions are numbered starting with 0, so s1.charAt(0) is actually the first character, s1.charAt(1) is the second, and so on. The final position is s1.length() - 1. For example, the value of "cat".charAt(1) is 'a'. An error occurs if the value of the parameter is less than zero or greater than s1.length() - 1. ● s1.substring(N,M), where N and M are integers, returns a value of type String. The returned value consists of the characters in s1 in positions N, N+1,..., M-1. Note that the character in position M is not included. The returned value is called a substring of s1. ● s1.indexOf(s2) returns an integer. If s2 occurs as a substring of s1, then the returned value is the http://math.hws.edu/eck/cs124/javanotes4/c2/s3.html (4 of 5) [6/11/2004 11:08:14 AM] Java Programming: Section 2.3 starting position of that substring. Otherwise, the returned value is -1. You can also use s1.indexOf(ch) to search for a particular character, ch, in s1. To find the first occurrence of x at or after position N, you can use s1.indexOf(x,N). ● s1.compareTo(s2) is an integer-valued function that compares the two strings. If the strings are equal, the value returned is zero. If s1 is less than s2, the value returned is a number less than zero, and if s1 is greater than s2, the value returned is some number greater than zero. (If both of the strings consist entirely of lowercase letters, then "less than" and "greater than" refer to alphabetical order. Otherwise, the ordering is more complicated.) ● s1.toUpperCase() is a String-valued function that returns a new string that is equal to s1, except that any lower case letters in s1 have been converted to upper case. For example, "Cat".toUpperCase() is the string "CAT". There is also a function s1.toLowerCase(). ● s1.trim() is a String-valued function that returns a new string that is equal to s1 except that any non-printing characters such as spaces and tabs have been trimmed from the beginning and from the end of the string. Thus, if s1 has the value "fred ", then s1.trim() is the string "fred". For the functions s1.toUpperCase(), s1.toLowerCase(), and s1.trim(), note that the value of s1 is not changed. Instead a new string is created and returned as the value of the function. The returned value could be used, for example, in an assignment statement such as "s2 = s1.toLowerCase();". Here is another extremely useful fact about strings: You can use the plus operator, +, to concatenate two strings. The concatenation of two strings is a new string consisting of all the characters of the first string followed by all the characters of the second string. For example, "Hello" + "World" evaluates to "HelloWorld". (Gotta watch those spaces, of course.) Let's suppose that name is a variable of type String and that it already refers to the name of the person using the program. Then, the program could greet the user by executing the statement: System.out.println("Hello, " + name + ". Pleased to meet you!"); Even more surprising is that you can concatenate values belonging to one of the primitive types onto a String using the + operator. The value of primitive type is converted to a string, just as it would be if you printed it to the standard output, and then it is concatenated onto the string. For example, the expression "Number" + 42 evaluates to the string "Number42". And the statements System.out.print("After "); System.out.print(years); System.out.print(" years, the value is "); System.out.print(principal); can be replaced by the single statement: System.out.print("After " + years + " years, the value is " + principal); Obviously, this is very convenient. It would have shortened several of the examples used earlier in this chapter. [ Next Section | Previous Section | Chapter Index | Main Index ] http://math.hws.edu/eck/cs124/javanotes4/c2/s3.html (5 of 5) [6/11/2004 11:08:14 AM] Java Programming: Section 2.4 Section 2.4 Text Input and Output FOR SOME UNFATHOMABLE REASON, Java seems to lack any reasonable built-in subroutines for reading data typed in by the user. You've already seen that output can be displayed to the user using the subroutine System.out.print. This subroutine is part of a pre-defined object called System.out. The purpose of this object is precisely to display output to the user. There is a corresponding object called System.in that exists to read data input by the user, but it provides only very primitive input facilities, and it requires some advanced Java programming skills to use it effectively. Java 1.5 Note: Java 1.5, finally, makes input a little easier with a new Scanner class. However, it requires some knowledge of object-oriented programming to use this class. There is also a new formatted output capability that makes it easy to produce nicely formatted output. This capability is provided, for example, in a new System.out.printf function. For those of you familiar with the C programming language, System.out.printf works much like C's printf function. There is some excuse for this lack of concern with input, since Java is meant mainly to write programs for Graphical User Interfaces, and those programs have their own style of input/output, which is implemented in Java. However, basic support is needed for input/output in old-fashioned non-GUI programs. Fortunately, it is possible to extend Java by creating new classes that provide subroutines that are not available in the classes which are a standard part of the language. As soon as a new class is available, the subroutines that it contains can be used in exactly the same way as built-in routines. For example, I've written a class called TextIO that defines subroutines for reading values typed by the user. The subroutines in this class make it possible to get input from the standard input object, System.in, without knowing about the advanced aspects of Java that are needed to use System.in directly. TextIO also contains a set of output subroutines. The output subroutines are similar to those provided in System.out, but they provide a few additional features. You can use whichever set of output subroutines you prefer, and you can even mix them in the same program. To use the TextIO class, you must make sure that the class is available to your program. What this means depends on the Java programming environment that you are using. See Appendix 2 for information about programming environments. In general, you just have to add the compiled file, TextIO.class, to the directory that contains your main program. You can obtain the compiled class file by compiling the source code, TextIO.java. The input routines in the TextIO class are static member functions. (Static member functions were introduced in the previous section.) Let's suppose that you want your program to read an integer typed in by the user. The TextIO class contains a static member function named getInt that you can use for this purpose. Since this function is contained in the TextIO class, you have to refer to it in your program as TextIO.getInt. The function has no parameters, so a call to the function takes the form "TextIO.getInt()". This function call represents the int value typed by the user, and you have to do something with the returned value, such as assign it to a variable. For example, if userInput is a variable of type int (created with a declaration statement "int userInput;"), then you could use the assignment statement userInput = TextIO.getInt(); When the computer executes this statement, it will wait for the user to type in an integer value. The value typed will be returned by the function, and it will be stored in the variable, userInput. Here is a complete program that uses TextIO.getInt to read a number typed by the user and then prints out the square of http://math.hws.edu/eck/cs124/javanotes4/c2/s4.html (1 of 5) [6/11/2004 11:08:15 AM] Java Programming: Section 2.4 the number that the user types: public class PrintSquare { public static void main(String[] args) { // A program that computes and prints the square // of a number input by the user. int userInput; // the number input by the user int square; // the userInput, multiplied by itself System.out.print("Please type a number: "); userInput = TextIO.getInt(); square = userInput * userInput; System.out.print("The square of that number is "); System.out.println(square); } // end of main() } //end of class PrintSquare Here's an applet that simulates this program. When you run the program, it will display the message "Please type a number: " and will pause until you type a response. (If the applet does not respond to your typing, you might have to click on it to activate it. In some browsers, you might also need to leave the mouse cursor inside the applet for it to recognize your typing.) (Applet "PrintSquareConsole" would be displayed here if Java were available.) The TextIO class contains static member subroutines TextIO.put and TextIO.putln that can be used in the same way as System.out.print and System.out.println. For example, although there is no particular advantage in doing so in this case, you could replace the two lines System.out.print("The square of that number is "); System.out.println(square); with TextIO.put("The square of that number is "); TextIO.putln(square); For the next few chapters, I will use TextIO for input in all my examples, and I will often use it for output. Keep in mind that TextIO can only be used in a program if TextIO.class is available to that program. It is not built into Java, as the System class is. Let's look a little more closely at the built-in output subroutines System.out.print and System.out.println. Each of these subroutines can be used with one parameter, where the parameter can be any value of type byte, short, int, long, float, double, char, boolean, or String. (These are the eight primitive types plus the type String.) That is, you can say "System.out.print(x);" or "System.out.println(x);", where x is any expression whose value is of one of these types. The expression can be a constant, a variable, or even something more complicated such as 2*distance*time. In fact, there are actually several different subroutines to handle the different parameter types. There is one System.out.print for printing values of type double, one for values of type int, another for values of type String, and so on. These subroutines can have the same name since the computer can tell which one you mean in a given subroutine call statement, depending on the type of parameter that you supply. Having several subroutines of the same name that differ in the types of their parameters is called overloading. Many programming languages do not permit overloading, but it is http://math.hws.edu/eck/cs124/javanotes4/c2/s4.html (2 of 5) [6/11/2004 11:08:15 AM] Java Programming: Section 2.4 common in Java programs. The difference between System.out.print and System.out.println is that System.out.println outputs a carriage return after it outputs the specified parameter value. There is a version of System.out.println that has no parameters. This version simply outputs a carriage return, and nothing else. Of course, a subroutine call statement for this version of the program looks like "System.out.println();", with empty parentheses. Note that "System.out.println(x);" is exactly equivalent to "System.out.print(x); System.out.println();". (There is no version of System.out.print without parameters. Do you see why?) As mentioned above, the TextIO subroutines TextIO.put and TextIO.putln can be used as replacements for System.out.print and System.out.println. However, TextIO goes beyond System.out by providing additional, two-parameter versions of put and putln. You can use subroutine call statements of the form "TextIO.put(x,n);" and "TextIO.putln(x,n);", where the second parameter, n, is an integer-valued expression. The idea is that n is the number of characters that you want to output. If x takes up fewer than n characters, then the computer will add some spaces at the beginning to bring the total up to n. (If x already takes up more than n characters, the computer will just print out more characters than you ask for.) This feature is useful, for example, when you are trying to output neat columns of numbers, and you know just how many characters you need in each column. The TextIO class is a little more versatile at doing output than is System.out. However, it's input for which we really need it. With TextIO, input is done using functions. For example, TextIO.getInt(), which was discussed above, makes the user type in a value of type int and returns that input value so that you can use it in your program. TextIO includes several functions for reading different types of input values. Here are examples of using each of them: b = TextIO.getByte(); // value read is a byte i = TextIO.getShort(); // value read is a short j = TextIO.getInt(); // value read is an int k = TextIO.getLong(); // value read is a long x = TextIO.getFloat(); // value read is a float y = TextIO.getDouble(); // value read is a double a = TextIO.getBoolean(); // value read is a boolean c = TextIO.getChar(); // value read is a char w = TextIO.getWord(); // value read is a String s = TextIO.getln(); // value read is a String For these statements to be legal, the variables on the left side of each assignment statement must be of the same type as that returned by the function on the right side. When you call one of these functions, you are guaranteed that it will return a legal value of the correct type. If the user types in an illegal value as input -- for example, if you ask for a byte and the user types in a number that is outside the legal range of -128 to 127 -- then the computer will ask the user to re-enter the value, and your program never sees the first, illegal value that the user entered. You'll notice that there are two input functions that return Strings. The first, getWord(), returns a string consisting of non-blank characters only. When it is called, it skips over any spaces and carriage returns typed in by the user. Then it reads non-blank characters until it gets to the next space or carriage return. It returns a String consisting of all the non-blank characters that it has read. The second input function, getln(), simply returns a string consisting of all the characters typed in by the user, including spaces, up to the next carriage return. It gets an entire line of input text. The carriage return itself is not returned as part of the input string, but it is read and discarded by the computer. Note that the String returned by this function might be the empty string, "", which contains no characters at all. http://math.hws.edu/eck/cs124/javanotes4/c2/s4.html (3 of 5) [6/11/2004 11:08:15 AM] Java Programming: Section 2.4 All the other input functions listed -- getByte(), getShort(), getInt(), getLong(), getFloat(), getDouble(), getBoolean(), and getChar() -- behave like getWord(). That is, they will skip past any blanks and carriage returns in the input before reading a value. However, they will not skip past other characters. If you try to read two ints and the user types "2,3", the computer will read the first number correctly, but when it tries to read the second number, it will see the comma. It will regard this as an error and will force the user to retype the number. If you want to input several numbers from one line, you should make sure that the user knows to separate them with spaces, not commas. Alternatively, if you want to require a comma between the numbers, use getChar() to read the comma before reading the second number. There is another character input function, TextIO.getAnyChar(), which does not skip past blanks or carriage returns. It simply reads and returns the next character typed by the user. This could be any character, including a space or a carriage return. If the user typed a carriage return, then the char returned by getChar() is the special linefeed character '\n'. There is also a function, TextIO.peek(), that let's you look ahead at the next character in the input without actually reading it. After you "peek" at the next character, it will still be there when you read the next item from input. This allows you to look ahead and see what's coming up in the input, so that you can take different actions depending on what's there. The semantics of input is much more complicated than the semantics of output. The first time the program tries to read input from the user, the computer will wait while the user types in an entire line of input. TextIO stores that line in a chunk of internal memory called the input buffer. Input is actually read from the buffer, not directly from the user's typing. The user only gets to type when the buffer is empty. This lets you read several numbers from one line of input. However, if you only want to read in one number and the user types in extra stuff on the line, then you could be in trouble. The extra stuff will still be there the next time you try to read something from input. (The symptom of this trouble is that the computer doesn't pause where you think it should to let the user type something in. The computer had stuff left over in the input buffer from the previous line that the user typed.) To help you avoid this, there are versions of the TextIO input functions that read a data value and then discard any leftover stuff on the same line: b = TextIO.getlnByte(); // value read is a byte i = TextIO.getlnShort(); // value read is a short j = TextIO.getlnInt(); // value read is an int k = TextIO.getlnLong(); // value read is a long x = TextIO.getlnFloat(); // value read is a float y = TextIO.getlnDouble(); // value read is a double a = TextIO.getlnBoolean(); // value read is a boolean c = TextIO.getlnChar(); // value read is a char w = TextIO.getlnWord(); // value read is a String Note that calling getlnDouble(), for example, is equivalent to first calling getDouble() and then calling getln() to read any remaining data on the same line, including the end-of-line character itself. I strongly advise you to use the "getln" versions of the input routines, rather than the "get" versions, unless you really want to read several items from the same line of input. You might be wondering why there are only two output routines, put and putln, which can output data values of any type, while there is a separate input routine for each data type. As noted above, in reality there are many put and putln routines. The computer can tell them apart based on the type of the parameter that you provide. However, the input routines don't have parameters, so the different input routines can only be distinguished by having different names. Using TextIO for input and output, we can now improve the program from Section 2 for computing the value of an investment. We can have the user type in the initial value of the investment and the interest rate. The result is a much more useful program -- for one thing, it makes sense to run more than once! public class Interest2 { http://math.hws.edu/eck/cs124/javanotes4/c2/s4.html (4 of 5) [6/11/2004 11:08:15 AM] Java Programming: Section 2.4 /* This class implements a simple program that will compute the amount of interest that is earned on an investment over a period of one year. The initial amount of the investment and the interest rate are input by the user. The value of the investment at the end of the year is output. The rate must be input as a decimal, not a percentage (for example, 0.05, rather than 5). */ public static void main(String[] args) { double principal; // the value of the investment double rate; // the annual interest rate double interest; // the interest earned during the year TextIO.put("Enter the initial investment: "); principal = TextIO.getlnDouble(); TextIO.put("Enter the annual interest rate: "); rate = TextIO.getlnDouble(); interest = principal * rate; // compute this year's interest principal = principal + interest; // add it to principal TextIO.put("The value of the investment after one year is $"); TextIO.putln(principal); } // end of main() } // end of class Interest2 Try out an equivalent applet here. (If the applet does not respond to your typing, you might have to click on it to activate it.) (Applet "Interest2Console" would be displayed here if Java were available.) By the way, the applets on this page don't actually use TextIO. The TextIO class is only for use in programs, not applets. For applets, I have written a separate class that provides similar input/output capabilities in a Graphical User Interface program. [ Next Section | Previous Section | Chapter Index | Main Index ] http://math.hws.edu/eck/cs124/javanotes4/c2/s4.html (5 of 5) [6/11/2004 11:08:15 AM] Java Programming: Section 2.5 Section 2.5 Details of Expressions THIS SECTION TAKES A CLOSER LOOK at expressions. Recall that an expression is a piece of program code that represents or computes a value. An expression can be a literal, a variable, a function call, or several of these things combined with operators such as + and >. The value of an expression can be assigned to a variable, used as the output value in an output routine, or combined with other values into a more complicated expression. (The value can even, in some cases, be ignored, if that's what you want to do; this is more common than you might think.) Expressions are an essential part of programming. So far, these notes have dealt only informally with expressions. This section tells you the more-or-less complete story. The basic building blocks of expressions are literals (such as 674, 3.14, true, and 'X'), variables, and function calls. Recall that a function is a subroutine that returns a value. You've already seen some examples of functions: the input routines from the TextIO class and the mathematical functions from the Math class. Literals, variables, and function calls are simple expressions. More complex expressions can be built up by using operators to combine simpler expressions. Operators include + for adding two numbers, > for comparing two values, and so on. When several operators appear in an expression, there is a question of precedence, which determines how the operators are grouped for evaluation. For example, in the expression "A + B * C", B*C is computed first and then the result is added to A. We say that multiplication (*) has higher precedence than addition (+). If the default precedence is not what you want, you can use parentheses to explicitly specify the grouping you want. For example, you could use "(A + B) * C" if you want to add A to B first and then multiply the result by C. The rest of this section gives details of operators in Java. The number of operators in Java is quite large, and I will not cover them all here. Most of the important ones are here; a few will be covered in later chapters as they become relevant. Arithmetic Operators Arithmetic operators include addition, subtraction, multiplication, and division. They are indicated by +, -, *, and /. These operations can be used on values of any numeric type: byte, short, int, long, float, or double. When the computer actually calculates one of these operations, the two values that it combines must be of the same type. If your program tells the computer to combine two values of different types, the computer will convert one of the values from one type to another. For example, to compute 37.4 + 10, the computer will convert the integer 10 to a real number 10.0 and will then compute 37.4 + 10.0. (The computer's internal representations for 10 and 10.0 are very different, even though people think of them as representing the same number.) Ordinarily, you don't have to worry about type conversion, because the computer does it automatically. When two numerical values are combined (after doing type conversion on one of them, if necessary), the answer will be of the same type. If you multiply two ints, you get an int; if you multiply two doubles, you get a double. This is what you would expect, but you have to be very careful when you use the division operator /. When you divide two integers, the answer will always be an integer; if the quotient has a fractional part, it is discarded. For example, the value of 7/2 is 3, not 3.5. If N is an integer variable, then N/100 is an integer, and 1/N is equal to zero for any N greater than one! This fact is a common source of programming errors. You can force the computer to compute a real number as the answer by making one of the operands real: For example, when the computer evaluates 1.0/N, it first converts N to a real number in order to match the type of 1.0, so you get a real number as the answer. Java also has an operator for computing the remainder when one integer is divided by another. This http://math.hws.edu/eck/cs124/javanotes4/c2/s5.html (1 of 6) [6/11/2004 11:08:16 AM] Java Programming: Section 2.5 operator is indicated by %. If A and B are integers, then A % B represents the remainder when A is divided by B. For example, 7 % 2 is 1, while 34577 % 100 is 77, and 50 % 8 is 2. A common use of % is to test whether a given integer is even or odd. N is even if N % 2 is zero, and it is odd if N % 2 is 1. More generally, you can check whether an integer N is evenly divisible by an integer M by checking whether N % M is zero. Finally, you might need the unary minus operator, which takes the negative of a number. For example, -X has the same value as (-1)*X. For completeness, Java also has a unary plus operator, as in +X, even though it doesn't really do anything. Increment and Decrement You'll find that adding 1 to a variable is an extremely common operation in programming. Subtracting 1 from a variable is also pretty common. You might perform the operation of adding 1 to a variable with assignment statements such as: counter = counter + 1; goalsScored = goalsScored + 1; The effect of the assignment statement x = x + 1 is to take the old value of the variable x, compute the result of adding 1 to that value, and store the answer as the new value of x. The same operation can be accomplished by writing x++ (or, if you prefer, ++x). This actually changes the value of x, so that it has the same effect as writing "x = x + 1". The two statements above could be written counter++; goalsScored++; Similarly, you could write x-- (or --x) to subtract 1 from x. That is, x-- performs the same computation as x = x - 1. Adding 1 to a variable is called incrementing that variable, and subtracting 1 is called decrementing. The operators ++ and -- are called the increment operator and the decrement operator, respectively. These operators can be used on variables belonging to any of the numerical types and also on variables of type char. Usually, the operators ++ or --, are used in statements like "x++;" or "x--;". These statements are commands to change the value of x. However, it is also legal to use x++, ++x, x--, or --x as expressions, or as parts of larger expressions. That is, you can write things like: y = x++; y = ++x; TextIO.putln(--x); z = (++x) * (y--); The statement "y = x++;" has the effects of adding 1 to the value of x and, in addition, assigning some value to y. The value assigned to y is the value of the expression x++, which is defined to be the old value of x, before the 1 is added. Thus, if the value of x is 6, the statement "y = x++;" will change the value of x to 7, but it will change the value of y to 6 since the value assigned to y is the old value of x. On the other hand, the value of ++x is defined to be the new value of x, after the 1 is added. So if x is 6, then the statement "y = ++x;" changes the values of both x and y to 7. The decrement operator, --, works in a similar way. This can be confusing. My advice is: Don't be confused. Use ++ and -- only in stand-alone statements, not in expressions. I will follow this advice in all the examples in these notes. http://math.hws.edu/eck/cs124/javanotes4/c2/s5.html (2 of 6) [6/11/2004 11:08:16 AM] Java Programming: Section 2.5 Relational Operators Java has boolean variables and boolean-valued expressions that can be used to express conditions that can be either true or false. One way to form a boolean-valued expression is to compare two values using a relational operator. Relational operators are used to test whether two values are equal, whether one value is greater than another, and so forth. The relation operators in Java are: ==, !=, <, >, <=, and >=. The meanings of these operators are: A == B Is A "equal to" B? A != B Is A "not equal to" B? A < B Is A "less than" B? A > B Is A "greater than" B? A <= B Is A "less than or equal to" B? A >= B Is A "greater than or equal to" B? These operators can be used to compare values of any of the numeric types. They can also be used to compare values of type char. For characters, < and > are defined according the numeric Unicode values of the characters. (This might not always be what you want. It is not the same as alphabetical order because all the upper case letters come before all the lower case letters.) When using boolean expressions, you should remember that as far as the computer is concerned, there is nothing special about boolean values. In the next chapter, you will see how to use them in loop and branch statements. But you can also assign boolean-valued expressions to boolean variables, just as you can assign numeric values to numeric variables. By the way, the operators == and != can be used to compare boolean values. This is occasionally useful. For example, can you figure out what this does: boolean sameSign; sameSign = ((x > 0) == (y > 0)); One thing that you cannot do with the relational operators <, >, <=, and <= is to use them to compare values of type String. You can legally use == and != to compare Strings, but because of peculiarities in the way objects behave, they might not give the results you want. (The == operator checks whether two objects are stored in the same memory location, rather than whether they contain the same value. Occasionally, for some objects, you do want to make such a check -- but rarely for strings. I'll get back to this in a later chapter.) Instead, you should use the subroutines equals(), equalsIgnoreCase(), and compareTo(), which were described in Section 3, to compare two Strings. Boolean Operators In English, complicated conditions can be formed using the words "and", "or", and "not." For example, "If there is a test and you did not study for it...". "And", "or", and "not" are boolean operators, and they exist in Java as well as in English. In Java, the boolean operator "and" is represented by &&. The && operator is used to combine two boolean values. The result is also a boolean value. The result is true if both of the combined values are true, and the result is false if either of the combined values is false. For example, "(x == 0) && (y == 0)" is true if and only if both x is equal to 0 and y is equal to 0. The boolean operator "or" is represented by ||. (That's supposed to be two of the vertical line characters, |.) The expression "A || B" is true if either A is true or B is true, or if both are true. "A || B" is false only if both A and B are false. The operators && and || are said to be short-circuited versions of the boolean operators. This means that http://math.hws.edu/eck/cs124/javanotes4/c2/s5.html (3 of 6) [6/11/2004 11:08:16 AM] Java Programming: Section 2.5 the second operand of && or || is not necessarily evaluated. Consider the test (x != 0) && (y/x > 1) Suppose that the value of x is in fact zero. In that case, the division y/x is illegal, since division by zero is not allowed. However, the computer will never perform the division, since when the computer evaluates (x != 0), it finds that the result is false, and so it knows that ((x != 0) && anything) has to be false. Therefore, it doesn't bother to evaluate the second operand, (y/x > 1). The evaluation has been short-circuited and the division by zero is avoided. Without the short-circuiting, there would have been a division-by-zero error. (This may seem like a technicality, and it is. But at times, it will make your programming life a little easier. To be even more technical: There are actually non-short-circuited versions of && and ||, which are written as & and |. Don't use them unless you have a particular reason to do so.) The boolean operator "not" is a unary operator. In Java, it is indicated by ! and is written in front of its single operand. For example, if test is a boolean variable, then test = ! test; will reverse the value of test, changing it from true to false, or from false to true. Conditional Operator Any good programming language has some nifty little features that aren't really necessary but that let you feel cool when you use them. Java has the conditional operator. It's a ternary operator -- that is, it has three operands -- and it comes in two pieces, ? and :, that have to be used together. It takes the form boolean-expression ? expression-1 : expression-2 The computer tests the value of boolean-expression. If the value is true, it evaluates expression-1; otherwise, it evaluates expression-2. For example: next = (N % 2 == 0) ? (N/2) : (3*N+1); will assign the value N/2 to next if N is even (that is, if N % 2 == 0 is true), and it will assign the value (3*N+1) to next if N is odd. Assignment Operators and Type-Casts You are already familiar with the assignment statement, which uses the symbol "=" to assign the value of an expression to a variable. In fact, = is really an operator in the sense that an assignment can itself be used as an expression or as part of a more complex expression. The value of an assignment such as A=B is the same as the value that is assigned to A. So, if you want to assign the value of B to A and test at the same time whether that value is zero, you could say: if ( (A=B) == 0 ) Usually, I would say, don't do things like that! In general, the type of the expression on the right-hand side of an assignment statement must be the same as the type of the variable on the left-hand side. However, in some cases, the computer will automatically convert the value computed by the expression to match the type of the variable. Consider the list of numeric types: byte, short, int, long, float, double. A value of a type that occurs earlier in this list can be converted automatically to a value that occurs later. For example: http://math.hws.edu/eck/cs124/javanotes4/c2/s5.html (4 of 6) [6/11/2004 11:08:16 AM] Java Programming: Section 2.5 int A; double X; short B; A = 17; X = A; // OK; A is converted to a double B = A; // illegal; no automatic conversion // from int to short The idea is that conversion should only be done automatically when it can be done without changing the semantics of the value. Any int can be converted to a double with the same numeric value. However, there are int values that lie outside the legal range of shorts. There is simply no way to represent the int 100000 as a short, for example, since the largest value of type short is 32767. In some cases, you might want to force a conversion that wouldn't be done automatically. For this, you can use what is called a type cast. A type cast is indicated by putting a type name, in parentheses, in front of the value you want to convert. For example, int A; short B; A = 17; B = (short)A; // OK; A is explicitly type cast // to a value of type short You can do type casts from any numeric type to any other numeric type. However, you should note that you might change the numeric value of a number by type-casting it. For example, (short)100000 is -31072. (The 34464 is obtained by taking the 4-byte int 100000 and throwing away two of those bytes to obtain a short -- you've lost the real information that was in those two bytes.) As another example of type casts, consider the problem of getting a random integer between 1 and 6. The function Math.random() gives a real number between 0.0 and 0.9999..., and so 6*Math.random() is between 0.0 and 5.999.... The type-cast operator, (int), can be used to convert this to an integer: (int)(6*Math.random()). A real number is cast to an integer by discarding the fractional part. Thus, (int)(6*Math.random()) is one of the integers 0, 1, 2, 3, 4, and 5. To get a number between 1 and 6, we can add 1: "(int)(6*Math.random()) + 1". You can also type-cast between the type char and the numeric types. The numeric value of a char is its Unicode code number. For example, (char)97 is 'a', and (int)'+' is 43. Java has several variations on the assignment operator, which exist to save typing. For example, "A += B" is defined to be the same as "A = A + B". Every operator in Java that applies to two operands gives rise to a similar assignment operator. For example: x -= y; // same as: x = x - y; x *= y; // same as: x = x * y; x /= y; // same as: x = x / y; x %= y; // same as: x = x % y; (for integers x and y) q &&= p; // same as: q = q && p; (for booleans q and p) The combined assignment operator += even works with strings. You will recall from Section 3 that when the + operator is used with a string as the first operand, it represents concatenation. Since str += x is equivalent to str = str + x, when += is used with a string on the left-hand side, it appends the value on the right-hand side onto the string. For example, if str has the value "tire", then the statement str += 'd'; changes the value of str to "tired". http://math.hws.edu/eck/cs124/javanotes4/c2/s5.html (5 of 6) [6/11/2004 11:08:16 AM] Java Programming: Section 2.5 Precedence Rules If you use several operators in one expression, and if you don't use parentheses to explicitly indicate the order of evaluation, then you have to worry about the precedence rules that determine the order of evaluation. (Advice: don't confuse yourself or the reader of your program; use parentheses liberally.) Here is a listing of the operators discussed in this section, listed in order from highest precedence (evaluated first) to lowest precedence (evaluated last): Unary operators: ++, --, !, unary - and +, type-cast Multiplication and division: *, /, % Addition and subtraction: +, - Relational operators: <, >, <=, >= Equality and inequality: ==, != Boolean and: && Boolean or: || Conditional operator: ?: Assignment operators: =, +=, -=, *=, /=, %= Operators on the same line have the same precedence. When they occur together, unary operators and assignment operators are evaluated right-to-left, and the remaining operators are evaluated left-to-right. For example, A*B/C means (A*B)/C, while A=B=C means A=(B=C). (Can you see how the expression A=B=C might be useful, given that the value of B=C as an expression is the same as the value that is assigned to B?) End of Chapter 2 [ Next Chapter | Previous Section | Chapter Index | Main Index ] http://math.hws.edu/eck/cs124/javanotes4/c2/s5.html (6 of 6) [6/11/2004 11:08:16 AM] Java Programming: Chapter 2 Exercises Programming Exercises For Chapter 2 THIS PAGE CONTAINS programming exercises based on material from Chapter 2 of this on-line Java textbook. Each exercise has a link to a discussion of one possible solution of that exercise. Exercise 2.1: Write a program that will print your initials to standard output in letters that are nine lines tall. Each big letter should be made up of a bunch of *'s. For example, if your initials were "DJE", then the output would look something like: ****** ************* ********** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ******** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ***** **** ********** See the solution! Exercise 2.2: Write a program that simulates rolling a pair of dice. You can simulate rolling one die by choosing one of the integers 1, 2, 3, 4, 5, or 6 at random. The number you pick represents the number on the die after it is rolled. As pointed out in Section 5, The expression (int)(Math.random()*6) + 1 does the computation you need to select a random integer between 1 and 6. You can assign this value to a variable to represent one of the dice that are being rolled. Do this twice and add the results together to get the total roll. Your program should report the number showing on each die as well as the total roll. For example: The first die comes up 3 The second die comes up 5 Your total roll is 8 (Note: The word "dice" is a plural, as in "two dice." The singular is "die.") See the solution! Exercise 2.3: Write a program that asks the user's name, and then greets the user by name. Before outputting the user's name, convert it to upper case letters. For example, if the user's name is Fred, then the program should respond "Hello, FRED, nice to meet you!". See the solution! Exercise 2.4: Write a program that helps the user count his change. The program should ask how many quarters the user has, then how many dimes, then how many nickels, then how many pennies. Then the http://math.hws.edu/eck/cs124/javanotes4/c2/exercises.html (1 of 2) [6/11/2004 11:08:17 AM] Java Programming: Chapter 2 Exercises program should tell the user how much money he has, expressed in dollars. See the solution! Exercise 2.5: If you have N eggs, then you have N/12 dozen eggs, with N%12 eggs left over. (This is essentially the definition of the / and % operators for integers.) Write a program that asks the user how many eggs she has and then tells the user how many dozen eggs she has and how many extra eggs are left over. A gross of eggs is equal to 144 eggs. Extend your program so that it will tell the user how many gross, how many dozen, and how many left over eggs she has. For example, if the user says that she has 1342 eggs, then your program would respond with Your number of eggs is 9 gross, 3 dozen, and 10 since 1342 is equal to 9*144 + 3*12 + 10. See the solution! [ Chapter Index | Main Index ] http://math.hws.edu/eck/cs124/javanotes4/c2/exercises.html (2 of 2) [6/11/2004 11:08:17 AM] Java Programming: Chapter 2 Quiz Quiz Questions For Chapter 2 THIS PAGE CONTAINS A SAMPLE quiz on material from Chapter 2 of this on-line Java textbook. You should be able to answer these questions after studying that chapter. Sample answers to all the quiz questions can be found here. Question 1: Briefly explain what is meant by the syntax and the semantics of a programming language. Give an example to illustrate the difference between a syntax error and a semantics error. Question 2: What does the computer do when it executes a variable declaration statement. Give an example. Question 3: What is a type, as this term relates to programming? Question 4: One of the primitive types in Java is boolean. What is the boolean type? Where are boolean values used? What are its possible values? Question 5: Give the meaning of each of the following Java operators: a) ++ b) && c) != Question 6: Explain what is meant by an assignment statement, and give an example. What are assignment statements used for? Question 7: What is meant by precedence of operators? Question 8: What is a literal? Question 9: In Java, classes have two fundamentally different purposes. What are they? Question 10: What is the difference between the statement "x = TextIO.getDouble();" and the statement "x = TextIO.getlnDouble();" [ Answers | Chapter Index | Main Index ] http://math.hws.edu/eck/cs124/javanotes4/c2/quiz.html [6/11/2004 11:08:17 AM] Java Programming: Chapter 3 Index Chapter 3 Programming in the Small II Control THE BASIC BUILDING BLOCKS of programs -- variables, expressions, assignment statements, and subroutine call statements -- were covered in the previous chapter. Starting with this chapter, we look at how these building blocks can be put together to build complex programs with more interesting behavior. Since we are still working on the level of "programming in the small" in this chapter, we are interested in the kind of complexity that can occur within a single subroutine. On this level, complexity is provided by control structures. The two types of control structures, loop and branches, can be used to repeat a sequence of statements over and over or to choose among two or more possible courses of action. Java includes several control structures of each type, and we will look at each of them in some detail. This chapter will also begin the study of program design. Given a problem, how can you come up with a program to solve that problem? We'll look at a partial answer to this question in Section 2. In the following sections, we'll apply the techniques from Section 2 to a variety of examples. Contents of Chapter 3: ● Section 1:Blocks, Loops, and Branches ● Section 2:Algorithm Development ● Section 3:The while and do..while Statements ● Section 4:The for Statement ● Section 5:The if Statement ● Section 6:The switch Statement ● Section 7:Introduction to Applets and Graphics ● Programming Exercises ● Quiz on this Chapter [ First Section | Next Chapter | Previous Chapter | Main Index ] http://math.hws.edu/eck/cs124/javanotes4/c3/ [6/11/2004 11:08:49 AM] Java Programming: Section 3.1 Section 3.1 Blocks, Loops, and Branches THE ABILITY OF A COMPUTER TO PERFORM complex tasks is built on just a few ways of combining simple commands into control structures. In Java, there are just six such structures -- and, in fact, just three of them would be enough to write programs to perform any task. The six control structures are: the block, the while loop, the do..while loop, the for loop, the if statement, and the switch statement. Each of these structures is considered to be a single "statement," but each is in fact a structured statement that can contain one or more other statements inside itself. The block is the simplest type of structured statement. Its purpose is simply to group a sequence of statements into a single statement. The format of a block is: { statements } That is, it consists of a sequence of statements enclosed between a pair of braces, "{" and "}". (In fact, it is possible for a block to contain no statements at all; such a block is called an empty block, and can actually be useful at times. An empty block consists of nothing but an empty pair of braces.) Block statements usually occur inside other statements, where their purpose is to group together several statements into a unit. However, a block can be legally used wherever a statement can occur. There is one place where a block is required: As you might have already noticed in the case of the main subroutine of a program, the definition of a subroutine is a block, since it is a sequence of statements enclosed inside a pair of braces. I should probably note at this point that Java is what is called a free-format language. There are no syntax rules about how the language has to be arranged on a page. So, for example, you could write an entire block on one line if you want. But as a matter of good programming style, you should lay out your program on the page in a way that will make its structure as clear as possible. In general, this means putting one statement per line and using indentation to indicate statements that are contained inside control structures. This is the format that I will generally use in my examples. Here are two examples of blocks: { System.out.print("The answer is "); System.out.println(ans); } { // This block exchanges the values of x and y int temp; // A temporary variable for use in this block. temp = x; // Save a copy of the value of x in temp. x = y; // Copy the value of y into x. y = temp; // Copy the value of temp into y. } In the second example, a variable, temp, is declared inside the block. This is perfectly legal, and it is good style to declare a variable inside a block if that variable is used nowhere else but inside the block. A variable declared inside a block is completely inaccessible and invisible from outside that block. When the computer executes the variable declaration statement, it allocates memory to hold the value of the variable. When the block ends, that memory is discarded (that is, made available for reuse). The variable is said to be local to the block. There is a general concept called the "scope" of an identifier. The scope of an identifier is the part of the program in which that identifier is valid. The scope of a variable defined inside a block is limited to that http://math.hws.edu/eck/cs124/javanotes4/c3/s1.html (1 of 5) [6/11/2004 11:08:50 AM] Java Programming: Section 3.1 block, and more specifically to the part of the block that comes after the declaration of the variable. The block statement by itself really doesn't affect the flow of control in a program. The five remaining control structures do. They can be divided into two classes: loop statements and branching statements. You really just need one control structure from each category in order to have a completely general-purpose programming language. More than that is just convenience. In this section, I'll introduce the while loop and the if statement. I'll give the full details of these statements and of the other three control structures in later sections. A while loop is used to repeat a given statement over and over. Of course, its not likely that you would want to keep repeating it forever. That would be an infinite loop, which is generally a bad thing. (There is an old story about computer pioneer Grace Murray Hopper, who read instructions on a bottle of shampoo telling her to "lather, rinse, repeat." As the story goes, she claims that she tried to follow the directions, but she ran out of shampoo. (In case you don't get it, this is a joke about the way that computers mindlessly follow instructions.)) To be more specific, a while loop will repeat a statement over and over, but only so long as a specified condition remains true. A while loop has the form: while (boolean-expression) statement Since the statement can be, and usually is, a block, many while loops have the form: while (boolean-expression) { statements } The semantics of this statement go like this: When the computer comes to a while statement, it evaluates the boolean-expression, which yields either true or false as the value. If the value is false, the computer skips over the rest of the while loop and proceeds to the next command in the program. If the value of the expression is true, the computer executes the statement or block of statements inside the loop. Then it returns to the beginning of the while loop and repeats the process. That is, it re-evaluates the boolean-expression, ends the loop if the value is false, and continues it if the value is true. This will continue over and over until the value of the expression is false; if that never happens, then there will be an infinite loop. Here is an example of a while loop that simply prints out the numbers 1, 2, 3, 4, 5: int number; // The number to be printed. number = 1; // Start with 1. while ( number < 6 ) { // Keep going as long as number is < 6. System.out.println(number); number = number + 1; // Go on to the next number. } System.out.println("Done!"); The variable number is initialized with the value 1. So the first time through the while loop, when the computer evaluates the expression "number < 6", it is asking whether 1 is less than 6, which is true. The computer therefor proceeds to execute the two statements inside the loop. The first statement prints out "1". The second statement adds 1 to number and stores the result back into the variable number; the value of number has been changed to 2. The computer has reached the end of the loop, so it returns to the beginning and asks again whether number is less than 6. Once again this is true, so the computer executes the loop again, this time printing out 2 as the value of number and then changing the value of number to 3. It continues in this way until eventually number becomes equal to 6. At that point, the expression "number < 6" evaluates to false. So, the computer jumps past the end of the loop to the next statement and prints out the message "Done!". Note that when the loop ends, the value of number is 6, but the last value that was http://math.hws.edu/eck/cs124/javanotes4/c3/s1.html (2 of 5) [6/11/2004 11:08:50 AM] Java Programming: Section 3.1 printed was 5. By the way, you should remember that you'll never see a while loop standing by itself in a real program. It will always be inside a subroutine which is itself defined inside some class. As an example of a while loop used inside a complete program, here is a little program that computes the interest on an investment over several years. This is an improvement over examples from the previous chapter that just reported the results for one year: public class Interest3 { /* This class implements a simple program that will compute the amount of interest that is earned on an investment over a period of 5 years. The initial amount of the investment and the interest rate are input by the user. The value of the investment at the end of each year is output. */ public static void main(String[] args) { double principal; // The value of the investment. double rate; // The annual interest rate. /* Get the initial investment and interest rate from the user. */ TextIO.put("Enter the initial investment: "); principal = TextIO.getlnDouble(); TextIO.put("Enter the annual interest rate: "); rate = TextIO.getlnDouble(); /* Simulate the investment for 5 years. */ int years; // Counts the number of years that have passed. years = 0; while (years < 5) { double interest; // Interest for this year. interest = principal * rate; principal = principal + interest; // Add it to principal. years = years + 1; // Count the current year. System.out.print("The value of the investment after "); System.out.print(years); System.out.print(" years is $"); System.out.println(principal); } // end of while loop } // end of main() } // end of class Interest3 And here is the applet which simulates this program: (Applet "Interest3Console" would be displayed here http://math.hws.edu/eck/cs124/javanotes4/c3/s1.html (3 of 5) [6/11/2004 11:08:50 AM] Java Programming: Section 3.1 if Java were available.) You should study this program, and make sure that you understand what the computer does step-by-step as it executes the while loop. An if statement tells the computer to take one of two alternative courses of action, depending on whether the value of a given boolean-valued expression is true or false. It is an example of a "branching" or "decision" statement. An if statement has the form: if ( boolean-expression ) statement else statement When the computer executes an if statement, it evaluates the boolean expression. If the value is true, the computer executes the first statement and skips the statement that follows the "else". If the value of the expression is false, then the computer skips the first statement and executes the second one. Note that in any case, one and only one of the two statements inside the if statement is executed. The two statements represent alternative courses of action; the computer decides between these courses of action based on the value of the boolean expression. In many cases, you want the computer to choose between doing something and not doing it. You can do this with an if statement that omits the else part: if ( boolean-expression ) statement To execute this statement, the computer evaluates the expression. If the value is true, the computer executes the statement that is contained inside the if statement; if the value is false, the computer skips that statement. Of course, either or both of the statement's in an if statement can be a block, so that an if statement often looks like: if ( boolean-expression ) { statements } else { statements } or: if ( boolean-expression ) { statements } As an example, here is an if statement that exchanges the value of two variables, x and y, but only if x is greater than y to begin with. After this if statement has been executed, we can be sure that the value of x is definitely less than or equal to the value of y: if ( x > y ) { int temp; // A temporary variable for use in this block. temp = x; // Save a copy of the value of x in temp. x = y; // Copy the value of y into x. y = temp; // Copy the value of temp into y. } Finally, here is an example of an if statement that includes an else part. See if you can figure out what it http://math.hws.edu/eck/cs124/javanotes4/c3/s1.html (4 of 5) [6/11/2004 11:08:50 AM] Java Programming: Section 3.1 does, and why it would be used: if ( years > 1 ) { // handle case for 2 or more years System.out.print("The value of the investment after "); System.out.print(years); System.out.print(" years is $"); } else { // handle case for 1 year System.out.print("The value of the investment after 1 year is $"); } // end of if statement System.out.println(principal); // this is done in any case I'll have more to say about control structures later in this chapter. But you already know the essentials. If you never learned anything more about control structures, you would already know enough to perform any possible computing task. Simple looping and branching are all you really need! [ Next Section | Previous Chapter | Chapter Index | Main Index ] http://math.hws.edu/eck/cs124/javanotes4/c3/s1.html (5 of 5) [6/11/2004 11:08:50 AM] Java Programming: Section 3.2 Section 3.2 Algorithm Development PROGRAMMING IS DIFFICULT (like many activities that are useful and worthwhile -- and like most of those activities, it can also be rewarding and a lot of fun). When you write a program, you have to tell the computer every small detail of what to do. And you have to get everything exactly right, since the computer will blindly follow your program exactly as written. How, then, do people write any but the most simple programs? It's not a big mystery, actually. It's a matter of learning to think in the right way. A program is an expression of an idea. A programmer starts with a general idea of a task for the computer to perform. Presumably, the programmer has some idea of how to perform the task by hand, at least in general outline. The problem is to flesh out that outline into a complete, unambiguous, step-by-step procedure for carrying out the task. Such a procedure is called an "algorithm." (Technically, an algorithm is an unambiguous, step-by-step procedure that terminates after a finite number of steps; we don't want to count procedures that go on forever.) An algorithm is not the same as a program. A program is written in some particular programming language. An algorithm is more like the idea behind the program, but it's the idea of the steps the program will take to perform its task, not just the idea of the task itself. The steps of the algorithm don't have to be filled in in complete detail, as long as the steps are unambiguous and it's clear that carrying out the steps will accomplish the assigned task. An algorithm can be expressed in any language, including English. Of course, an algorithm can only be expressed as a program if all the details have been filled in. So, where do algorithms come from? Usually, they have to be developed, often with a lot of thought and hard work. Skill at algorithm development is something that comes with practice, but there are techniques and guidelines that can help. I'll talk here about some techniques and guidelines that are relevant to "programming in the small," and I will return to the subject several times in later chapters. When programming in the small, you have a few basics to work with: variables, assignment statements, and input-output routines. You might also have some subroutines, objects, or other building blocks that have already been written by you or someone else. (Input/output routines fall into this class.) You can build sequences of these basic instructions, and you can also combine them into more complex control structures such as while loops and if statements. Suppose you have a task in mind that you want the computer to perform. One way to proceed is to write a description of the task, and take that description as an outline of the algorithm you want to develop. Then you can refine and elaborate that description, gradually adding steps and detail, until you have a complete algorithm that can be translated directly into programming language. This method is called stepwise refinement, and it is a type of top-down design. As you proceed through the stages of stepwise refinement, you can write out descriptions of your algorithm in pseudocode -- informal instructions that imitate the structure of programming languages without the complete detail and perfect syntax of actual program code. As an example, let's see how one might develop the program from the previous section, which computes the value of an investment over five years. The task that you want the program to perform is: "Compute and display the value of an investment for each of the next five years, where the initial investment and interest rate are to be specified by the user." You might then write -- or at least think -- that this can be expanded as: Get the user's input Compute the value of the investment after 1 year Display the value Compute the value after 2 years Display the value Compute the value after 3 years Display the value http://math.hws.edu/eck/cs124/javanotes4/c3/s2.html (1 of 7) [6/11/2004 11:08:51 AM] Java Programming: Section 3.2 Compute the value after 4 years Display the value Compute the value after 5 years Display the value This is correct, but rather repetitive. And seeing that repetition, you might notice an opportunity to use a loop. A loop would take less typing. More important, it would be more general: Essentially the same loop will work no matter how many years you want to process. So, you might rewrite the above sequence of steps as: Get the user's input while there are more years to process: Compute the value after the next year Display the value Now, for a computer, we'll have to be more explicit about how to "Get the user's input," how to "Compute the value after the next year," and what it means to say "there are more years to process." We can expand the step, "Get the user's input" into Ask the user for the initial investment Read the user's response Ask the user for the interest rate Read the user's response To fill in the details of the step "Compute the value after the next year," you have to know how to do the computation yourself. (Maybe you need to ask your boss or professor for clarification?) Let's say you know that the value is computed by adding some interest to the previous value. Then we can refine the while loop to: while there are more years to process: Compute the interest Add the interest to the value Display the value As for testing whether there are more years to process, the only way that we can do that is by counting the years ourselves. This displays a very common pattern, and you should expect to use something similar in a lot of programs: We have to start with zero years, add one each time we process a year, and stop when we reach the desired number of years. So the while loop becomes: years = 0 while years < 5: years = years + 1 Compute the interest Add the interest to the value Display the value We still have to know how to compute the interest. Let's say that the interest is to be computed by multiplying the interest rate by the current value of the investment. Putting this together with the part of the algorithm that gets the user's inputs, we have the complete algorithm: Ask the user for the initial investment Read the user's response Ask the user for the interest rate Read the user's response years = 0 while years < 5: years = years + 1 Compute interest = value * interest rate Add the interest to the value Display the value http://math.hws.edu/eck/cs124/javanotes4/c3/s2.html (2 of 7) [6/11/2004 11:08:51 AM] Java Programming: Section 3.2 Finally, we are at the point where we can translate pretty directly into proper programming-language syntax. We still have to choose names for the variables, decide exactly what we want to say to the user, and so forth. Having done this, we could express our algorithm in Java as: double principal, rate, interest; // declare the variables int years; System.out.print("Type initial investment: "); principal = TextIO.getlnDouble(); System.out.print("Type interest rate: "); rate = TextIO.getlnDouble(); years = 0; while (years < 5) { years = years + 1; interest = principal * rate; principal = principal + interest; System.out.println(principal); } This still needs to be wrapped inside a complete program, it still needs to be commented, and it really needs to print out more information for the user. But it's essentially the same program as the one in the previous section. (Note that the pseudocode algorithm uses indentation to show which statements are inside the loop. In Java, indentation is completely ignored by the computer, so you need a pair of braces to tell the computer which statements are in the loop. If you leave out the braces, the only statement inside the loop would be "years = years + 1;". The other statements would only be executed once, after the loop ends. The nasty thing is that the computer won't notice this error for you, like it would if you left out the parentheses around "(years < 5)". The parentheses are required by the syntax of the while statement. The braces are only required semantically. The computer can recognize syntax errors but not semantic errors.) One thing you should have noticed here is that my original specification of the problem -- "Compute and display the value of an investment for each of the next five years" -- was far from being complete. Before you start writing a program, you should make sure you have a complete specification of exactly what the program is supposed to do. In particular, you need to know what information the program is going to input and output and what computation it is going to perform. Here is what a reasonably complete specification of the problem might look like in this example: "Write a program that will compute and display the value of an investment for each of the next five years. Each year, interest is added to the value. The interest is computed by multiplying the current value by a fixed interest rate. Assume that the initial value and the rate of interest are to be input by the user when the program is run." Let's do another example, working this time with a program that you haven't already seen. The assignment here is an abstract mathematical problem that is one of my favorite programming exercises. This time, we'll start with a more complete specification of the task to be performed: "Given a positive integer, N, define the '3N+1' sequence starting from N as follows: If N is an even number, then divide N by two; but if N is odd, then multiply N by 3 and add 1. Continue to generate numbers in this way until N becomes equal to 1. For example, starting from N = 3, which is odd, we multiply by 3 and add 1, giving N = 3*3+1 = 10. Then, since N is even, we divide by 2, giving N = 10/2 = 5. We continue in this way, stopping when we reach 1, giving the complete sequence: 3, 10, 5, 16, 8, 4, 2, 1. "Write a program that will read a positive integer from the user and will print out the 3N+1 sequence starting from that integer. The program should also count and print out the number of terms in the sequence." A general outline of the algorithm for the program we want is: http://math.hws.edu/eck/cs124/javanotes4/c3/s2.html (3 of 7) [6/11/2004 11:08:51 AM] Java Programming: Section 3.2 Get a positive integer N from the user; Compute, print, and count each number in the sequence; Output the number of terms; The bulk of the program is in the second step. We'll need a loop, since we want to keep computing numbers until we get 1. To put this in terms appropriate for a while loop, we want to continue as long as the number is not 1. So, we can expand our pseudocode algorithm to: Get a positive integer N from the user; while N is not 1: Compute N = next term; Output N; Count this term; Output the number of terms; In order to compute the next term, the computer must take different actions depending on whether N is even or odd. We need an if statement to decide between the two cases: Get a positive integer N from the user; while N is not 1: if N is even: Compute N = N/2; else Compute N = 3 * N + 1; Output N; Count this term; Output the number of terms; We are almost there. The one problem that remains is counting. Counting means that you start with zero, and every time you have something to count, you add one. We need a variable to do the counting. (Again, this is a common pattern that you should expect to see over and over.) With the counter added, we get: Get a positive integer N from the user; Let counter = 0; while N is not 1: if N is even: Compute N = N/2; else Compute N = 3 * N + 1; Output N; Add 1 to counter; Output the counter; We still have to worry about the very first step. How can we get a positive integer from the user? If we just read in a number, it's possible that the user might type in a negative number or zero. If you follow what happens when the value of N is negative or zero, you'll see that the program will go on forever, since the value of N will never become equal to 1. This is bad. In this case, the problem is probably no big deal, but in general you should try to write programs that are foolproof. One way to fix this is to keep reading in numbers until the user types in a positive number: Ask user to input a positive number; Let N be the user's response; while N is not positive: Print an error message; Read another value for N; Let counter = 0; while N is not 1: if N is even: Compute N = N/2; http://math.hws.edu/eck/cs124/javanotes4/c3/s2.html (4 of 7) [6/11/2004 11:08:51 AM] Java Programming: Section 3.2 else Compute N = 3 * N + 1; Output N; Add 1 to counter; Output the counter; The first while loop will end only when N is a positive number, as required. (A common beginning programmer's error is to use an if statement instead of a while statement here: "If N is not positive, ask the user to input another value." The problem arises if the second number input by the user is also non-positive. The if statement is only executed once, so the second input number is never tested. With the while loop, after the second number is input, the computer jumps back to the beginning of the loop and tests whether the second number is positive. If not, it asks the user for a third number, and it will continue asking for numbers until the user enters an acceptable input.) Here is a Java program implementing this algorithm. It uses the operators <= to mean "is less than or equal to" and != to mean "is not equal to." To test whether N is even, it uses "N % 2 == 0". All the operators used here were discussed in Section 2.5. public class ThreeN { /* This program prints out a 3N+1 sequence starting from a positive integer specified by the user. It also counts the number of terms in the sequence, and prints out that number. */ public static void main(String[] args) { int N; // for computing terms in the sequence int counter; // for counting the terms TextIO.put("Starting point for sequence: "); N = TextIO.getlnInt(); while (N <= 0) { TextIO.put("The starting point must be positive. " + " Please try again: "); N = TextIO.getlnInt(); } // At this point, we know that N > 0 counter = 0; while (N != 1) { if (N % 2 == 0) N = N / 2; else N = 3 * N + 1; TextIO.putln(N); counter = counter + 1; } TextIO.putln(); TextIO.put("There were "); TextIO.put(counter); TextIO.putln(" terms in the sequence."); http://math.hws.edu/eck/cs124/javanotes4/c3/s2.html (5 of 7) [6/11/2004 11:08:51 AM] Java Programming: Section 3.2 } // end of main() } // end of class ThreeN As usual, you can try this out in an applet that simulates the program. Try different starting values for N, including some negative values: (Applet "ThreeN1Console" would be displayed here if Java were available.) Two final notes on this program: First, you might have noticed that the first term of the sequence -- the value of N input by the user -- is not printed or counted by this program. Is this an error? It's hard to say. Was the specification of the program careful enough to decide? This is the type of thing that might send you back to the boss/professor for clarification. The problem (if it is one!) can be fixed easily enough. Just replace the line "counter = 0" before the while loop with the two lines: TextIO.putln(N); // print out initial term counter = 1; // and count it Second, there is the question of why this problem is at all interesting. Well, it's interesting to mathematicians and computer scientists because of a simple question about the problem that they haven't been able to answer: Will the process of computing the 3N+1 sequence finish after a finite number of steps for all possible starting values of N? Although individual sequences are easy to compute, no one has been able to answer the general question. (To put this another way, no one knows whether the process of computing 3N+1 sequences can properly be called an algorithm, since an algorithm is required to terminate after a finite number of steps!) Coding, Testing, Debugging It would be nice if, having developed an algorithm for your program, you could relax, press a button, and get a perfectly working program. Unfortunately, the process of turning an algorithm into Java source code doesn't always go smoothly. And when you do get to the stage of a working program, it's often only working in the sense that it does something. Unfortunately not what you want it to do. After program design comes coding: translating the design into a program written in Java or some other language. Usually, no matter how careful you are, a few syntax errors will creep in from somewhere, and the Java compiler will reject your program with some kind of error message. Unfortunately, while a compiler will always detect syntax errors, it's not very good about telling you exactly what's wrong. Sometimes, it's not even good about telling you where the real error is. A spelling error or missing "{" on line 45 might cause the compiler to choke on line 105. You can avoid lots of errors by making sure that you really understand the syntax rules of the language and by following some basic programming guidelines. For example, I never type a "{" without typing the matching "}". Then I go back and fill in the statements between the braces. A missing or extra brace can be one of the hardest errors to find in a large program. Always, always indent your program nicely. If you change the program, change the indentation to match. It's worth the trouble. Use a consistent naming scheme, so you don't have to struggle to remember whether you called that variable interestrate or interestRate. In general, when the compiler gives multiple error messages, don't try to fix the second error message from the compiler until you've fixed the first one. Once the compiler hits an error in your program, it can get confused, and the rest of the error messages might just be guesses. Maybe the best advice is: Take the time to understand the error before you try to fix it. Programming is not an experimental science. When your program compiles without error, you are still not done. You have to test the program to make sure it works correctly. Remember that the goal is not to get the right output for the two sample inputs that the professor gave in class. The goal is a program that will work correctly for all reasonable inputs. Ideally, when faced with an unreasonable input, it will respond by gently chiding the user rather than by crashing. http://math.hws.edu/eck/cs124/javanotes4/c3/s2.html (6 of 7) [6/11/2004 11:08:51 AM] Java Programming: Section 3.2 Test your program on a wide variety of inputs. Try to find a set of inputs that will test the full range of functionality that you've coded into your program. As you begin writing larger programs, write them in stages and test each stage along the way. You might even have to write some extra code to do the testing -- for example to call a subroutine that you've just written. You don't want to be faced, if you can avoid it, with 500 newly written lines of code that have an error in there somewhere. The point of testing is to find bugs -- semantic errors that show up as incorrect behavior rather than as compilation errors. And the sad fact is that you will probably find them. Again, you can minimize bugs by careful design and careful coding, but no one has found a way to avoid them altogether. Once you've detected a bug, it's time for debugging. You have to track down the cause of the bug in the program's source code and eliminate it. Debugging is a skill that, like other aspects of programming, requires practice to master. So don't be afraid of bugs. Learn from them. One essential debugging skill is the ability to read source code -- the ability to put aside preconceptions about what you think it does and to follow it the way the computer does -- mechanically, step-by-step -- to see what it really does. This is hard. I can still remember the time I spent hours looking for a bug only to find that a line of code that I had looked at ten times had a "1" where it should have had an "i", or the time when I wrote a subroutine named WindowClosing which would have done exactly what I wanted except that the computer was looking for windowClosing (with a lower case "w"). Sometimes it can help to have someone who doesn't share your preconceptions look at your code. Often, it's a problem just to find the part of the program that contains the error. Most programming environments come with a debugger, which is a program that can help you find bugs. Typically, your program can be run under the control of the debugger. The debugger allows you to set "breakpoints" in your program. A breakpoint is a point in the program where the debugger will pause the program so you can look at the values of the program's variables. The idea is to track down exactly when things start to go wrong during the program's execution. The debugger will also let you execute your program one line at a time, so that you can watch what happens in detail once you know the general area in the program where the bug is lurking. I will confess that I only rarely use debuggers myself. A more traditional approach to debugging is to insert debugging statements into your program. These are output statements that print out information about the state of the program. Typically, a debugging statement would say something like System.out.println("At start of while loop, N = " + N). You need to be able to tell where in your program the output is coming from, and you want to know the value of important variables. Sometimes, you will find that the computer isn't even getting to a part of the program that you think it should be executing. Remember that the goal is to find the first point in the program where the state is not what you expect it to be. That's where the bug is. And finally, remember the golden rule of debugging: If you are absolutely sure that everything in your program is right, and if it still doesn't work, then one of the things that you are absolutely sure of is wrong. [ Next Section | Previous Section | Chapter Index | Main Index ] http://math.hws.edu/eck/cs124/javanotes4/c3/s2.html (7 of 7) [6/11/2004 11:08:51 AM] Java Programming: Section 3.3 Section 3.3 The while and do..while Statements STATEMENTS IN JAVA CAN BE either simple statements or compound statements. Simple statements, such as assignments statements and subroutine call statements, are the basic building blocks of a program. Compound statements, such as while loops and if statements, are used to organize simple statements into complex structures, which are called control structures because they control the order in which the statements are executed. The next four sections explore the details of all the control structures that are available in Java, starting with the while statement and the do..while statement in this section. At the same time, we'll look at examples of programming with each control structure and apply the techniques for designing algorithms that were introduced in the previous section. The while Statement The while statement was already introduced in Section 1. A while loop has the form while ( boolean-expression ) statement The statement can, of course, be a block statement consisting of several statements grouped together between a pair of braces. This statement is called the body of the loop. The body of the loop is repeated as long as the boolean-expression is true. This boolean expression is called the continuation condition, or more simply the test, of the loop. There are a few points that might need some clarification. What happens if the condition is false in the first place, before the body of the loop is executed even once? In that case, the body of the loop is never executed at all. The body of a while loop can be executed any number of times, including zero. What happens if the condition is true, but it becomes false somewhere in the middle of the loop body? Does the loop end as soon as this happens? It does not, because the computer continues executing the body of the loop until it gets to the end. Only then does it jump back to the beginning of the loop and test the condition, and only then can the loop end. Let's look at a typical problem that can be solved using a while loop: finding the average of a set of positive integers entered by the user. The average is the sum of the integers, divided by the number of integers. The program will ask the user to enter one integer at a time. It will keep count of the number of integers entered, and it will keep a running total of all the numbers it has read so far. Here is a pseudocode algorithm for the program: Let sum = 0 Let count = 0 while there are more integers to process: Read an integer Add it to the sum Count it Divide sum by count to get the average Print out the average But how can we test whether there are more integers to process? A typical solution is to tell the user to type in zero after all the data have been entered. This will work because we are assuming that all the data are positive numbers, so zero is not a legal data value. The zero is not itself part of the data to be averaged. It's just there to mark the end of the real data. A data value used in this way is sometimes called a sentinel value. So now the test in the while loop becomes "while the input integer is not zero". But there is another problem! The first time the test is evaluated, before the body of the loop has ever been executed, no integer has yet been read. There is no "input integer" yet, so testing whether the input integer is zero doesn't make http://math.hws.edu/eck/cs124/javanotes4/c3/s3.html (1 of 6) [6/11/2004 11:08:52 AM] Java Programming: Section 3.3 sense. So, we have to do something before the while loop to make sure that the test makes sense. Setting things up so that the test in a while loop makes sense the first time it is executed is called priming the loop. In this case, we can simply read the first integer before the beginning of the loop. Here is a revised algorithm: Let sum = 0 Let count = 0 Read an integer while the integer is not zero: Add the integer to the sum Count it Read an integer Divide sum by count to get the average Print out the average Notice that I've rearranged the body of the loop. Since an integer is read before the loop, the loop has to begin by processing that integer. At the end of the loop, the computer reads a new integer. The computer then jumps back to the beginning of the loop and tests the integer that it has just read. Note that when the computer finally reads the sentinel value, the loop ends before the sentinel value is processed. It is not added to the sum, and it is not counted. This is the way it's supposed to work. The sentinel is not part of the data. The original algorithm, even if it could have been made to work without priming, was incorrect since it would have summed and counted all the integers, including the sentinel. (Since the sentinel is zero, the sum would still be correct, but the count would be off by one. Such so-called off-by-one errors are very common. Counting turns out to be harder than it looks!) We can easily turn the algorithm into a complete program. Note that the program cannot use the statement "average = sum/count;" to compute the average. Since sum and count are both variables of type int, the value of sum/count is an integer. The average should be a real number. We've seen this problem before: we have to convert one of the int values to a double to force the computer to compute the quotient as a real number. This can be done by type-casting one of the variables to type double. The type cast "(double)sum" converts the value of sum to a real number, so in the program the average is computed as "average = ((double)sum) / count;". Another solution in this case would have been to declare sum to be a variable of type double in the first place. One other issue is addressed by the program: If the user enters zero as the first input value, there are no data to process. We can test for this case by checking whether count is still equal to zero after the while loop. This might seem like a minor point, but a careful programmer should cover all the bases. Here is the program and an applet that simulates it: public class ComputeAverage { /* This program reads a sequence of positive integers input by the user, and it will print out the average of those integers. The user is prompted to enter one integer at a time. The user must enter a 0 to mark the end of the data. (The zero is not counted as part of the data to be averaged.) The program does not check whether the user's input is positive, so it will actually work for both positive and negative input values. */ public static void main(String[] args) { int inputNumber; // One of the integers input by the user. int sum; // The sum of the positive integers. http://math.hws.edu/eck/cs124/javanotes4/c3/s3.html (2 of 6) [6/11/2004 11:08:52 AM] Java Programming: Section 3.3 int count; // The number of positive integers. double average; // The average of the positive integers. /* Initialize the summation and counting variables. */ sum = 0; count = 0; /* Read and process the user's input. */ TextIO.put("Enter your first positive integer: "); inputNumber = TextIO.getlnInt(); while (inputNumber != 0) { sum += inputNumber; // Add inputNumber to running sum. count++; // Count the input by adding 1 to count. TextIO.put("Enter your next positive integer, or 0 to end: "); inputNumber = TextIO.getlnInt(); } /* Display the result. */ if (count == 0) { TextIO.putln("You didn't enter any data!"); } else { average = ((double)sum) / count; TextIO.putln(); TextIO.putln("You entered " + count + " positive integers."); TextIO.putln("Their average is " + average + "."); } } // end main() } // end class ComputeAverage (Applet "ComputeAverageConsole" would be displayed here if Java were available.) The do..while Statement Sometimes it is more convenient to test the continuation condition at the end of a loop, instead of at the beginning, as is done in the while loop. The do..while statement is very similar to the while statement, except that the word "while," along with the condition that it tests, has been moved to the end. The word "do" is added to mark the beginning of the loop. A do..while statement has the form do statement while ( boolean-expression ); or, since, as usual, the statement can be a block, do { statements } while ( boolean-expression ); http://math.hws.edu/eck/cs124/javanotes4/c3/s3.html (3 of 6) [6/11/2004 11:08:52 AM] Java Programming: Section 3.3 Note the semicolon, ';', at the end. This semicolon is part of the statement, just as the semicolon at the end of an assignment statement or declaration is part of the statement. Omitting it is a syntax error. (More generally, every statement in Java ends either with a semicolon or a right brace, '}'.) To execute a do loop, the computer first executes the body of the loop -- that is, the statement or statements inside the loop -- and then it evaluates the boolean expression. If the value of the expression is true, the computer returns to the beginning of the do loop and repeats the process; if the value is false, it ends the loop and continues with the next part of the program. Since the condition is not tested until the end of the loop, the body of a do loop is executed at least once. For example, consider the following pseudocode for a game-playing program. The do loop makes sense here instead of a while loop because with the do loop, you know there will be at least one game. Also, the test that is used at the end of the loop wouldn't even make sense at the beginning: do { Play a Game Ask user if he wants to play another game Read the user's response } while ( the user's response is yes ); Let's convert this into proper Java code. Since I don't want to talk about game playing at the moment, let's say that we have a class named Checkers, and that the Checkers class contains a static member subroutine named playGame() that plays one game of checkers against the user. Then, the pseudocode "Play a game" can be expressed as the subroutine call statement "Checkers.playGame();". We need a variable to store the user's response. The TextIO class makes it convenient to use a boolean variable to store the answer to a yes/no question. The input function TextIO.getlnBoolean() allows the user to enter the value as "yes" or "no". "Yes" is considered to be true, and "no" is considered to be false. So, the algorithm can be coded as boolean wantsToContinue; // True if user wants to play again. do { Checkers.playGame(); TextIO.put("Do you want to play again? "); wantsToContinue = TextIO.getlnBoolean(); } while (wantsToContinue == true); When the value of the boolean variable is set to false, it is a signal that the loop should end. When a boolean variable is used in this way -- as a signal that is set in one part of the program and tested in another part -- it is sometimes called a flag or flag variable (in the sense of a signal flag). By the way, a more-than-usually-pedantic programmer would sneer at the test "while (wantsToContinue == true)". This test is exactly equivalent to "while (wantsToContinue)". Testing whether "wantsToContinue == true" is true amounts to the same thing as testing whether "wantsToContinue" is true. A little less offensive is an expression of the form "flag == false", where flag is a boolean variable. The value of "flag == false" is exactly the same as the value of "!flag", where ! is the boolean negation operator. So you can write "while (!flag)" instead of "while (flag == false)", and you can write "if (!flag)" instead of "if (flag == false)". Although a do..while statement is sometimes more convenient than a while statement, having two kinds of loops does not make the language more powerful. Any problem that can be solved using do..while loops can also be solved using only while statements, and vice versa. In fact, if doSomething represents any block of program code, then do { doSomething } while ( boolean-expression ); http://math.hws.edu/eck/cs124/javanotes4/c3/s3.html (4 of 6) [6/11/2004 11:08:52 AM] Java Programming: Section 3.3 has exactly the same effect as doSomething while ( boolean-expression ) { doSomething } Similarly, while ( boolean-expression ) { doSomething } can be replaced by if ( boolean-expression ) { do { doSomething } while ( boolean-expression ); } without changing the meaning of the program in any way. The break and continue Statements The syntax of the while and do..while loops allows you to test the continuation condition at either the beginning of a loop or at the end. Sometimes, it is more natural to have the test in the middle of the loop, or to have several tests at different places in the same loop. Java provides a general method for breaking out of the middle of any loop. It's called the break statement, which takes the form break; When the computer executes a break statement in a loop, it will immediately jump out of the loop. It then continues on to whatever follows the loop in the program. Consider for example: while (true) { // looks like it will run forever! TextIO.put("Enter a positive number: "); N = TextIO.getlnlnt(); if (N > 0) // input is OK; jump out of loop break; TextIO.putln("Your answer must be > 0."); } // continue here after break If the number entered by the user is greater than zero, the break statement will be executed and the computer will jump out of the loop. Otherwise, the computer will print out "Your answer must be > 0." and will jump back to the start of the loop to read another input value. (The first line of the loop, "while (true)" might look a bit strange, but it's perfectly legitimate. The condition in a while loop can be any boolean-valued expression. The computer evaluates this expression and checks whether the value is true or false. The boolean literal "true" is just a boolean expression that always evaluates to true. So "while (true)" can be used to write an infinite loop, or one that can be terminated only by a break statement.) A break statement terminates the loop that immediately encloses the break statement. It is possible to have nested loops, where one loop statement is contained inside another. If you use a break statement inside a nested loop, it will only break out of that loop, not out of the loop that contains the nested loop. http://math.hws.edu/eck/cs124/javanotes4/c3/s3.html (5 of 6) [6/11/2004 11:08:52 AM] Java Programming: Section 3.3 There is something called a "labeled break" statement that allows you to specify which loop you want to break. I won't give the details here; you can look them up if you ever need them. The continue statement is related to break, but less commonly used. A continue statement tells the computer to skip the rest of the current iteration of the loop. However, instead of jumping out of the loop altogether, it jumps back to the beginning of the loop and continues with the next iteration (after evaluating the loop's continuation condition to see whether any further iterations are required). break and continue can be used in while loops and do..while loops. They can also be used in for loops, which are covered in the next section. In Section 6, we'll see that break can also be used to break out of a switch statement. Note that when a break occurs inside an if statement, it breaks out of the loop or switch statement that contains the if statement. If the if statement is not contained inside a loop or switch, then the if statement cannot legally contain a break statement. A similar consideration applies to continue statements. [ Next Section | Previous Section | Chapter Index | Main Index ] http://math.hws.edu/eck/cs124/javanotes4/c3/s3.html (6 of 6) [6/11/2004 11:08:52 AM] Java Programming: Section 3.4 Section 3.4 The for Statement WE TURN IN THIS SECTION to another type of loop, the for statement. Any for loop is equivalent to some while loop, so the language doesn't get any additional power by having for statements. But for a certain type of problem, a for loop can be easier to construct and easier to read than the corresponding while loop. It's quite possible that in real programs, for loops actually outnumber while loops. The for statement makes a common type of while loop easier to write. Many while loops have the general form: initialization while ( continuation-condition ) { statements update } For example, consider this example, copied from an example in Section 2: years = 0; // initialize the variable years while ( years < 5 ) { // condition for continuing loop interest = principal * rate; // principal += interest; // do three statements System.out.println(principal); // years++; // update the value of the variable, years } This loop can be written as the following equivalent for statement: for ( years = 0; years < 5; years++ ) { interest = principal * rate; principal += interest; System.out.println(principal); } The initialization, continuation condition, and updating have all been combined in the first line of the for loop. This keeps everything involved in the "control" of the loop in one place, which helps makes the loop easier to read and understand. The for loop is executed in exactly the same way as the original code: The initialization part is executed once, before the loop begins. The continuation condition is executed before each execution of the loop, and the loop ends when this condition is false. The update part is executed at the end of each execution of the loop, just before jumping back to check the condition. The formal syntax of the for statement is as follows: for ( initialization; continuation-condition; update ) statement or, using a block statement: for ( initialization; continuation-condition; update ) { statements } The continuation-condition must be a boolean-valued expression. The initialization can be any expression, http://math.hws.edu/eck/cs124/javanotes4/c3/s4.html (1 of 8) [6/11/2004 11:08:53 AM] Java Programming: Section 3.4 as can the update. Any of the three can be empty. If the continuation condition is empty, it is treated as if it were "true," so the loop will be repeated forever or until it ends for some other reason, such as a break statement. (Some people like to begin an infinite loop with "for (;;)" instead of "while (true)".) Usually, the initialization part of a for statement assigns a value to some variable, and the update changes the value of that variable with an assignment statement or with an increment or decrement operation. The value of the variable is tested in the continuation condition, and the loop ends when this condition evaluates to false. A variable used in this way is called a loop control variable. In the for statement given above, the loop control variable is years. Certainly, the most common type of for loop is the counting loop, where a loop control variable takes on all integer values between some minimum and some maximum value. A counting loop has the form for ( variable = min; variable <= max; variable++ ) { statements } where min and max are integer-valued expressions (usually constants). The variable takes on the values min, min+1, min+2, ...,max. The value of the loop control variable is often used in the body of the loop. The for loop at the beginning of this section is a counting loop in which the loop control variable, years, takes on the values 1, 2, 3, 4, 5. Here is an even simpler example, in which the numbers 1, 2, ..., 10 are displayed on standard output: for ( N = 1 ; N <= 10 ; N++ ) System.out.println( N ); For various reasons, Java programmers like to start counting at 0 instead of 1, and they tend to use a "<" in the condition, rather than a "<=". The following variation of the above loop prints out the ten numbers 0, 1, 2, ..., 9: for ( N = 0 ; N < 10 ; N++ ) System.out.println( N ); Using < instead of <= in the test, or vice versa, is a common source of off-by-one errors in programs. You should always stop and think, do I want the final value to be processed or not? It's easy to count down from 10 to 1 instead of counting up. Just start with 10, decrement the loop control variable instead of incrementing it, and continue as long as the variable is greater than or equal to one. for ( N = 10 ; N >= 1 ; N-- ) System.out.println( N ); Now, in fact, the official syntax of a for statemenent actually allows both the initialization part and the update part to consist of several expressions, separated by commas. So we can even count up from 1 to 10 and count down from 10 to 1 at the same time! for ( i=1, j=10; i <= 10; i++, j-- ) { TextIO.put(i,5); // Output i in a 5-character wide column. TextIO.putln(j,5); // Output j in a 5-character column // and end the line. } As a final example, let's say that we want to use a for loop that prints out just the even numbers between 2 and 20, that is: 2, 4, 6, 8, 10, 12, 14, 16, 18, 20. There are several ways to do this. Just to show how even a very simple problem can be solved in many ways, here are four different solutions (three of which would get full credit): (1) // There are 10 numbers to print. // Use a for loop to count 1, 2, // ..., 10. The numbers we want http://math.hws.edu/eck/cs124/javanotes4/c3/s4.html (2 of 8) [6/11/2004 11:08:53 AM] Java Programming: Section 3.4 // to print are 2*1, 2*2, ... 2*10. for (N = 1; N <= 10; N++) { System.out.println( 2*N ); } (2) // Use a for loop that counts // 2, 4, ..., 20 directly by // adding 2 to N each time through // the loop. for (N = 2; N <= 20; N = N + 2) { System.out.println( N ); } (3) // Count off all the numbers // 2, 3, 4, ..., 19, 20, but // only print out the numbers // that are even. for (N = 2; N <= 20; N++) { if ( N % 2 == 0 ) // is N even? System.out.println( N ); } (4) // Irritate the professor with // a solution that follows the // letter of this silly assignment // while making fun of it. for (N = 1; N <= 1; N++) { System.out.print("2 4 6 8 10 12 "); System.out.println("14 16 18 20"); } Perhaps it is worth stressing one more time that a for statement, like any statement, never occurs on its own in a real program. A statement must be inside the main routine of a program or inside some other subroutine. And that subroutine must be defined inside a class. I should also remind you that every variable must be declared before it can be used, and that includes the loop control variable in a for statement. In all the examples that you have seen so far in this section, the loop control variables should be declared to be of type int. It is not required that a loop control variable be an integer. Here, for example, is a for loop in which the variable, ch, is of type char: // Print out the alphabet on one line of output. char ch; // The loop control variable; // one of the letters to be printed. for ( ch = 'A'; ch <= 'Z'; ch++ ) System.out.print(ch); System.out.println(); Let's look at a less trivial problem that can be solved with a for loop. If N and D are positive integers, we say that D is a divisor of N if the remainder when D is divided into N is zero. (Equivalently, we could say that http://math.hws.edu/eck/cs124/javanotes4/c3/s4.html (3 of 8) [6/11/2004 11:08:53 AM] Java Programming: Section 3.4 N is an even multiple of D.) In terms of Java programming, D is a divisor of N if N % D is zero. Let's write a program that inputs a positive integer, N, from the user and computes how many different divisors N has. The numbers that could possibly be divisors of N are 1, 2, ...,N. To compute the number of divisors of N, we can just test each possible divisor of N and count the ones that actually do divide N evenly. In pseudocode, the algorithm takes the form Get a positive integer, N, from the user Let divisorCount = 0 for each number, testDivisor, in the range from 1 to N: if testDivisor is a divisor of N: Count it by adding 1 to divisorCount Output the count This algorithm displays a common programming pattern that is used when some, but not all, of a sequence of items are to be processed. The general pattern is for each item in the sequence: if the item passes the test: process it The for loop in our divisor-counting algorithm can be translated into Java code as for (testDivisor = 1; testDivisor <= N; testDivisor++) { if ( N % testDivisor == 0 ) divisorCount++; } On a modern computer, this loop can be executed very quickly. It is not impossible to run it even for the largest legal int value, 2147483647. (If you wanted to run it for even larger values, you could use variables of type long rather than int.) However, it does take a noticeable amount of time for very large numbers. So when I implemented this algorithm, I decided to output a period every time the computer has tested one million possible divisors. In the improved version of the program, there are two types of counting going on. We have to count the number of divisors and we also have to count the number of possible divisors that have been tested. So the program needs two counters. When the second counter reaches 1000000, we output a '.' and reset the counter to zero so that we can start counting the next group of one million. Reverting to pseudocode, the algorithm now looks like Get a positive integer, N, from the user Let divisorCount = 0 // Number of divisors found. Let numberTested = 0 // Number of possible divisors tested // since the last period was output. for each number, testDivisor, in the range from 1 to N: if testDivisor is a divisor of N: Count it by adding 1 to divisorCount Add 1 to numberTested if numberTested is 1000000: print out a '.' Let numberTested = 0 Output the count Finally, we can translate the algorithm into a complete Java program. Here it is, followed by an applet that simulates it: public class CountDivisors { /* This program reads a positive integer from the user. It counts how many divisors that number has, and then it prints the result. http://math.hws.edu/eck/cs124/javanotes4/c3/s4.html (4 of 8) [6/11/2004 11:08:53 AM] Java Programming: Section 3.4 */ public static void main(String[] args) { int N; // A positive integer entered by the user. // Divisors of this number will be counted. int testDivisor; // A number between 1 and N that is a // possible divisor of N. int divisorCount; // Number of divisors of N that have been found. int numberTested; // Used to count how many possible divisors // of N have been tested. When the number // reaches 1000000, a period is output and // the value of numberTested is reset to zero. /* Get a positive integer from the user. */ while (true) { TextIO.put("Enter a positive integer: "); N = TextIO.getlnInt(); if (N > 0) break; TextIO.putln("That number is not positive. Please try again."); } /* Count the divisors, printing a "." after every 1000000 tests. */ divisorCount = 0; numberTested = 0; for (testDivisor = 1; testDivisor <= N; testDivisor++) { if ( N % testDivisor == 0 ) divisorCount++; numberTested++; if (numberTested == 1000000) { TextIO.put('.'); numberTested = 0; } } /* Display the result. */ TextIO.putln(); TextIO.putln("The number of divisors of " + N + " is " + divisorCount); } // end main() } // end class CountDivisors (Applet "CountDivisorsConsole" would be displayed here if Java were available.) http://math.hws.edu/eck/cs124/javanotes4/c3/s4.html (5 of 8) [6/11/2004 11:08:53 AM] Java Programming: Section 3.4 Nested Loops Control structures in Java are statements that contain statements. In particular, control structures can contain control structures. You've already seen several examples of if statements inside loops, but any combination of one control structure inside another is possible. We say that one structure is nested inside another. You can even have multiple levels of nesting, such as a while loop inside an if statement inside another while loop. The syntax of Java does not set a limit on the number of levels of nesting. As a practical matter, though, it's difficult to understand a program that has more than a few levels of nesting. Nested for loops arise naturally in many algorithms, and it is important to understand how they work. Let's look at a couple of examples. First, consider the problem of printing out a multiplication table like this one: 1 2 3 4 5 6 7 8 9 10 11 12 2 4 6 8 10 12 14 16 18 20 22 24 3 6 9 12 15 18 21 24 27 30 33 36 4 8 12 16 20 24 28 32 36 40 44 48 5 10 15 20 25 30 35 40 45 50 55 60 6 12 18 24 30 36 42 48 54 60 66 72 7 14 21 28 35 42 49 56 63 70 77 84 8 16 24 32 40 48 56 64 72 80 88 96 9 18 27 36 45 54 63 72 81 90 99 108 10 20 30 40 50 60 70 80 90 100 110 120 11 22 33 44 55 66 77 88 99 110 121 132 12 24 36 48 60 72 84 96 108 120 132 144 The data in the table are arranged into 12 rows and 12 columns. The process of printing them out can be expressed in a pseudocode algorithm as for each rowNumber = 1, 2, 3, ..., 12: Print the first twelve multiples of rowNumber on one line Output a carriage return The first step in the for loop can itself be expressed as a for loop: for N = 1, 2, 3, ..., 12: Print N * rowNumber so a refined algorithm for printing the table has one for loop nested inside another: for each rowNumber = 1, 2, 3, ..., 12: for N = 1, 2, 3, ..., 12: Print N * rowNumber Output a carriage return Assuming that rowNumber and N have been declared to be variables of type int, this can be expressed in Java as for ( rowNumber = 1; rowNumber <= 12; rowNumber++ ) { for ( N = 1; N <= 12; N++ ) { // print in 4-character columns TextIO.put( N * rowNumber, 4 ); } TextIO.putln(); } This section has been weighed down with lots of examples of numerical processing. For our final example, let's do some text processing. Consider the problem of finding which of the 26 letters of the alphabet occur in a given string. For example, the letters that occur in "Hello World" are D, E, H, L, O, R, and W. More specifically, we will write a program that will list all the letters contained in a string and will also count the http://math.hws.edu/eck/cs124/javanotes4/c3/s4.html (6 of 8) [6/11/2004 11:08:53 AM] Java Programming: Section 3.4 number of different letters. The string will be input by the user. Let's start with a pseudocode algorithm for the program. Ask the user to input a string Read the response into a variable, str Let count = 0 (for counting the number of different letters) for each letter of the alphabet: if the letter occurs in str: Print the letter Add 1 to count Output the count Since we want to process the entire line of text that is entered by the user, we'll use TextIO.getln() to read it. The line of the algorithm that reads "for each letter of the alphabet" can be expressed as "for (letter='A'; letter<='Z'; letter++)". But the body of this for loop needs more thought. How do we check whether the given letter, letter, occurs in str? One idea is to look at each letter in the string in turn, and check whether that letter is equal to letter. We can get the i-th character of str with the function call str.charAt(i), where i ranges from 0 to str.length() - 1. One more difficulty: A letter such as 'A' can occur in str in either upper or lower case, 'A' or 'a'. We have to check for both of these. But we can avoid this difficulty by converting str to upper case before processing it. Then, we only have to check for the upper case letter. We can now flesh out the algorithm fully. Note the use of break in the nested for loop. It is required to avoid printing or counting a given letter more than once. The break statement breaks out of the inner for loop, but not the outer for loop. Upon executing the break, the computer continues the outer loop with the next value of letter. Ask the user to input a string Read the response into a variable, str Convert str to upper case Let count = 0 for letter = 'A', 'B', ..., 'Z': for i = 0, 1, ..., str.length()-1: if letter == str.charAt(i): Print letter Add 1 to count break // jump out of the loop Output the count Here is the complete program and an applet to simulate it: public class ListLetters { /* This program reads a line of text entered by the user. It prints a list of the letters that occur in the text, and it reports how many different letters were found. */ public static void main(String[] args) { String str; // Line of text entered by the user. int count; // Number of different letters found in str. char letter; // A letter of the alphabet. TextIO.putln("Please type in a line of text."); str = TextIO.getln(); http://math.hws.edu/eck/cs124/javanotes4/c3/s4.html (7 of 8) [6/11/2004 11:08:53 AM] Java Programming: Section 3.4 str = str.toUpperCase(); count = 0; TextIO.putln("Your input contains the following letters:"); TextIO.putln(); TextIO.put(" "); for ( letter = 'A'; letter <= 'Z'; letter++ ) { int i; // Position of a character in str. for ( i = 0; i < str.length(); i++ ) { if ( letter == str.charAt(i) ) { TextIO.put(letter); TextIO.put(' '); count++; break; } } } TextIO.putln(); TextIO.putln(); TextIO.putln("There were " + count + " different letters."); } // end main() } // end class ListLetters (Applet "ListLettersConsole" would be displayed here if Java were available.) In fact, there is an easier way to determine whether a given letter occurs in a string, str. The built-in function str.indexOf(letter) will return -1 if letter does not occur in the string. It returns a number greater than or equal to zero if it does occur. So, we could check whether letter occurs in str simply by checking "if (str.indexOf(letter) >= 0)". If we used this technique in the above program, we wouldn't need a nested for loop. This gives you preview of how subroutines can be used to deal with complexity. [ Next Section | Previous Section | Chapter Index | Main Index ] http://math.hws.edu/eck/cs124/javanotes4/c3/s4.html (8 of 8) [6/11/2004 11:08:53 AM] Java Programming: Section 3.5 Section 3.5 The if Statement THE FIRST OF THE TWO BRANCHING STATEMENTS in Java is the if statement, which you have already seen in Section 1. It takes the form if (boolean-expression) statement-1 else statement-2 As usual, the statements inside an if statements can be blocks. The if statement represents a two-way branch. The else part of an if statement -- consisting of the word "else" and the statement that follows it -- can be omitted. Now, an if statement is, in particular, a statement. This means that either statement-1 or statement-2 in the above if statement can itself be an if statement. A problem arises, however, if statement-1 is an if statement that has no else part. This special case is effectively forbidden by the syntax of Java. Suppose, for example, that you type if ( x > 0 ) if (y > 0) System.out.println("First case"); else System.out.println("Second case"); Now, remember that the way you've indented this doesn't mean anything at all to the computer. You might think that the else part is the second half of your "if (x > 0)" statement, but the rule that the computer follows attaches the else to "if (y > 0)", which is closer. That is, the computer reads your statement as if it were formatted: if ( x > 0 ) if (y > 0) System.out.println("First case"); else System.out.println("Second case"); You can force the computer to use the other interpretation by enclosing the nested if in a block: if ( x > 0 ) { if (y > 0) System.out.println("First case"); } else System.out.println("Second case"); These two statements have different meanings: If x <= 0, the first statement doesn't print anything, but the second statement prints "Second case.". Much more interesting than this technicality is the case where statement-2, the else part of the if statement, is itself an if statement. The statement would look like this (perhaps without the final else part): if (boolean-expression-1) statement-1 else if (boolean-expression-2) http://math.hws.edu/eck/cs124/javanotes4/c3/s5.html (1 of 6) [6/11/2004 11:08:54 AM] Java Programming: Section 3.5 statement-2 else statement-3 However, since the computer doesn't care how a program is laid out on the page, this is almost always written in the format: if (boolean-expression-1) statement-1 else if (boolean-expression-2) statement-2 else statement-3 You should think of this as a single statement representing a three-way branch. When the computer executes this, one and only one of the three statements -- statement-1, statement-2, or statement-3 -- will be executed. The computer starts by evaluating boolean-expression-1. If it is true, the computer executes statement-1 and then jumps all the way to the end of the outer if statement, skipping the other two statements. If boolean-expression-1 is false, the computer skips statement-1 and executes the second, nested if statement. To do this, it tests the value of boolean-expression-2 and uses it to decide between statement-2 and statement-3. Here is an example that will print out one of three different messages, depending on the value of a variable named temperature: if (temperature < 50) System.out.println("It's cold."); else if (temperature < 80) System.out.println("It's nice."); else System.out.println("It's hot."); If temperature is, say, 42, the first test is true. The computer prints out the message "It's cold", and skips the rest -- without even evaluating the second condition. For a temperature of 75, the first test is false, so the computer goes on to the second test. This test is true, so the computer prints "It's nice" and skips the rest. If the temperature is 173, both of the tests evaluate to false, so the computer says "It's hot" (unless its circuits have been fried by the heat, that is). You can go on stringing together "else-if's" to make multi-way branches with any number of cases: if (boolean-expression-1) statement-1 else if (boolean-expression-2) statement-2 else if (boolean-expression-3) statement-3 . . // (more cases) . else if (boolean-expression-N) statement-N else statement-(N+1) The computer evaluates boolean expressions one after the other until it comes to one that is true. It executes the associated statement and skips the rest. If none of the boolean expressions evaluate to true, then the statement in the else part is executed. This statement is called a multi-way branch because only one of the statements will be executed. The final else part can be omitted. In that case, if all the boolean expressions are false, none of the statements is executed. Of course, each of the statements can be a block, http://math.hws.edu/eck/cs124/javanotes4/c3/s5.html (2 of 6) [6/11/2004 11:08:54 AM] Java Programming: Section 3.5 consisting of a number of statements enclosed between { and }. (Admittedly, there is lot of syntax here; as you study and practice, you'll become comfortable with it.) As an example of using if statements, lets suppose that x, y, and z are variables of type int, and that each variable has already been assigned a value. Consider the problem of printing out the values of the three variables in increasing order. For examples, if the values are 42, 17, and 20, then the output should be in the order 17, 20, 42. One way to approach this is to ask, where does x belong in the list? It comes first if it's less than both y and z. It comes last if it's greater than both y and z. Otherwise, it comes in the middle. We can express this with a 3-way if statement, but we still have to worry about the order in which y and z should be printed. In pseudocode, if (x < y && x < z) { output x, followed by y and z in their correct order } else if (x > y && x > z) { output y and z in their correct order, followed by x } else { output x in between y and z in their correct order } Determining the relative order of y and z requires another if statement, so this becomes if (x < y && x < z) { // x comes first if (y < z) System.out.println( x + " " + y + " " + z ); else System.out.println( x + " " + z + " " + y ); } else if (x > y && x > z) { // x comes last if (y < z) System.out.println( y + " " + z + " " + x ); else System.out.println( z + " " + y + " " + x ); } else { // x in the middle if (y < z) System.out.println( y + " " + x + " " + z); else System.out.println( z + " " + x + " " + y); } You might check that this code will work correctly even if some of the values are the same. If the values of two variables are the same, it doesn't matter which order you print them in. Note, by the way, that even though you can say in English "if x is less than y and z,", you can't say in Java "if (x < y && z)". The && operator can only be used between boolean values, so you have to make separate tests, x<y and x<z, and then combine the two tests with &&. There is an alternative approach to this problem that begins by asking, "which order should x and y be printed in?" Once that's known, you only have to decide where to stick in z. This line of thought leads to different Java code: if ( x < y ) { // x comes before y if ( z < x ) System.out.println( z + " " + x + " " + y); http://math.hws.edu/eck/cs124/javanotes4/c3/s5.html (3 of 6) [6/11/2004 11:08:54 AM] Java Programming: Section 3.5 else if ( z > y ) System.out.println( x + " " + y + " " + z); else System.out.println( x + " " + z + " " + y); } else { // y comes before x if ( z < y ) System.out.println( z + " " + y + " " + x); else if ( z > x ) System.out.println( y + " " + x + " " + z); else System.out.println( y + " " + z + " " + x); } Once again, we see how the same problem can be solved in many different ways. The two approaches to this problem have not exhausted all the possibilities. For example, you might start by testing whether x is greater than y. If so, you could swap their values. Once you've done that, you know that x should be printed before y. Finally, let's write a complete program that uses an if statement in an interesting way. I want a program that will convert measurements of length from one unit of measurement to another, such as miles to yards or inches to feet. So far, the problem is extremely under-specified. Let's say that the program will only deal with measurements in inches, feet, yards, and miles. It would be easy to extend it later to deal with other units. The user will type in a measurement in one of these units, such as "17 feet" or "2.73 miles". The output will show the length in terms of each of the four units of measure. (This is easier than asking the user which units to use in the output.) An outline of the process is Read the user's input measurement and units of measure Express the measurement in inches, feet, yards, and miles Display the four results The program can read both parts of the user's input from the same line by using TextIO.getDouble() to read the numerical measurement and TextIO.getlnWord() to read the units of measure. The conversion into different units of measure can be simplified by first converting the user's input into inches. From there, it can be converted into feet, yards, and miles. We still have to test the input to determine which unit of measure the user has specified: Let measurement = TextIO.getDouble() Let units = TextIO.getlnWord() if the units are inches Let inches = measurement else if the units are feet Let inches = measurement * 12 // 12 inches per foot else if the units are yards Let inches = measurement * 36 // 36 inches per yard else if the units are miles Let inches = measurement * 12 * 5280 // 5280 feet per mile else The units are illegal! Print an error message and stop processing Let feet = inches / 12.0 Let yards = inches / 36.0 Let miles = inches / (12.0 * 5280.0) Display the results Since units is a String, we can use units.equals("inches") to check whether the specified unit of measure is "inches". However, it would be nice to allow the units to be specified as "inch" or abbreviated http://math.hws.edu/eck/cs124/javanotes4/c3/s5.html (4 of 6) [6/11/2004 11:08:54 AM] Java Programming: Section 3.5 to "in". To allow these three possibilities, we can check if (units.equals("inches") || units.equals("inch") || units.equals("in")). It would also be nice to allow upper case letters, as in "Inches" or "IN". We can do this by converting units to lower case before testing it or by substituting the function units.equalsIgnoreCase for units.equals. In my final program, I decided to make things more interesting by allowing the user to enter a whole sequence of measurements. The program will end only when the user inputs 0. To do this, I just have to wrap the above algorithm inside a while loop, and make sure that the loop ends when the user inputs a 0. Here's the complete program, followed by an applet that simulates it. public class LengthConverter { /* This program will convert measurements expressed in inches, feet, yards, or miles into each of the possible units of measure. The measurement is input by the user, followed by the unit of measure. For example: "17 feet", "1 inch", "2.73 mi". Abbreviations in, ft, yd, and mi are accepted. The program will continue to read and convert measurements until the user enters an input of 0. */ public static void main(String[] args) { double measurement; // Numerical measurement, input by user. String units; // The unit of measure for the input, also // specified by the user. double inches, feet, yards, miles; // Measurement expressed in // each possible unit of // measure. TextIO.putln("Enter measurements in inches, feet, yards, or miles."); TextIO.putln("For example: 1 inch 17 feet 2.73 miles"); TextIO.putln("You can use abbreviations: in ft yd mi"); TextIO.putln("I will convert your input into the other units"); TextIO.putln("of measure."); TextIO.putln(); while (true) { /* Get the user's input, and convert units to lower case. */ TextIO.put("Enter your measurement, or 0 to end: "); measurement = TextIO.getDouble(); if (measurement == 0) break; // terminate the while loop units = TextIO.getlnWord(); units = units.toLowerCase(); /* Convert the input measurement to inches. */ if (units.equals("inch") || units.equals("inches") || units.equals("in")) { inches = measurement; } http://math.hws.edu/eck/cs124/javanotes4/c3/s5.html (5 of 6) [6/11/2004 11:08:54 AM] Java Programming: Section 3.5 else if (units.equals("foot") || units.equals("feet") || units.equals("ft")) { inches = measurement * 12; } else if (units.equals("yard") || units.equals("yards") || units.equals("yd")) { inches = measurement * 36; } else if (units.equals("mile") || units.equals("miles") || units.equals("mi")) { inches = measurement * 12 * 5280; } else { TextIO.putln("Sorry, but I don't understand \"" + units + "\"."); continue; // back to start of while loop } /* Convert measurement in inches to feet, yards, and miles. */ feet = inches / 12; yards = inches / 36; miles = inches / (12*5280); /* Output measurement in terms of each unit of measure. */ TextIO.putln(); TextIO.putln("That's equivalent to:"); TextIO.put(inches, 15); TextIO.putln(" inches"); TextIO.put(feet, 15); TextIO.putln(" feet"); TextIO.put(yards, 15); TextIO.putln(" yards"); TextIO.put(miles, 15); TextIO.putln(" miles"); TextIO.putln(); } // end while TextIO.putln(); TextIO.putln("OK! Bye for now."); } // end main() } // end class LengthConverter (Applet "LengthConverterConsole" would be displayed here if Java were available.) [ Next Section | Previous Section | Chapter Index | Main Index ] http://math.hws.edu/eck/cs124/javanotes4/c3/s5.html (6 of 6) [6/11/2004 11:08:54 AM] Java Programming: Section 3.6 Section 3.6 The switch Statement THE SECOND BRANCHING STATEMENT in Java is the switch statement, which is introduced in this section. The switch is used far less often than the if statement, but it is sometimes useful for expressing a certain type of multi-way branch. Since this section wraps up coverage of all of Java's control statements, I've included a complete list of Java's statement types at the end of the section. A switch statement allows you to test the value of an expression and, depending on that value, to jump to some location within the switch statement. The expression must be either integer-valued or character-valued. It cannot be a String or a real number. The positions that you can jump to are marked with "case labels" that take the form: "case constant:". This marks the position the computer jumps to when the expression evaluates to the given constant. As the final case in a switch statement you can, optionally, use the label "default:", which provides a default jump point that is used when the value of the expression is not listed in any case label. A switch statement has the form: switch (expression) { case constant-1: statements-1 break; case constant-2: statements-2 break; . . // (more cases) . case constant-N: statements-N break; default: // optional default case statements-(N+1) } // end of switch statement The break statements are technically optional. The effect of a break is to make the computer jump to the end of the switch statement. If you leave out the break statement, the computer will just forge ahead after completing one case and will execute the statements associated with the next case label. This is rarely what you want, but it is legal. (I will note here -- although you won't understand it until you get to the next chapter -- that inside a subroutine, the break statement is sometimes replaced by a return statement.) Note that you can leave out one of the groups of statements entirely (including the break). You then have two case labels in a row, containing two different constants. This just means that the computer will jump to the same place and perform the same action for each of the two constants. Here is an example of a switch statement. This is not a useful example, but it should be easy for you to follow. Note, by the way, that the constants in the case labels don't have to be in any particular order, as long as they are all different: switch (N) { // assume N is an integer variable case 1: System.out.println("The number is 1."); break; case 2: http://math.hws.edu/eck/cs124/javanotes4/c3/s6.html (1 of 4) [6/11/2004 11:08:55 AM] Java Programming: Section 3.6 case 4: case 8: System.out.println("The number is 2, 4, or 8."); System.out.println("(That's a power of 2!)"); break; case 3: case 6: case 9: System.out.println("The number is 3, 6, or 9."); System.out.println("(That's a multiple of 3!)"); break; case 5: System.out.println("The number is 5."); break; default: System.out.println("The number is 7,"); System.out.println(" or is outside the range 1 to 9."); } The switch statement is pretty primitive as control structures go, and it's easy to make mistakes when you use it. Java takes all its control structures directly from the older programming languages C and C++. The switch statement is certainly one place where the designers of Java should have introduced some improvements. One application of switch statements is in processing menus. A menu is a list of options. The user selects one of the options. The computer has to respond to each possible choice in a different way. If the options are numbered 1, 2, ..., then the number of the chosen option can be used in a switch statement to select the proper response. In a TextIO-based program, the menu can be presented as a numbered list of options, and the user can choose an option by typing in its number. Here is an example that could be used in a variation of the LengthConverter example from the previous section: int optionNumber; // Option number from menu, selected by user. double measurement; // A numerical measurement, input by the user. // The unit of measurement depends on which // option the user has selected. double inches; // The same measurement, converted into inches. /* Display menu and get user's selected option number. */ TextIO.putln("What unit of measurement does your input use?"); TextIO.putln(); TextIO.putln(" 1. inches"); TextIO.putln(" 2. feet"); TextIO.putln(" 3. yards"); TextIO.putln(" 4. miles"); TextIO.putln(); TextIO.putln("Enter the number of your choice: "); optionNumber = TextIO.getlnInt(); /* Read user's measurement and convert to inches. */ switch ( optionNumber ) { case 1: http://math.hws.edu/eck/cs124/javanotes4/c3/s6.html (2 of 4) [6/11/2004 11:08:55 AM] Java Programming: Section 3.6 TextIO.putln("Enter the number of inches: ); measurement = TextIO.getlnDouble(); inches = measurement; break; case 2: TextIO.putln("Enter the number of feet: ); measurement = TextIO.getlnDouble(); inches = measurement * 12; break; case 3: TextIO.putln("Enter the number of yards: ); measurement = TextIO.getlnDouble(); inches = measurement * 36; break; case 4: TextIO.putln("Enter the number of miles: ); measurement = TextIO.getlnDouble(); inches = measurement * 12 * 5280; break; default: TextIO.putln("Error! Illegal option number! I quit!"); System.exit(1); } // end switch /* Now go on to convert inches to feet, yards, and miles... */ The Empty Statement As a final note in this section, I will mention one more type of statement in Java: the empty statement. This is a statement that consists simply of a semicolon. The existence of the empty statement makes the following legal, even though you would not ordinarily see a semicolon after a }. if (x < 0) { x = -x; }; The semicolon is legal after the }, but the computer considers it to be an empty statement, not part of the if statement. Occasionally, you might find yourself using the empty statement when what you mean is, in fact, "do nothing". I prefer, though, to use an empty block, consisting of { and } with nothing between, for such cases. Occasionally, stray empty statements can cause annoying, hard-to-find errors in a program. For example, the following program segment prints out "Hello" just once, not ten times: for (int i = 0; i < 10; i++); System.out.println("Hello"); Why? Because the ";" at the end of the first line is a statement, and it is this statement that is executed ten times. The System.out.println statement is not really inside the for statement at all, so it is executed just once, after the for loop has completed. http://math.hws.edu/eck/cs124/javanotes4/c3/s6.html (3 of 4) [6/11/2004 11:08:55 AM] Java Programming: Section 3.6 A List of Java Statement Types I mention the empty statement here mainly for completeness. You've now seen just about every type of Java statement. A complete list is given below for reference. The only new items in the list are the try..catch, throw, and synchronized statements, which are related to advanced aspects of Java known as exception-handling and multi-threading, and the return statement, which is used in subroutines. These will be covered in later sections. Another possible surprise is what I've listed as "other expression statement," which reflects the fact that any expression followed by a semicolon can be used as a statement. To execute such a statement, the computer simply evaluates the expression, and then ignores the value. Of course, this only makes sense when the evaluation has a side effect that makes some change in the state of the computer. An example of this is the expression statement "x++;", which has the side effect of adding 1 to the value of x. Similarly, the function call "TextIO.getln()", which reads a line of input, can be used as a stand-alone statement if you want to read a line of input and discard it. Note that, technically, assignment statements and subroutine call statements are also considered to be expression statements. Java statement types: ● declaration statement (for declaring variables) ● assignment statement ● subroutine call statement (including input/output routines) ● other expression statement (such as "x++;") ● empty statement ● block statement ● while statement ● do..while statement ● if statement ● for statement ● switch statement ● break statement (found in loops and switch statements only) ● continue statement (found in loops only) ● return statement (found in subroutine definitions only) ● try..catch statement ● throw statement ● synchronized statement [ Next Section | Previous Section | Chapter Index | Main Index ] http://math.hws.edu/eck/cs124/javanotes4/c3/s6.html (4 of 4) [6/11/2004 11:08:55 AM] Java Programming: Section 3.7 Section 3.7 Introduction to Applets and Graphics FOR THE PAST TWO CHAPTERS, you've been learning the sort of programming that is done inside a single subroutine. In the rest of the text, we'll be more concerned with the larger scale structure of programs, but the material that you've already learned will be an important foundation for everything to come. In this section, before moving on to programming-in-the-large, we'll take a look at how programming-in-the-small can be used in other contexts besides text-based, command-line-style programs. We'll do this by taking a short, introductory look at applets and graphical programming. An applet is a Java program that runs on a Web page. An applet is not a stand-alone application, and it does not have a main() routine. In fact, an applet is an object rather than a class. When an applet is placed on a Web page, it is assigned a rectangular area on the page. It is the job of the applet to draw the contents of that rectangle. When the region needs to be drawn, the Web page calls a subroutine in the applet to do so. This is not so different from what happens with stand-alone programs. When a program needs to be run, the system calls the main() routine of the program. Similarly, when an applet needs to be drawn, the Web page calls the paint() routine of the applet. The programmer specifies what happens when these routines are called by filling in the bodies of the routines. Programming in the small! Applets can do other things besides draw themselves, such as responding when the user clicks the mouse on the applet. Each of the applet's behaviors is defined by a subroutine in the applet object. The programmer specifies how the applet behaves by filling in the bodies of the appropriate subroutines. A very simple applet, which does nothing but draw itself, can be defined by a class that contains nothing but a paint() routine. The source code for the class would have the form: import java.awt.*; import java.applet.*; public class name-of-applet extends Applet { public void paint(Graphics g) { statements } } where name-of-applet is an identifier that names the class, and the statements are the code that actually draws the applet. This looks similar to the definition of a stand-alone program, but there are a few things here that need to be explained, starting with the first two lines. When you write a program, there are certain built-in classes that are available for you to use. These built-in classes include System and Math. If you want to use one of these classes, you don't have to do anything special. You just go ahead and use it. But Java also has a large number of standard classes that are there if you want them but that are not automatically available to your program. (There are just too many of them.) If you want to use these classes in your program, you have to ask for them first. The standard classes are grouped into so-called "packages." Two of these packages are called "java.awt" and "java.applet". The directive "import java.awt.*;" makes all the classes from the package java.awt available for use in your program. The java.awt package contains classes related to graphical user interface programming, including a class called Graphics. The Graphics class is referred to in the paint() routine above. The java.applet package contains classes specifically related to applets, including the class named Applet. The first line of the class definition above says that the class "extends Applet." Applet is a standard http://math.hws.edu/eck/cs124/javanotes4/c3/s7.html (1 of 7) [6/11/2004 11:08:56 AM] Java Programming: Section 3.7 class that is defined in the java.applet package. It defines all the basic properties and behaviors of applet objects. By extending the Applet class, the new class we are defining inherits all those properties and behaviors. We only have to define the ways in which our class differs from the basic Applet class. In our case, the only difference is that our applet will draw itself differently, so we only have to define the paint() routine. This is one of the main advantages of object-oriented programming. (Actually, most of our applets will be defined to extend JApplet rather than Applet. The JApplet class is itself an extension of Applet. The Applet class has existed since the original version of Java, while JApplet is part of the newer "Swing" set of graphical user interface components. For the moment, the distinction is not important.) One more thing needs to be mentioned -- and this is a point where Java's syntax gets unfortunately confusing. Applets are objects, not classes. Instead of being static members of a class, the subroutines that define the applet's behavior are part of the applet object. We say that they are "non-static" subroutines. Of course, objects are related to classes because every object is described by a class. Now here is the part that can get confusing: Even though a non-static subroutine is not actually part of a class (in the sense of being part of the behavior of the class), it is nevertheless defined in a class (in the sense that the Java code that defines the subroutine is part of the Java code that defines the class). Many objects can be described by the same class. Each object has its own non-static subroutine. But the common definition of those subroutines -- the actual Java source code -- is physically part of the class that describes all the objects. To put it briefly: static subroutines in a class definition say what the class does; non-static subroutines say what all the objects described by the class do. An applet's paint() routine is an example of a non-static subroutine. A stand-alone program's main() routine is an example of a static subroutine. The distinction doesn't really matter too much at this point: When working with stand-alone programs, mark everything with the reserved word, "static"; leave it out when working with applets. However, the distinction between static and non-static will become more important later in the course. Let's write an applet that draws something. In order to write an applet that draws something, you need to know what subroutines are available for drawing, just as in writing text-oriented programs you need to know what subroutines are available for reading and writing text. In Java, the built-in drawing subroutines are found in objects of the class Graphics, one of the classes in the java.awt package. In an applet's paint() routine, you can use the Graphics object g for drawing. (This object is provided as a parameter to the paint() routine when that routine is called.) Graphics objects contain many subroutines. I'll mention just three of them here. You'll find more listed in Section 6.3. g.setColor(c), is called to set the color that is used for drawing. The parameter, c is an object belonging to a class named Color, another one of the classes in the java.awt package. About a dozen standard colors are available as static member variables in the Color class. These standard colors include Color.black, Color.white, Color.red, Color.green, and Color.blue. For example, if you want to draw in red, you would say "g.setColor(Color.red);". The specified color is used for all drawing operations up until the next time setColor is called. g.drawRect(x,y,w,h) draws the outline of a rectangle. The parameters x, y, w, and h must be integers. This draws the outline of the rectangle whose top-left corner is x pixels from the left edge of the applet and y pixels down from the top of the applet. The width of the rectangle is w pixels, and the height is h pixels. g.fillRect(x,y,w,h) is similar to drawRect except that it fills in the inside of the rectangle instead of just drawing an outline. This is enough information to write the applet shown here: Sorry, Java is not available. http://math.hws.edu/eck/cs124/javanotes4/c3/s7.html (2 of 7) [6/11/2004 11:08:56 AM] Java Programming: Section 3.7 But here's the picture that the applet draws: This applet first fills its entire rectangular area with red. Then it changes the drawing color to black and draws a sequence of rectangles, where each rectangle is nested inside the previous one. The rectangles can be drawn with a while loop. Each time through the loop, the rectangle gets smaller and it moves down and over a bit. We'll need variables to hold the width and height of the rectangle and a variable to record how far the top-left corner of the rectangle is inset from the edges of the applet. The while loop ends when the rectangle shrinks to nothing. In general outline, the algorithm for drawing the applet is Set the drawing color to red (using the g.setColor subroutine) Fill in the entire applet (using the g.fillRect subroutine) Set the drawing color to black Set the top-left corner inset to be 0 Set the rectangle width and height to be as big as the applet while the width and height are greater than zero: draw a rectangle (using the g.drawRect subroutine) increase the inset decrease the width and the height In my applet, each rectangle is 15 pixels away from the rectangle that surrounds it, so the inset is increased by 15 each time through the while loop. The rectangle shrinks by 15 pixels on the left and by 15 pixels on the right, so the width of the rectangle shrinks by 30 each time through the loop. The height also shrinks by 30 pixels each time through the loop. It is not hard to code this algorithm into Java and use it to define the paint() method of an applet. I've assumed that the applet has a height of 160 pixels and a width of 300 pixels. The size is actually set in the source code of the Web page where the applet appears. In order for an applet to appear on a page, the source code for the page must include a command that specifies which applet to run and how big it should be. (The commands that can be used on a Web page are discussed in Section 6.2.) It's not a great idea to assume that we know how big the applet is going to be. On the other hand, it's also not a great idea to write an applet that does nothing but draw a static picture. I'll address both these issues before the end of this section. But for now, here is the source code for the applet: import java.awt.*; import java.applet.Applet; public class StaticRects extends Applet { public void paint(Graphics g) { // Draw a set of nested black rectangles on a red background. // Each nested rectangle is separated by 15 pixels on // all sides from the rectangle that encloses it. http://math.hws.edu/eck/cs124/javanotes4/c3/s7.html (3 of 7) [6/11/2004 11:08:56 AM] Java Programming: Section 3.7 int inset; // Gap between borders of applet // and one of the rectangles. int rectWidth, rectHeight; // The size of one of the rectangles. g.setColor(Color.red); g.fillRect(0,0,300,160); // Fill the entire applet with red. g.setColor(Color.black); // Draw the rectangles in black. inset = 0; rectWidth = 299; // Set size of first rect to size of applet. rectHeight = 159; while (rectWidth >= 0 && rectHeight >= 0) { g.drawRect(inset, inset, rectWidth, rectHeight); inset += 15; // Rects are 15 pixels apart. rectWidth -= 30; // Width decreases by 15 pixels // on left and 15 on right. rectHeight -= 30; // Height decreases by 15 pixels // on top and 15 on bottom. } } // end paint() } // end class StaticRects (You might wonder why the initial rectWidth is set to 299, instead of to 300, since the width of the applet is 300 pixels. It's because rectangles are drawn as if with a pen whose nib hangs below and to the right of the point where the pen is placed. If you run the pen exactly along the right edge of the applet, the line it draws is actually outside the applet and therefore is not seen. So instead, we run the pen along a line one pixel to the left of the edge of the applet. The same reasoning applies to rectHeight. Careful graphics programming demands attention to details like these.) When you write an applet, you get to build on the work of the people who wrote the Applet class. The Applet class provides a framework on which you can hang your own work. Any programmer can create additional frameworks that can be used by other programmers as a basis for writing specific types of applets or stand-alone programs. One example is the applets in previous sections that simulate text-based programs. All these applets are based on a class called ConsoleApplet, which itself is based on the standard Applet class. You can write your own console applet by filling in this simple framework (which leaves out just a couple of bells and whistles): public class name-of-applet extends ConsoleApplet { public void program() { statements } } The statements in the program() subroutine are executed when the user of the applet clicks the applet's "Run Program" button. This "program" can't use TextIO or System.out to do input and output. However, the ConsoleApplet framework provides an object named console for doing text http://math.hws.edu/eck/cs124/javanotes4/c3/s7.html (4 of 7) [6/11/2004 11:08:56 AM] Java Programming: Section 3.7 input/output. This object contains exactly the same set of subroutines as the TextIO class. For example, where you would say TextIO.putln("Hello World") in a stand-alone program, you could say console.putln("Hello World") in a console applet. The console object just displays the output on the applet instead of on standard output. Similarly, you can substitute x = console.getInt() for x = TextIO.getInt(), and so on. As a simple example, here's a console applet that gets two numbers from the user and prints their product: public class PrintProduct extends ConsoleApplet { public void program() { double x,y; // Numbers input by the user. double prod; // The product, x*y. console.put("What is your first number? "); x = console.getlnDouble(); console.put("What is your second number? "); y = console.getlnDouble(); prod = x * y; console.putln(); console.put("The product is "); console.putln(prod); } // end program() } // end class PrintProduct And here's what this applet looks like on a Web page: (Applet "PrintProduct" would be displayed here if Java were available.) Now, any console-style applet that you write depends on the ConsoleApplet class, which is not a standard part of Java. This means that the compiled class file, ConsoleApplet.class must be available to your applet when it is run. As a matter of fact, ConsoleApplet uses two other non-standard classes, ConsolePanel and ConsoleCanvas, so the compiled class files ConsolePanel.class and ConsoleCanvas.class must also be available to your applet. This just means that all four class files -- your own class and the three classes it depends on -- must be in the same directory with the source code for the Web page on which your applet appears. I've written another framework that makes it possible to write applets that display simple animations. An example is given by the applet at the bottom of this page, which is an animated version of the nested squares applet from earlier in this section. A computer animation is really just a sequence of still images. The computer displays the images one after the other. Each image differs a bit from the preceding image in the sequence. If the differences are not too big and if the sequence is displayed quickly enough, the eye is tricked into perceiving continuous motion. In the example, rectangles shrink continually towards the center of the applet, while new rectangles appear at the edge. The perpetual motion is, of course, an illusion. If you think about it, you'll see that the applet loops through the same set of images over and over. In each image, there is a gap between the borders of the applet and the outermost rectangle. This gap gets wider and wider until a new rectangle appears at the border. Only it's not a new rectangle. What has really happened is that the applet has started over again with the first image in the sequence. http://math.hws.edu/eck/cs124/javanotes4/c3/s7.html (5 of 7) [6/11/2004 11:08:56 AM] Java Programming: Section 3.7 The problem of creating an animation is really just the problem of drawing each of the still images that make up the animation. Each still image is called a frame. In my framework for animation, which is based on a non-standard class called SimpleAnimationApplet2, all you have to do is fill in the code that says how to draw one frame. The basic format is as follows: import java.awt.*; public class name-of-class extends SimpleAnimationApplet2 { public void drawFrame(Graphics g) { statements // to draw one frame of the animation } } The "import java.awt.*;" is required to get access to graphics-related classes such as Graphics and Color. You get to fill in any name you want for the class, and you get to fill in the statements inside the subroutine. The drawFrame() subroutine will be called by the system each time a frame needs to be drawn. All you have to do is say what happens when this subroutine is called. Of course, you have to draw a different picture for each frame, and to do that you need to know which frame you are drawing. The class SimpleAnimationApplet2 provides a function named getFrameNumber() that you can call to find out which frame to draw. This function returns an integer value that represents the frame number. If the value returned is 0, you are supposed to draw the first frame; if the value is 1, you are supposed to draw the second frame, and so on. In the sample applet, the thing that differs from one frame to another is the distance between the edges of the applet and the outermost rectangle. Since the rectangles are 15 pixels apart, this distance increases from 0 to 14 and then jumps back to 0 when a "new" rectangle appears. The appropriate value can be computed very simply from the frame number, with the statement "inset = getFrameNumber() % 15;". The value of the expression getFrameNumber() % 15 is between 0 and 14. When the frame number reaches 15, the value of getFrameNumber() % 15 jumps back to 0. Drawing one frame in the sample animated applet is very similar to drawing the single image of the StaticRects applet, as given above. The paint() method in the StaticRects applet becomes, with only minor modification, the drawFrame() method of my MovingRects animation applet. I've chosen to make one improvement: The StaticRects applet assumes that the applet is 300 by 160 pixels. The MovingRects applet will work for any applet size. To implement this, the drawFrame() routine has to know how big the applet is. There are two functions that can be called to get this information. The function getWidth() returns an integer value representing the width of the applet, and the function getHeight() returns the height. The width and height, together with the frame number, are used to compute the size of the first rectangle that is drawn. Here is the complete source code: import java.awt.*; public class MovingRects extends SimpleAnimationApplet2 { public void drawFrame(Graphics g) { // Draw one frame in the animation by filling in the background // with a solid red and then drawing a set of nested black // rectangles. The frame number tells how much the first // rectangle is to be inset from the borders of the applet. int width; // Width of the applet, in pixels. int height; // Height of the applet, in pixels. http://math.hws.edu/eck/cs124/javanotes4/c3/s7.html (6 of 7) [6/11/2004 11:08:56 AM] Java Programming: Section 3.7 int inset; // Gap between borders of applet and a rectangle. // The inset for the outermost rectangle goes // from 0 to 14 then back to 0, and so on, // as the frameNumber varies. int rectWidth, rectHeight; // The size of one of the rectangles. width = getWidth(); // Find out the size of the drawing area. height = getHeight(); g.setColor(Color.red); // Fill the frame with red. g.fillRect(0,0,width,height); g.setColor(Color.black); // Switch color to black. inset = getFrameNumber() % 15; // Get the inset for the // outermost rect. rectWidth = width - 2*inset - 1; // Set size of outermost rect. rectHeight = height - 2*inset - 1; while (rectWidth >= 0 && rectHeight >= 0) { g.drawRect(inset,inset,rectWidth,rectHeight); inset += 15; // Rects are 15 pixels apart. rectWidth -= 30; // Width decreases by 15 pixels // on left and 15 on right. rectHeight -= 30; // Height decreases by 15 pixels // on top and 15 on bottom. } } // end drawFrame() } // end class MovingRects (The SimpleAnimationApplet2 class uses Swing and requires Java version 1.3 or better. There is an older version named SimpleAnimationApplet that provides the same functionality but works with any version of Java. You could use SimpleAnimationApplet to write animations that will work on older Web browsers.) The main point here is that by building on an existing framework, you can do interesting things using the type of local, inside-a-subroutine programming that was covered in Chapters 2 and 3. As you learn more about programming and more about Java, you'll be able to do more on your own -- but no matter how much you learn, you'll always be dependent on other people's work to some extent. End of Chapter 3 [ Next Chapter | Previous Section | Chapter Index | Main Index ] http://math.hws.edu/eck/cs124/javanotes4/c3/s7.html (7 of 7) [6/11/2004 11:08:56 AM] Java Programming: Chapter 3 Exercises Programming Exercises For Chapter 3 THIS PAGE CONTAINS programming exercises based on material from Chapter 3 of this on-line Java textbook. Each exercise has a link to a discussion of one possible solution of that exercise. Exercise 3.1: How many times do you have to roll a pair of dice before they come up snake eyes? You could do the experiment by rolling the dice by hand. Write a computer program that simulates the experiment. The program should report the number of rolls that it makes before the dice come up snake eyes. (Note: "Snake eyes" means that both dice show a value of 1.) Exercise 2.2 explained how to simulate rolling a pair of dice. See the solution! Exercise 3.2: Which integer between 1 and 10000 has the largest number of divisors, and how many divisors does it have? Write a program to find the answers and print out the results. It is possible that several integers in this range have the same, maximum number of divisors. Your program only has to print out one of them. One of the examples from Section 3.4 discussed divisors. The source code for that example is CountDivisors.java. You might need some hints about how to find a maximum value. The basic idea is to go through all the integers, keeping track of the largest number of divisors that you've seen so far. Also, keep track of the integer that had that number of divisors. See the solution! Exercise 3.3: Write a program that will evaluate simple expressions such as 17 + 3 and 3.14159 * 4.7. The expressions are to be typed in by the user. The input always consist of a number, followed by an operator, followed by another number. The operators that are allowed are +, -, *, and /. You can read the numbers with TextIO.getDouble() and the operator with TextIO.getChar(). Your program should read an expression, print its value, read another expression, print its value, and so on. The program should end when the user enters 0 as the first number on the line. See the solution! Exercise 3.4: Write a program that reads one line of input text and breaks it up into words. The words should be output one per line. A word is defined to be a sequence of letters. Any characters in the input that are not letters should be discarded. For example, if the user inputs the line He said, "That's not a good idea." then the output of the program should be He said that s not a http://math.hws.edu/eck/cs124/javanotes4/c3/exercises.html (1 of 3) [6/11/2004 11:08:56 AM] Java Programming: Chapter 3 Exercises good idea (An improved version of the program would list "that's" as a word. An apostrophe can be considered to be part of a word if there is a letter on each side of the apostrophe. But that's not part of the assignment.) To test whether a character is a letter, you might use (ch >= 'a' && ch <= 'z') || (ch >= 'A' && ch <= 'Z'). However, this only works in English and similar languages. A better choice is to call the standard function Character.isLetter(ch), which returns a boolean value of true if ch is a letter and false if it is not. This works for any Unicode character. For example, it counts an accented e, é, as a letter. See the solution! Exercise 3.5: Write an applet that draws a checkerboard. Assume that the size of the applet is 160 by 160 pixels. Each square in the checkerboard is 20 by 20 pixels. The checkerboard contains 8 rows of squares and 8 columns. The squares are red and black. Here is a tricky way to determine whether a given square is red or black: If the row number and the column number are either both even or both odd, then the square is red. Otherwise, it is black. Note that a square is just a rectangle in which the height is equal to the width, so you can use the subroutine g.fillRect() to draw the squares. Here is an image of the checkerboard: (To run an applet, you need a Web page to display it. A very simple page will do. Assume that your applet class is called Checkerboard, so that when you compile it you get a class file named Checkerboard.class Make a file that contains only the lines: <applet code="Checkerboard.class" width=160 height=160> </applet> Call this file Checkerboard.html. This is the source code for a simple Web page that shows nothing but your applet. You can open the file in a Web browser or with Sun's appletviewer program. The compiled class file, Checkerboard.class, must be in the same directory with the Web-page file, Checkerboard.html.) See the solution! Exercise 3.6: Write an animation applet that shows a checkerboard pattern in which the even numbered rows slide to the left while the odd numbered rows slide to the right. You can assume that the applet is 160 by 160 pixels. Each row should be offset from its usual position by the amount getFrameNumber() % 40. Hints: Anything you draw outside the boundaries of the applet will be invisible, so you can draw more than 8 squares in a row. You can use negative values of x in g.fillRect(x,y,w,h). Here is a working solution to this exercise: http://math.hws.edu/eck/cs124/javanotes4/c3/exercises.html (2 of 3) [6/11/2004 11:08:56 AM] Java Programming: Chapter 3 Exercises Your applet will extend the non-standard class, SimpleAnimationApplet2, which was introduced in Section 7. When you run your applet, the compiled class files, SimpleAnimationApplet2.class and SimpleAnimationApplet2$1.class, must be in the same directory as your Web-page source file and the compiled class file for your own class. These files are produced when you compile SimpleAnimationApplet2.java. Assuming that the name of your class is SlidingCheckerboard, then the source file for the Web page should contain the lines: <applet code="SlidingCheckerboard.class" width=160 height=160> </applet> See the solution! [ Chapter Index | Main Index ] http://math.hws.edu/eck/cs124/javanotes4/c3/exercises.html (3 of 3) [6/11/2004 11:08:56 AM] Java Programming: Chapter 3 Quiz Quiz Questions For Chapter 3 THIS PAGE CONTAINS A SAMPLE quiz on material from Chapter 3 of this on-line Java textbook. You should be able to answer these questions after studying that chapter. Sample answers to all the quiz questions can be found here. Question 1: Explain briefly what is meant by "pseudocode" and how is it useful in the development of algorithms. Question 2: What is a block statement? How are block statements used in Java programs. Question 3: What is the main difference between a while loop and a do..while loop? Question 4: What does it mean to prime a loop? Question 5: Explain what is meant by an animation and how a computer displays an animation. Question 6: Write a for loop that will print out all the multiples of 3 from 3 to 36, that is: 3 6 9 12 15 18 21 24 27 30 33 36. Question 7: Fill in the following main() routine so that it will ask the user to enter an integer, read the user's response, and tell the user whether the number entered is even or odd. (You can use TextIO.getInt() to read the integer. Recall that an integer n is even if n % 2 == 0.) public static void main(String[] args) { // Fill in the body of this subroutine! } Question 8: Show the exact output that would be produced by the following main() routine: public static void main(String[] args) { int N; N = 1; while (N <= 32) { N = 2 * N; System.out.println(N); } } Question 9: Show the exact output produced by the following main() routine: public static void main(String[] args) { int x,y; x = 5; y = 1; while (x > 0) { x = x - 1; y = y * x; System.out.println(y); http://math.hws.edu/eck/cs124/javanotes4/c3/quiz.html (1 of 2) [6/11/2004 11:08:57 AM] Java Programming: Chapter 3 Quiz } } Question 10: What output is produced by the following program segment? Why? (Recall that name.charAt(i) is the i-th character in the string, name.) String name; int i; boolean startWord; name = "Richard M. Nixon"; startWord = true; for (i = 0; i < name.length(); i++) { if (startWord) System.out.println(name.charAt(i)); if (name.charAt(i) == ' ') startWord = true; else startWord = false; } [ Answers | Chapter Index | Main Index ] http://math.hws.edu/eck/cs124/javanotes4/c3/quiz.html (2 of 2) [6/11/2004 11:08:57 AM] Java Programming: Chapter 4 Index Chapter 4 Programming in the Large I Subroutines ONE WAY TO BREAK UP A COMPLEX PROGRAM into manageable pieces is to use subroutines. A subroutine consists of the instructions for carrying out a certain task, grouped together and given a name. Elsewhere in the program, that name can be used as a stand-in for the whole set of instructions. As a computer executes a program, whenever it encounters a subroutine name, it executes all the instructions necessary to carry out the task associated with that subroutine. Subroutines can be used over and over, at different places in the program. A subroutine can even be used inside another subroutine. This allows you to write simple subroutines and then use them to help write more complex subroutines, which can then be used in turn in other subroutines. In this way, very complex programs can be built up step-by-step, where each step in the construction is reasonably simple. As mentioned in Section 3.7, subroutines in Java can be either static or non-static. This chapter covers static subroutines only. Non-static subroutines, which are used in true object-oriented programming, will be covered in the next chapter. Contents of Chapter 4: ● Section 1: Black Boxes ● Section 2: Static Subroutines and Static Variables ● Section 3: Parameters ● Section 4: Return Values ● Section 5: Toolboxes, API's, and Packages ● Section 6: More on Program Design ● Section 7: The Truth about Declarations ● Programming Exercises ● Quiz on this Chapter [ First Section | Next Chapter | Previous Chapter | Main Index ] http://math.hws.edu/eck/cs124/javanotes4/c4/ [6/11/2004 11:09:17 AM] Java Programming: Section 4.1 Section 4.1 Black Boxes A SUBROUTINE CONSISTS OF INSTRUCTIONS for performing some task, chunked together and given a name. "Chunking" allows you to deal with a potentially very complicated task as a single concept. Instead of worrying about the many, many steps that the computer might have to go though to perform that task, you just need to remember the name of the subroutine. Whenever you want your program to perform the task, you just call the subroutine. Subroutines are a major tool for dealing with complexity. A subroutine is sometimes said to be a "black box" because you can't see what's "inside" it (or, to be more precise, you usually don't want to see inside it, because then you would have to deal with all the complexity that the subroutine is meant to hide). Of course, a black box that has no way of interacting with the rest of the world would be pretty useless. A black box needs some kind of interface with the rest of the world, which allows some interaction between what's inside the box and what's outside. A physical black box might have buttons on the outside that you can push, dials that you can set, and slots that can be used for passing information back and forth. Since we are trying to hide complexity, not create it, we have the first rule of black boxes: The interface of a black box should be fairly straightforward, well-defined, and easy to understand. Are there any examples of black boxes in the real world? Yes; in fact, you are surrounded by them. Your television, your car, your VCR, your refrigerator... You can turn your television on and off, change channels, and set the volume by using elements of the television's interface -- dials, remote control, don't forget to plug in the power -- without understanding anything about how the thing actually works. The same goes for a VCR, although if stories about how hard people find it to set the time on a VCR are true, maybe the VCR violates the simple interface rule. Now, a black box does have an inside -- the code in a subroutine that actually performs the task, all the electronics inside your television set. The inside of a black box is called its implementation. The second rule of black boxes is that To use a black box, you shouldn't need to know anything about its implementation; all you need to know is its interface. In fact, it should be possible to change the implementation, as long as the behavior of the box, as seen from the outside, remains unchanged. For example, when the insides of TV sets went from using vacuum tubes to using transistors, the users of the sets didn't even need to know about it -- or even know what it means. Similarly, it should be possible to rewrite the inside of a subroutine, to use more efficient code, for example, without affecting the programs that use that subroutine. Of course, to have a black box, someone must have designed and built the implementation in the first place. The black box idea works to the advantage of the implementor as well as of the user of the black box. After all, the black box might be used in an unlimited number of different situations. The implementor of the black box doesn't need to know about any of that. The implementor just needs to make sure that the box performs its assigned task and interfaces correctly with the rest of the world. This is the third rule of black boxes: The implementor of a black box should not need to know anything about the larger systems in which the box will be used. In a way, a black box divides the world into two parts: the inside (implementation) and the outside. The interface is at the boundary, connecting those two parts. http://math.hws.edu/eck/cs124/javanotes4/c4/s1.html (1 of 2) [6/11/2004 11:09:18 AM] Java Programming: Section 4.1 By the way, you should not think of an interface as just the physical connection between the box and the rest of the world. The interface also includes a specification of what the box does and how it can be controlled by using the elements of the physical interface. It's not enough to say that a TV set has a power switch; you need to specify that the power switch is used to turn the TV on and off! To put this in computer science terms, the interface of a subroutine has a semantic as well as a syntactic component. The syntactic part of the interface tells you just what you have to type in order to call the subroutine. The semantic component specifies exactly what task the subroutine will accomplish. To write a legal program, you need to know the syntactic specification of the subroutine. To understand the purpose of the subroutine and to use it effectively, you need to know the subroutine's semantic specification. I will refer to both parts of the interface -- syntactic and semantic -- collectively as the contract of the subroutine. The contract of a subroutine says, essentially, "Here is what you have to do to use me, and here is what I will do for you, guaranteed." When you write a subroutine, the comments that you write for the subroutine should make the contract very clear. (I should admit that in practice, subroutines' contracts are often inadequately specified, much to the regret and annoyance of the programmers who have to use them.) For the rest of this chapter, I turn from general ideas about black boxes and subroutines in general to the specifics of writing and using subroutines in Java. But keep the general ideas and principles in mind. They are the reasons that subroutines exist in the first place, and they are your guidelines for using them. This should be especially clear in Section 6, where I will discuss subroutines as a tool in program development. You should keep in mind that subroutines are not the only example of black boxes in programming. For example, a class is also a black box. We'll see that a class can have a "public" part, representing its interface, and a "private" part that is entirely inside its hidden implementation. All the principles of black boxes apply to classes as well as to subroutines. [ Next Section | Previous Chapter | Chapter Index | Main Index ] http://math.hws.edu/eck/cs124/javanotes4/c4/s1.html (2 of 2) [6/11/2004 11:09:18 AM] Java Programming: Section 4.2 Section 4.2 Static Subroutines and Static Variables EVERY SUBROUTINE IN JAVA MUST BE DEFINED inside some class. This makes Java rather unusual among programming languages, since most languages allow free-floating, independent subroutines. One purpose of a class is to group together related subroutines and variables. Perhaps the designers of Java felt that everything must be related to something. As a less philosophical motivation, Java's designers wanted to place firm controls on the ways things are named, since a Java program potentially has access to a huge number of subroutines scattered all over the Internet. The fact that those subroutines are grouped into named classes (and classes are grouped into named "packages") helps control the confusion that might result from so many different names. A subroutine that is a member of a class is often called a method, and "method" is the term that most people prefer for subroutines in Java. I will start using the term "method" occasionally; however, I will continue to prefer the term "subroutine" for static subroutines. I will use the term "method" most often to refer to non-static subroutines, which belong to objects rather than to classes. This chapter will deal with static subroutines almost exclusively. We'll turn to non-static methods and object-oriented programming in the next chapter. A subroutine definition in Java takes the form: modifiers return-type subroutine-name ( parameter-list ) { statements } It will take us a while -- most of the chapter -- to get through what all this means in detail. Of course, you've already seen examples of subroutines in previous chapters, such as the main() routine of a program and the paint() routine of an applet. So you are familiar with the general format. The statements between the braces, { and }, make up the body of the subroutine. These statements are the inside, or implementation part, of the "black box", as discussed in the previous section. They are the instructions that the computer executes when the method is called. Subroutines can contain any of the statements discussed in Chapter 2 and Chapter 3. The modifiers that can occur at the beginning of a subroutine definition are words that set certain characteristics of the method, such as whether it is static or not. The modifiers that you've seen so far are "static" and "public". There are only about a half-dozen possible modifiers altogether. If the subroutine is a function, whose job is to compute some value, then the return-type is used to specify the type of value that is returned by the function. We'll be looking at functions and return types in some detail in Section 4. If the subroutine is not a function, then the return-type is replaced by the special value void, which indicates that no value is returned. The term "void" is meant to indicate that the return value is empty or non-existent. Finally, we come to the parameter-list of the method. Parameters are part of the interface of a subroutine. They represent information that is passed into the subroutine from outside, to be used by the subroutine's internal computations. For a concrete example, imagine a class named Television that includes a method named changeChannel(). The immediate question is: What channel should it change to? A parameter can be used to answer this question. Since the channel number is an integer, the type of the parameter would be int, and the declaration of the changeChannel() method might look like public void changeChannel(int channelNum) {...} http://math.hws.edu/eck/cs124/javanotes4/c4/s2.html (1 of 8) [6/11/2004 11:09:19 AM] Java Programming: Section 4.2 This declaration specifies that changeChannel() has a parameter named channelNum of type int. However, channelNum does not yet have any particular value. A value for channelNum is provided when the subroutine is called; for example: changeChannel(17); The parameter list in a subroutine can be empty, or it can consist of one or more parameter declarations of the form type parameter-name. If there are several declarations, they are separated by commas. Note that each declaration can name only one parameter. For example, if you want two parameters of type double, you have to say "double x, double y", rather than "double x, y". Parameters are covered in more detail in the next section. Here are a few examples of subroutine definitions, leaving out the statements that define what the subroutines do: public static void playGame() { // "public" and "static" are modifiers; "void" is the // return-type; "playGame" is the subroutine-name; // the parameter-list is empty . . . // statements that define what playGame does go here } int getNextN(int N) { // there are no modifiers; "int" in the return-type // "getNextN" is the subroutine-name; the parameter-list // includes one parameter whose name is "N" and whose // type is "int" . . . // statements that define what getNextN does go here } static boolean lessThan(double x, double y) { // "static" is a modifier; "boolean" is the // return-type; "lessThan" is the subroutine-name; the // parameter-list includes two parameters whose names are // "x" and "y", and the type of each of these parameters // is "double" . . . // statements that define what lessThan does go here } In the second example given here, getNextN, is a non-static method, since its definition does not include the modifier "static" -- and so it's not an example that we should be looking at in this chapter! The other modifier shown in the examples is "public". This modifier indicates that the method can be called from anywhere in a program, even from outside the class where the method is defined. There is another modifier, "private", which indicates that the method can be called only from inside the same class. The modifiers public and private are called access specifiers. If no access specifier is given for a method, then by default, that method can be called from anywhere in the "package" that contains the class, but not from outside that package. (Packages were mentioned in Section 3.7, and you'll learn more about packages in this chapter, in Section 5.) There is one other access modifier, protected, which will only become relevant when we turn to object-oriented programming in Chapter 5. Note, by the way, that the main() routine of a program follows the usual syntax rules for a subroutine. In public static void main(String[] args) { .... } the modifiers are public and static, the return type is void, the subroutine name is main, and the parameter list is "String[] args". The only question might be about "String[]", which has to be a type if it is to match the format of a parameter list. In fact, String[] represents a so-called "array type", so http://math.hws.edu/eck/cs124/javanotes4/c4/s2.html (2 of 8) [6/11/2004 11:09:19 AM] Java Programming: Section 4.2 the syntax is valid. We will cover arrays in Chapter 8. (The parameter, args, represents information provided to the program when the main() routine is called by the system. In case you know the term, the information consists of any "command-line arguments" specified in the command that the user typed to run the program.) You've already had some experience with filling in the statements of a subroutine. In this chapter, you'll learn all about writing your own complete subroutine definitions, including the interface part. When you define a subroutine, all you are doing is telling the computer that the subroutine exists and what it does. The subroutine doesn't actually get executed until it is called. (This is true even for the main() routine in a class -- even though you don't call it, it is called by the system when the system runs your program.) For example, the playGame() method defined above could be called using the following subroutine call statement: playGame(); This statement could occur anywhere in the same class that includes the definition of playGame(), whether in a main() method or in some other subroutine. Since playGame() is a public method, it can also be called from other classes, but in that case, you have to tell the computer which class it comes from. Let's say, for example, that playGame() is defined in a class named Poker. Then to call playGame() from outside the Poker class, you would have to say Poker.playGame(); The use of the class name here tells the computer which class to look in to find the method. It also lets you distinguish between Poker.playGame() and other potential playGame() methods defined in other classes, such as Roulette.playGame() or Blackjack.playGame(). More generally, a subroutine call statement takes the form subroutine-name(parameters); if the subroutine that is being called is in the same class, or class-name.subroutine-name(parameters); if the subroutine is a static subroutine defined elsewhere, in a different class. (Non-static methods belong to objects rather than classes, and they are called using object names instead of class names. More on that later.) Note that the parameter list can be empty, as in the playGame() example, but the parentheses must be there even if there is nothing between them. It's time to give an example of what a complete program looks like, when it includes other subroutines in addition to the main() routine. Let's write a program that plays a guessing game with the user. The computer will choose a random number between 1 and 100, and the user will try to guess it. The computer tells the user whether the guess is high or low or correct. If the user gets the number after six guesses or fewer, the user wins the game. After each game, the user has the option of continuing with another game. Since playing one game can be thought of as a single, coherent task, it makes sense to write a subroutine that will play one guessing game with the user. The main() routine will use a loop to call the playGame() subroutine over and over, as many times as the user wants to play. We approach the problem of designing the playGame() subroutine the same way we write a main() routine: Start with an outline of the algorithm and apply stepwise refinement. Here is a short pseudocode algorithm for a guessing game program: Pick a random number while the game is not over: Get the user's guess http://math.hws.edu/eck/cs124/javanotes4/c4/s2.html (3 of 8) [6/11/2004 11:09:19 AM] Java Programming: Section 4.2 Tell the user whether the guess is high, low, or correct. The test for whether the game is over is complicated, since the game ends if either the user makes a correct guess or the number of guesses is six. As in many cases, the easiest thing to do is to use a "while (true)" loop and use break to end the loop whenever we find a reason to do so. Also, if we are going to end the game after six guesses, we'll have to keep track of the number of guesses that the user has made. Filling out the algorithm gives: Let computersNumber be a random number between 1 and 100 Let guessCount = 0 while (true): Get the user's guess Count the guess by adding 1 to guess count if the user's guess equals computersNumber: Tell the user he won break out of the loop if the number of guesses is 6: Tell the user he lost break out of the loop if the user's guess is less than computersNumber: Tell the user the guess was low else if the user's guess is higher than computersNumber: Tell the user the guess was high With variable declarations added and translated into Java, this becomes the definition of the playGame() routine. A random integer between 1 and 100 can be computed as (int)(100 * Math.random()) + 1. I've cleaned up the interaction with the user to make it flow better. static void playGame() { int computersNumber; // A random number picked by the computer. int usersGuess; // A number entered by user as a guess. int guessCount; // Number of guesses the user has made. computersNumber = (int)(100 * Math.random()) + 1; // The value assigned to computersNumber is a randomly // chosen integer between 1 and 100, inclusive. guessCount = 0; TextIO.putln(); TextIO.put("What is your first guess? "); while (true) { usersGuess = TextIO.getInt(); // get the user's guess guessCount++; if (usersGuess == computersNumber) { TextIO.putln("You got it in " + guessCount + " guesses! My number was " + computersNumber); break; // the game is over; the user has won } if (guessCount == 6) { TextIO.putln("You didn't get the number in 6 guesses."); TextIO.putln("You lose. My number was " + computersNumber); break; // the game is over; the user has lost } // If we get to this point, the game continues. // Tell the user if the guess was too high or too low. if (usersGuess < computersNumber) TextIO.put("That's too low. Try again: "); else if (usersGuess > computersNumber) TextIO.put("That's too high. Try again: "); http://math.hws.edu/eck/cs124/javanotes4/c4/s2.html (4 of 8) [6/11/2004 11:09:19 AM] Java Programming: Section 4.2 } TextIO.putln(); } // end of playGame() Now, where exactly should you put this? It should be part of the same class as the main() routine, but not inside the main routine. It is not legal to have one subroutine physically nested inside another. The main() routine will call playGame(), but not contain it physically. You can put the definition of playGame() either before or after the main() routine. Java is not very picky about having the members of a class in any particular order. It's pretty easy to write the main routine. You've done things like this before. Here's what the complete program looks like (except that a serious program needs more comments than I've included here). public class GuessingGame { public static void main(String[] args) { TextIO.putln("Let's play a game. I'll pick a number between"); TextIO.putln("1 and 100, and you try to guess it."); boolean playAgain; do { playGame(); // call subroutine to play one game TextIO.put("Would you like to play again? "); playAgain = TextIO.getlnBoolean(); } while (playAgain); TextIO.putln("Thanks for playing. Goodbye."); } // end of main() static void playGame() { int computersNumber; // A random number picked by the computer. int usersGuess; // A number entered by user as a guess. int guessCount; // Number of guesses the user has made. computersNumber = (int)(100 * Math.random()) + 1; // The value assigned to computersNumber is a randomly // chosen integer between 1 and 100, inclusive. guessCount = 0; TextIO.putln(); TextIO.put("What is your first guess? "); while (true) { usersGuess = TextIO.getInt(); // get the user's guess guessCount++; if (usersGuess == computersNumber) { TextIO.putln("You got it in " + guessCount + " guesses! My number was " + computersNumber); break; // the game is over; the user has won } if (guessCount == 6) { TextIO.putln("You didn't get the number in 6 guesses."); TextIO.putln("You lose. My number was " + computersNumber); break; // the game is over; the user has lost } // If we get to this point, the game continues. // Tell the user if the guess was too high or too low. if (usersGuess < computersNumber) TextIO.put("That's too low. Try again: "); else if (usersGuess > computersNumber) TextIO.put("That's too high. Try again: "); http://math.hws.edu/eck/cs124/javanotes4/c4/s2.html (5 of 8) [6/11/2004 11:09:19 AM] Java Programming: Section 4.2 } TextIO.putln(); } // end of playGame() } // end of class GuessingGame Take some time to read the program carefully and figure out how it works. And try to convince yourself that even in this relatively simple case, breaking up the program into two methods makes the program easier to understand and probably made it easier to write each piece. You can try out a simulation of this program here: (Applet "GuessingGameConsole" would be displayed here if Java were available.) A class can include other things besides subroutines. In particular, it can also include variable declarations. Of course, you can have variable declarations inside subroutines. Those are called local variables. However, you can also have variables that are not part of any subroutine. To distinguish such variables from local variables, we call them member variables, since they are members of a class. Just as with subroutines, member variables can be either static or non-static. In this chapter, we'll stick to static variables. A static member variable belongs to the class itself, and it exists as long as the class exists. Memory is allocated for the variable when the class is first loaded by the Java interpreter. Any assignment statement that assigns a value to the variable changes the content of that memory, no matter where that assignment statement is located in the program. Any time the variable is used in an expression, the value is fetched from that same memory, no matter where the expression is located in the program. This means that the value of a static member variable can be set in one subroutine and used in another subroutine. Static member variables are "shared" by all the static subroutines in the class. A local variable in a subroutine, on the other hand, exists only while that subroutine is being executed, and is completely inaccessible from outside that one subroutine. The declaration of a member variable looks just like the declaration of a local variable except for two things: The member variable is declared outside any subroutine (although it still has to be inside a class), and the declaration can be marked with modifiers such as static, public, and private. Since we are only working with static member variables for now, every declaration of a member variable in this chapter will include the modifier static. For example: static int numberOfPlayers; static String usersName; static double velocity, time; A static member variable that is not declared to be private can be accessed from outside the class where it is defined, as well as inside. When it is used in some other class, it must be referred to with a compound identifier of the form class-name.variable-name. For example, the System class contains the public static member variable named out, and you use this variable in your own classes by referring to System.out. If numberOfPlayers is a public static member variable in a class named Poker, subroutines in the Poker class would refer to it simply as numberOfPlayers. Subroutines in another class would refer to it as Poker.numberOfPlayers. As an example, let's add a static member variable to the GuessingGame class that we wrote earlier in this section. This variable will be used to keep track of how many games the user wins. We'll call the variable gamesWon and declare it with the statement "static int gamesWon;" In the playGame() routine, we add 1 to gamesWon if the user wins the game. At the end of the main() routine, we print out the value of gamesWon. It would be impossible to do the same thing with a local variable, since we need access to the same variable from both subroutines. http://math.hws.edu/eck/cs124/javanotes4/c4/s2.html (6 of 8) [6/11/2004 11:09:19 AM] Java Programming: Section 4.2 When you declare a local variable in a subroutine, you have to assign a value to that variable before you can do anything with it. Member variables, on the other hand are automatically initialized with a default value. For numeric variables, the default value is zero. For boolean variables, the default is false. And for char variables, it's the unprintable character that has Unicode code number zero. (For objects, such as Strings, the default initial value is a special value called null, which we won't encounter officially until later.) Since it is of type int, the static member variable gamesWon automatically gets assigned an initial value of zero. This happens to be the correct initial value for a variable that is being used as a counter. You can, of course, assign a different value to the variable at the beginning of the main() routine if you are not satisfied with the default initial value. Here's a revised version of GuessingGame.java that includes the gamesWon variable. The changes from the above version are shown in red: public class GuessingGame2 { static int gamesWon; // The number of games won by // the user. public static void main(String[] args) { gamesWon = 0; // This is actually redundant, since 0 is // the default initial value. TextIO.putln("Let's play a game. I'll pick a number between"); TextIO.putln("1 and 100, and you try to guess it."); boolean playAgain; do { playGame(); // call subroutine to play one game TextIO.put("Would you like to play again? "); playAgain = TextIO.getlnBoolean(); } while (playAgain); TextIO.putln(); TextIO.putln("You won " + gamesWon + " games."); TextIO.putln("Thanks for playing. Goodbye."); } // end of main() static void playGame() { int computersNumber; // A random number picked by the computer. int usersGuess; // A number entered by user as a guess. int guessCount; // Number of guesses the user has made. computersNumber = (int)(100 * Math.random()) + 1; // The value assigned to computersNumber is a randomly // chosen integer between 1 and 100, inclusive. guessCount = 0; TextIO.putln(); TextIO.put("What is your first guess? "); while (true) { usersGuess = TextIO.getInt(); // get the user's guess guessCount++; if (usersGuess == computersNumber) { TextIO.putln("You got it in " + guessCount + " guesses! My number was " + computersNumber); gamesWon++; // Count this game by incrementing gamesWon. break; // the game is over; the user has won } http://math.hws.edu/eck/cs124/javanotes4/c4/s2.html (7 of 8) [6/11/2004 11:09:19 AM] Java Programming: Section 4.2 if (guessCount == 6) { TextIO.putln("You didn't get the number in 6 guesses."); TextIO.putln("You lose. My number was " + computersNumber); break; // the game is over; the user has lost } // If we get to this point, the game continues. // Tell the user if the guess was too high or too low. if (usersGuess < computersNumber) TextIO.put("That's too low. Try again: "); else if (usersGuess > computersNumber) TextIO.put("That's too high. Try again: "); } TextIO.putln(); } // end of playGame() } // end of class GuessingGame2 [ Next Section | Previous Section | Chapter Index | Main Index ] http://math.hws.edu/eck/cs124/javanotes4/c4/s2.html (8 of 8) [6/11/2004 11:09:19 AM] Java Programming: Section 4.3 Section 4.3 Parameters IF A SUBROUTINE IS A BLACK BOX, then a parameter provides a mechanism for passing information from the outside world into the box. Parameters are part of the interface of a subroutine. They allow you to customize the behavior of a subroutine to adapt it to a particular situation. As an analogy, consider a thermostat -- a black box whose task it is to keep your house at a certain temperature. The thermostat has a parameter, namely the dial that is used to set the desired temperature. The thermostat always performs the same task: maintaining a constant temperature. However, the exact task that it performs -- that is, which temperature it maintains -- is customized by the setting on its dial. As an example, let's go back to the "3N+1" problem that was discussed in Section 3.2. (Recall that a 3N+1 sequence is computed according to the rule, "if N is odd, multiply by 3 and add 1; if N is even, divide by 2; continue until N is equal to 1." For example, starting from N=3 we get the sequence: 3, 10, 5, 16, 8, 4, 2, 1.) Suppose that we want to write a subroutine to print out such sequences. The subroutine will always perform the same task: Print out a 3N+1 sequence. But the exact sequence it prints out depends on the starting value of N. So, the starting value of N would be a parameter to the subroutine. The subroutine could be written like this: static void Print3NSequence(int startingValue) { // Prints a 3N+1 sequence to standard output, using // startingValue as the initial value of N. It also // prints the number of terms in the sequence. // The value of the parameter, startingValue, must // be a positive integer. int N; // One of the terms in the sequence. int count; // The number of terms. N = startingValue; // The first term is whatever value // is passed to the subroutine as // a parameter. int count = 1; // We have one term, the starting value, so far. TextIO.putln("The 3N+1 sequence starting from " + N); TextIO.putln(); TextIO.putln(N); // print initial term of sequence while (N > 1) { if (N % 2 == 1) // is N odd? N = 3 * N + 1; else N = N / 2; count++; // count this term TextIO.putln(N); // print this term } TextIO.putln(); TextIO.putln("There were " + count + " terms in the sequence."); http://math.hws.edu/eck/cs124/javanotes4/c4/s3.html (1 of 6) [6/11/2004 11:09:20 AM] Java Programming: Section 4.3 } // end of Print3NSequence() The parameter list of this subroutine, "(int startingValue)", specifies that the subroutine has one parameter, of type int. When the subroutine is called, a value must be provided for this parameter. This value is assigned to the parameter, startingValue, before the body of the subroutine is executed. For example, the subroutine could be called using the subroutine call statement "Print3NSequence(17);". When the computer executes this statement, the computer assigns the value 17 to startingValue and then executes the statements in the subroutine. This prints the 3N+1 sequence starting from 17. If K is a variable of type int, then when the computer executes the subroutine call statement "Print3NSequence(K);", it will take the value of the variable K, assign that value to startingValue, and execute the body of the subroutine. The class that contains Print3NSequence can contain a main() routine (or other subroutines) that call Print3NSequence. For example, here is a main() program that prints out 3N+1 sequences for various starting values specified by the user: public static void main(String[] args) { TextIO.putln("This program will print out 3N+1 sequences"); TextIO.putln("for starting values that you specify."); TextIO.putln(); int K; // Input from user; loop ends when K < 0. do { TextIO.putln("Enter a starting value;") TextIO.put("To end the program, enter 0: "); K = TextIO.getInt(); // get starting value from user if (K > 0) // print sequence, but only if K is > 0 Print3NSequence(K); } while (K > 0); // continue only if K > 0 } // end main() Note that the term "parameter" is used to refer to two different, but related, concepts. There are parameters that are used in the definitions of subroutines, such as startingValue in the above example. And there are parameters that are used in subroutine call statements, such as the K in the statement "Print3NSequence(K);". Parameters in a subroutine definition are called formal parameters or dummy parameters. The parameters that are passed to a subroutine when it is called are called actual parameters. When a subroutine is called, the actual parameters in the subroutine call statement are evaluated and the values are assigned to the formal parameters in the subroutine's definition. Then the body of the subroutine is executed. A formal parameter must be an identifier, that is, a name. A formal parameter is very much like a variable, and -- like a variable -- it has a specified type such as int, boolean, or String. An actual parameter is a value, and so it can be specified by any expression, provided that the expression computes a value of the correct type. The type of the actual parameter must be one that could legally be assigned to the formal parameter with an assignment statement. For example, if the formal parameter is of type double, then it would be legal to pass an int as the actual parameter since ints can legally be assigned to doubles. When you call a subroutine, you must provide one actual parameter for each formal parameter in the subroutine's definition. Consider, for example, a subroutine static void doTask(int N, double x, boolean test) { // statements to perform the task go here } This subroutine might be called with the statement http://math.hws.edu/eck/cs124/javanotes4/c4/s3.html (2 of 6) [6/11/2004 11:09:20 AM] Java Programming: Section 4.3 doTask(17, Math.sqrt(z+1), z >= 10); When the computer executes this statement, it has essentially the same effect as the block of statements: { int N; // Allocate memory locations for the formal parameters. double x; boolean test; N = 17; // Assign 17 to the first formal parameter, N. x = Math.sqrt(z+1); // Compute Math.sqrt(z+1), and assign it to // the second formal parameter, x. test = (z >= 10); // Evaluate "z >= 10" and assign the resulting // true/false value to the third formal // parameter, test. // statements to perform the task go here } (There are a few technical differences between this and "doTask(17,Math.sqrt(z+1),z>=10);" -- besides the amount of typing -- because of questions about scope of variables and what happens when several variables or parameters have the same name.) Beginning programming students often find parameters to be surprisingly confusing. Calling a subroutine that already exists is not a problem -- the idea of providing information to the subroutine in a parameter is clear enough. Writing the subroutine definition is another matter. A common mistake is to assign values to the formal parameters at the beginning of the subroutine, or to ask the user to input their values. This represents a fundamental misunderstanding. When the statements in the subroutine are executed, the formal parameters will already have values. The values come from the subroutine call statement. Remember that a subroutine is not independent. It is called by some other routine, and it is the calling routine's responsibility to provide appropriate values for the parameters. In order to call a subroutine legally, you need to know its name, you need to know how many formal parameters it has, and you need to know the type of each parameter. This information is called the subroutine's signature. The signature of the subroutine doTask can be expressed as as: doTask(int,double,boolean). Note that the signature does not include the names of the parameters; in fact, if you just want to use the subroutine, you don't even need to know what the formal parameter names are, so the names are not part of the interface. Java is somewhat unusual in that it allows two different subroutines in the same class to have the same name, provided that their signatures are different. (The language C++ on which Java is based also has this feature.) We say that the name of the subroutine is overloaded because it has several different meanings. The computer doesn't get the subroutines mixed up. It can tell which one you want to call by the number and types of the actual parameters that you provide in the subroutine call statement. You have already seen overloading used in the TextIO class. This class includes many different methods named putln, for example. These methods all have different signatures, such as: putln(int) putln(int,int) putln(double) putln(String) putln(String,int) putln(char) putln(boolean) putln(boolean,int) putln() Of course all these different subroutines are semantically related, which is why it is acceptable programming style to use the same name for them all. But as far as the computer is concerned, printing out an int is very different from printing out a String, which is different from printing out a boolean, and so forth -- so that each of these operations requires a different method. Note, by the way, that the signature does not include the subroutine's return type. It is illegal to have two subroutines in the same class that have the same signature but that have different return types. For example, it would be a syntax error for a class to contain two methods defined as: http://math.hws.edu/eck/cs124/javanotes4/c4/s3.html (3 of 6) [6/11/2004 11:09:20 AM] Java Programming: Section 4.3 int getln() { ... } double getln() { ... } So it should be no surprise that in the TextIO class, the methods for reading different types are not all named getln(). In a given class, there can only be one routine that has the name getln and has no parameters. The input routines in TextIO are distinguished by having different names, such as getlnInt() and getlnDouble(). Let's do a few examples of writing small subroutines to perform assigned tasks. Of course, this is only one side of programming with subroutines. The task performed by a subroutine is always a subtask in a larger program. The art of designing those programs -- of deciding how to break them up into subtasks -- is the other side of programming with subroutines. We'll return to the question of program design in Section 6. As a first example, let's write a subroutine to compute and print out all the divisors of a given positive integer. The integer will be a parameter to the subroutine. Remember that the format of any subroutine is modifiers return-type subroutine-name ( parameter-list ) { statements } Writing a subroutine always means filling out this format. The assignment tells us that there is one parameter, of type int, and it tells us what the statements in the body of the subroutine should do. Since we are only working with static subroutines for now, we'll need to use static as a modifier. We could add an access modifier (public or private), but in the absence of any instructions, I'll leave it out. Since we are not told to return a value, the return type is void. Since no names are specified, we'll have to make up names for the formal parameter and for the subroutine itself. I'll use N for the parameter and printDivisors for the subroutine name. The subroutine will look like static void printDivisors( int N ) { statements } and all we have left to do is to write the statements that make up the body of the routine. This is not difficult. Just remember that you have to write the body assuming that N already has a value! The algorithm is: "For each possible divisor D in the range from 1 to N, if D evenly divides N, then print D." Written in Java, this becomes: static void printDivisors( int N ) { // Print all the divisors of N. // We assume that N is a positive integer. int D; // One of the possible divisors of N. System.out.println("The divisors of " + N + " are:"); for ( D = 1; D <= N; D++ ) { if ( N % D == 0 ) System.out.println(D); } } I've added comments indicating the contract of the subroutine -- that is, what it does and what assumptions it makes. The contract includes the assumption that N is a positive integer. It is up to the caller of the subroutine to make sure that this assumption is satisfied. As a second short example, consider the assignment: Write a subroutine named printRow. It should have a parameter ch of type char and a parameter N of type int. The subroutine should print out a line of text containing N copies of the character ch. http://math.hws.edu/eck/cs124/javanotes4/c4/s3.html (4 of 6) [6/11/2004 11:09:20 AM] Java Programming: Section 4.3 Here, we are told the name of the subroutine and the names of the two parameters, so we don't have much choice about the first line of the subroutine definition. The task in this case is pretty simple, so the body of the subroutine is easy to write. The complete subroutine is given by static void printRow( char ch, int N ) { // Write one line of output containing N copies of the // character ch. If N <= 0, an empty line is output. int i; // Loop-control variable for counting off the copies. for ( i = 1; i <= N; i++ ) { System.out.print( ch ); } System.out.println(); } Note that in this case, the contract makes no assumption about N, but it makes it clear what will happen in all cases, including the unexpected case that N < 0. Finally, let's do an example that shows how one subroutine can build on another. Let's write a subroutine that takes a String as a parameter. For each character in the string, it will print a line of output containing 25 copies of that character. It should use the printRow() subroutine to produce the output. Again, we get to choose a name for the subroutine and a name for the parameter. I'll call the subroutine printRowsFromString and the parameter str. The algorithm is pretty clear: For each position i in the string str, call printRow(str.charAt(i),25) to print one line of the output. So, we get: static void printRowsFromString( String str ) { // For each character in str, write a line of output // containing 25 copies of that character. int i; // Loop-control variable for counting off the chars. for ( i = 0; i < str.length(); i++ ) { printRow( str.charAt(i), 25 ); } } We could use printRowsFromString in a main() routine such as public static void main(String[] args) { String inputLine; // Line of text input by user. TextIO.put("Enter a line of text: "); inputLine = TextIO.getln(); TextIO.putln(); printRowsFromString( inputLine ); } Of course, the three routines, main(), printRowsFromString(), and printRow(), would have to be collected together inside the same class.The program is rather useless, but it does demonstrate the use of subroutines. You'll find the program in the file RowsOfChars.java, if you want to take a look. Here's an applet that simulates the program: (Applet "RowsOfCharsConsole" would be displayed here if Java were available.) I'll finish this section on parameters by noting that we now have three different sorts of variables that can be used inside a subroutine: local variables defined in the subroutine, formal parameter names, and static member variables that are defined outside the subroutine but inside the same class as the subroutine. Local variables have no connection to the outside world; they are purely part of the internal working of the http://math.hws.edu/eck/cs124/javanotes4/c4/s3.html (5 of 6) [6/11/2004 11:09:20 AM] Java Programming: Section 4.3 subroutine. Parameters are used to "drop" values into the subroutine when it is called, but once the subroutine starts executing, parameters act much like local variables. Changes made inside a subroutine to a formal parameter have no effect on the rest of the program (at least if the type of the parameter is one of the primitive types -- things are more complicated in the case of objects, as we'll see later). Things are different when a subroutine uses a variable that is defined outside the subroutine. That variable exists independently of the subroutine, and it is accessible to other parts of the program, as well as to the subroutine. Such a variable is said to be global to the subroutine, as opposed to the "local" variables defined inside the subroutine. The scope of a global variable includes the entire class in which it is defined. Changes made to a global variable can have effects that extend outside the subroutine where the changes are made. You've seen how this works in the last example in the previous section, where the value of the global variable, gamesWon, is computed inside a subroutine and is used in the main() routine. It's not always bad to use global variables in subroutines, but you should realize that the global variable then has to be considered part of the subroutine's interface. The subroutine uses the global variable to communicate with the rest of the program. This is a kind of sneaky, back-door communication that is less visible than communication done through parameters, and it risks violating the rule that the interface of a black box should be straightforward and easy to understand. So before you use a global variable in a subroutine, you should consider whether it's really necessary. I don't advise you to take an absolute stand against using global variables inside subroutines. There is at least one good reason to do it: If you think of the class as a whole as being a kind of black box, it can be very reasonable to let the subroutines inside that box be a little sneaky about communicating with each other, if that will make the class as a whole look simpler from the outside. [ Next Section | Previous Section | Chapter Index | Main Index ] http://math.hws.edu/eck/cs124/javanotes4/c4/s3.html (6 of 6) [6/11/2004 11:09:20 AM] Java Programming: Section 4.4 Section 4.4 Return Values A SUBROUTINE THAT RETURNS A VALUE is called a function. A given function can only return a value of a specified type, called the return type of the function. A function call generally occurs in a position where the computer is expecting to find a value, such as the right side of an assignment statement, as an actual parameter in a subroutine call, or in the middle of some larger expression. A boolean-valued function can even be used as the test condition in an if, while, or do..while statement. (It is also legal to use a function call as a stand-alone statement, just as if it were a regular subroutine. In this case, the computer ignores the value computed by the subroutine. Sometimes this makes sense. For example, the function TextIO.getln(), with a return type of String, reads and returns a line of input typed in by the user. Usually, the line that is returned is assigned to a variable to be used later in the program, as in the statement "name = TextIO.getln();". However, this function is also useful as a subroutine call statement "TextIO.getln();", which still reads all input up to and including the next carriage return. Since this input is not assigned to a variable or used in an expression, it is simply discarded. Sometimes, discarding unwanted input is exactly what you need to do.) You've already seen how functions such as Math.sqrt() and TextIO.getInt() can be used. What you haven't seen is how to write functions of your own. A function takes the same form as a regular subroutine, except that you have to specify the value that is to be returned by the subroutine. This is done with a return statement, which takes the form: return expression; Such a return statement can only occur inside the definition of a function, and the type of the expression must match the return type that was specified for the function. (More exactly, it must be legal to assign the expression to a variable whose type is specified by the return type.) When the computer executes this return statement, it evaluates the expression, terminates execution of the function, and uses the value of the expression as the returned value of the function. For example, consider the function definition static double pythagorus(double x, double y) { // Computes the length of the hypotenuse of a right // triangle, where the sides of the triangle are x and y. return Math.sqrt(x*x + y*y); } Suppose the computer executes the statement "totalLength = 17 + pythagorus(12,5);". When it gets to the term pythagorus(12,5), it assigns the actual parameters 12 and 5 to the formal parameters x and y in the function. In the body of the function, it evaluates Math.sqrt(12.0*12.0 + 5.0*5.0), which works out to 13.0. This value is returned, so it replaces the function call in the statement "totalLength = 17 + pythagorus(12,5);". The return value is added to 17, and the result, 30.0, is stored in the variable, totalLength. The effect is the same as if the statement had been "totalLength = 17 + 13.0;". Inside an ordinary subroutine -- with declared return type "void" -- you can use a return statement with no expression to immediately terminate execution of the subroutine and return control back to the point in the program from which the subroutine was called. This can be convenient if you want to terminate execution somewhere in the middle of the subroutine, but return statements are optional in non-function subroutines. In a function, on the other hand, a return statement, with expression, is always required. Here is a very simple function that could be used in a program to compute 3N+1 sequences. (The 3N+1 http://math.hws.edu/eck/cs124/javanotes4/c4/s4.html (1 of 6) [6/11/2004 11:09:21 AM] Java Programming: Section 4.4 sequence problem is one we've looked at several times already.) Given one term in a 3N+1 sequence, this function computes the next term of the sequence: static int nextN(int currentN) { if (currentN % 2 == 1) // test if current N is odd return 3*currentN + 1; // if so, return this value else return currentN / 2; // if not, return this instead } Exactly one of the two return statements is executed to give the value of the function. A return statement can occur anywhere in a function. Some people, however, prefer to use a single return statement at the very end of the function. This allows the reader to find the return statement easily. You might choose to write nextN() like this, for example: static int nextN(int currentN) { int answer; // answer will be the value returned if (currentN % 2 == 1) // test if current N is odd answer = 3*currentN+1; // if so, this is the answer else answer = currentN / 2; // if not, this is the answer return answer; // (Don't forget to return the answer!) } Here is a subroutine that uses this nextN function. In this case, the improvement from the version in Section 3 is not great, but if nextN() were a long function that performed a complex computation, then it would make a lot of sense to hide that complexity inside a function: static void Print3NSequence(int startingValue) { // Prints a 3N+1 sequence to standard output, using // startingValue as the initial value of N. It also // prints the number of terms in the sequence. // The value of startingValue must be a positive integer. int N; // One of the terms in the sequence. int count; // The number of terms found. N = startingValue; // Start the sequence with startingValue; count = 1; TextIO.putln("The 3N+1 sequence starting from " + N); TextIO.putln(); TextIO.putln(N); // print initial term of sequence while (N > 1) { N = nextN( N ); // Compute next term, // using the function nextN. count++; // Count this term. TextIO.putln(N); // Print this term. } TextIO.putln(); TextIO.putln("There were " + count + " terms in the sequence."); } // end of Print3NSequence() http://math.hws.edu/eck/cs124/javanotes4/c4/s4.html (2 of 6) [6/11/2004 11:09:21 AM] Java Programming: Section 4.4 Here are a few more examples of functions. The first one computes a letter grade corresponding to a given numerical grade, on a typical grading scale: static char letterGrade(int numGrade) { // Returns the letter grade corresponding to // the numerical grade, numGrade. if (numGrade >= 90) return 'A'; // 90 or above gets an A else if (numGrade >= 80) return 'B'; // 80 to 89 gets a B else if (numGrade >= 65) return 'C'; // 65 to 79 gets a C else if (numGrade >= 50) return 'D'; // 50 to 64 gets a D else return 'F'; // anything else gets an F } // end of function letterGrade() The type of the return value of letterGrade() is char. Functions can return values of any type at all. Here's a function whose return value is of type boolean. It demonstrates some interesting programming points, so you should read the comments: static boolean isPrime(int N) { // Returns true if N is a prime number. A prime number // is an integer greater than 1 that is not divisible // by any positive integer, except itself and 1. If N has // any divisor, D, in the range 1 < D < N, then it // has a divisor in the range 2 to Math.sqrt(N), namely // either D itself or N/D. So we only test possible // divisors from 2 to Math.sqrt(N). int divisor; // A number we will test to see whether it // evenly divides N. if (N <= 1) return false; // No number <= 1 is a prime. int maxToTry = (int)Math.sqrt(N); // We will try to divide N by numbers between // 2 and maxToTry; If N is not evenly divisible // by any of these numbers, then N is prime. // (Note that since Math.sqrt(N) is defined to // return a value of type double, the value // must be typecast to type int before it can // be assigned to maxToTry.) for (divisor = 2; divisor <= maxToTry; divisor++) { if ( N % divisor == 0 ) // Test if divisor evenly divides N. return false; // If so, we know N is not prime. // No need to continue testing. } // If we get to this point, N must be prime. Otherwise, http://math.hws.edu/eck/cs124/javanotes4/c4/s4.html (3 of 6) [6/11/2004 11:09:21 AM] Java Programming: Section 4.4 // the function would already have been terminated by // a return statement in the previous for loop. return true; // Yes, N is prime. } // end of function isPrime() Finally, here is a function with return type String. This function has a String as parameter. The returned value is a reversed copy of the parameter. For example, the reverse of "Hello World" is "dlroW olleH". The algorithm for computing the reverse of a string, str, is to start with an empty string and then to append each character from str, starting from the last character of str and working backwards to the first. static String reverse(String str) { // Returns a reversed copy of str. String copy; // The reversed copy. int i; // One of the positions in str, // from str.length() - 1 down to 0. copy = ""; // Start with an empty string. for ( i = str.length() - 1; i >= 0; i-- ) { // Append i-th char of str to copy. copy = copy + str.charAt(i); } return copy; } A palindrome is a string that reads the same backwards and forwards, such as "radar". The reverse() function could be used to check whether a string, word, is a palindrome by testing "if (word.equals(reverse(word)))". By the way, a typical beginner's error in writing functions is to print out the answer, instead of returning it. This represents a fundamental misunderstanding. The task of a function is to compute a value and return it to the point in the program where the function was called. That's where the value is used. Maybe it will be printed out. Maybe it will be assigned to a variable. Maybe it will be used in an expression. But it's not for the function to decide. I'll finish this section with a complete new version of the 3N+1 program. This will give me a chance to show the function nextN(), which was defined above, used in a complete program. I'll also take the opportunity to improve the program by getting it to print the terms of the sequence in columns, with five terms on each line. This will make the output more presentable. This idea is this: Keep track of how many terms have been printed on the current line; when that number gets up to 5, start a new line of output. To make the terms line up into columns, I will use the version of TextIO.put() with signature put(int,int). The second int parameter tells how wide the columns should be. public class ThreeN2 { /* A program that computes and displays several 3N+1 sequences. Starting values for the sequences are input by the user. Terms in a sequence are printed in columns, with five terms on each line of output. After a sequence has been displayed, the number of terms in that sequence is reported to the user. http://math.hws.edu/eck/cs124/javanotes4/c4/s4.html (4 of 6) [6/11/2004 11:09:21 AM] Java Programming: Section 4.4 */ public static void main(String[] args) { TextIO.putln("This program will print out 3N+1 sequences"); TextIO.putln("for starting values that you specify."); TextIO.putln(); int K; // Starting point for sequence, specified by the user. do { TextIO.putln("Enter a starting value;"); TextIO.put("To end the program, enter 0: "); K = TextIO.getInt(); // get starting value from user if (K > 0) // print sequence, but only if K is > 0 Print3NSequence(K); } while (K > 0); // continue only if K > 0 } // end main() static void Print3NSequence(int startingValue) { // Prints a 3N+1 sequence to standard output, using // startingValue as the initial value of N. Terms are // printed five to a line. The subroutine also // prints the number of terms in the sequence. // The value of startingValue must be a positive integer. int N; // One of the terms in the sequence. int count; // The number of terms found. int onLine; // The number of terms that have been output // so far on the current line. N = startingValue; // Start the sequence with startingValue; count = 1; // We have one term so far. TextIO.putln("The 3N+1 sequence starting from " + N); TextIO.putln(); TextIO.put(N, 8); // Print initial term, using 8 characters. onLine = 1; // There's now 1 term on current output line. while (N > 1) { N = nextN(N); // compute next term count++; // count this term if (onLine == 5) { // If current output line is full TextIO.putln(); // ...then output a carriage return onLine = 0; // ...and note that there are no terms // on the new line. } TextIO.put(N, 8); // Print this term in an 8-char column. onLine++; // Add 1 to the number of terms on this line. } TextIO.putln(); // end current line of output TextIO.putln(); // and then add a blank line TextIO.putln("There were " + count + " terms in the sequence."); http://math.hws.edu/eck/cs124/javanotes4/c4/s4.html (5 of 6) [6/11/2004 11:09:21 AM] Java Programming: Section 4.4 } // end of Print3NSequence() static int nextN(int currentN) { // Computes and returns the next term in a 3N+1 sequence, // given that the current term is currentN. if (currentN % 2 == 1) return 3 * currentN + 1; else return currentN / 2; } // end of nextN() } // end of class ThreeN2 You should read this program carefully and try to understand how it works. Here is an applet version for you to try: (Applet "ThreeN2Console" would be displayed here if Java were available.) [ Next Section | Previous Section | Chapter Index | Main Index ] http://math.hws.edu/eck/cs124/javanotes4/c4/s4.html (6 of 6) [6/11/2004 11:09:21 AM] Java Programming: Section 4.5 Section 4.5 Toolboxes, API's, and Packages AS COMPUTERS AND THEIR USER INTERFACES have become easier to use, they have also become more complex for programmers to deal with. You can write programs for a simple console-style user interface using just a few subroutines that write output to the console and read the user's typed replies. A modern graphical user interface, with windows, buttons, scroll bars, menus, text-input boxes, and so on, might make things easier for the user. But it forces the programmer to cope with a hugely expanded array of possibilities. The programmer sees this increased complexity in the form of great numbers of subroutines that are provided for managing the user interface, as well as for other purposes. Someone who wants to program for Macintosh computers -- and to produce programs that look and behave the way users expect them to -- must deal with the Macintosh Toolbox, a collection of well over a thousand different subroutines. There are routines for opening and closing windows, for drawing geometric figures and text to windows, for adding buttons to windows, and for responding to mouse clicks on the window. There are other routines for creating menus and for reacting to user selections from menus. Aside from the user interface, there are routines for opening files and reading data from them, for communicating over a network, for sending output to a printer, for handling communication between programs, and in general for doing all the standard things that a computer has to do. Windows 98 and Windows 2000 provide their own sets of subroutines for programmers to use, and they are quite a bit different from the subroutines used on the Mac. The analogy of a "toolbox" is a good one to keep in mind. Every programming project involves a mixture of innovation and reuse of existing tools. A programmer is given a set of tools to work with, starting with the set of basic tools that are built into the language: things like variables, assignment statements, if statements, and loops. To these, the programmer can add existing toolboxes full of routines that have already been written for performing certain tasks. These tools, if they are well-designed, can be used as true black boxes: They can be called to perform their assigned tasks without worrying about the particular steps they go through to accomplish those tasks. The innovative part of programming is to take all these tools and apply them to some particular project or problem (word-processing, keeping track of bank accounts, processing image data from a space probe, Web browsing, computer games,...). This is called applications programming. A software toolbox is a kind of black box, and it presents a certain interface to the programmer. This interface is a specification of what routines are in the toolbox, what parameters they use, and what tasks they perform. This information constitutes the API, or Applications Programming Interface, associated with the toolbox. The Macintosh API is a specification of all the routines available in the Macintosh Toolbox. A company that makes some hardware device -- say a card for connecting a computer to a network -- might publish an API for that device consisting of a list of routines that programmers can call in order to communicate with and control the device. Scientists who write a set of routines for doing some kind of complex computation -- such as solving "differential equations", say -- would provide an API to allow others to use those routines without understanding the details of the computations they perform. The Java programming language is supplemented by a large, standard API. You've seen part of this API already, in the form of mathematical subroutines such as Math.sqrt(), the String data type and its associated routines, and the System.out.print() routines. The standard Java API includes routines for working with graphical user interfaces, for network communication, for reading and writing files, and more. It's tempting to think of these routines as being built into the Java language, but they are technically subroutines that have been written and made available for use in Java programs. Java is platform-independent. That is, the same program can run on platforms as diverse as Macintosh, Windows, UNIX, and others. The same Java API must work on all these platforms. But notice that it is the http://math.hws.edu/eck/cs124/javanotes4/c4/s5.html (1 of 4) [6/11/2004 11:09:22 AM] Java Programming: Section 4.5 interface that is platform-independent; the implementation varies from one platform to another. A Java system on a particular computer includes implementations of all the standard API routines. A Java program includes only calls to those routines. When the Java interpreter executes a program and encounters a call to one of the standard routines, it will pull up and execute the implementation of that routine which is appropriate for the particular platform on which it is running. This is a very powerful idea. It means that you only need to learn one API to program for a wide variety of platforms. Like all subroutines in Java, the routines in the standard API are grouped into classes. To provide larger-scale organization, classes in Java can be grouped into packages. You can have even higher levels of grouping, since packages can also contain other packages. In fact, the entire standard Java API is implemented in several packages. One of these, which is named "java", contains the non-GUI packages as well as the original AWT graphics user interface classes. Another package, "javax", was added in Java version 1.2 and contains the classes used by the Swing graphical user interface. A package can contain both classes and other packages. A package that is contained in another package is sometimes called a "sub-package." Both the java package and the javax package contain sub-packages. One of the sub-packages of java, for example, is called "awt". Since awt is contained within java, its full name is actually java.awt. This is the package that contains classes related to the AWT graphical user interface, such as a Button class which represents push-buttons on the screen and the Graphics class which provides routines for drawing on the screen. Since these classes are contained in the package java.awt, their full names are actually java.awt.Button and java.awt.Graphics. (I hope that by now you've gotten the hang of how this naming thing works in Java.) Similarly, javax contains a sub-package named javax.swing, which includes such classes as javax.swing.JButton and javax.swing.JApplet. The java package includes several other sub-packages, such as java.io, which provides facilities for input/output, java.net, which deals with network communication, and java.applet, which implements the basic functionality of applets. The most basic package is called java.lang. This package contains fundamental classes such as String and Math. It might be helpful to look at a graphical representation of the levels of nesting in the java package, its sub-packages, the classes in those sub-packages, and the subroutines in those classes. This is not a complete picture, since it shows only a few of the many items in each element: http://math.hws.edu/eck/cs124/javanotes4/c4/s5.html (2 of 4) [6/11/2004 11:09:22 AM] Java Programming: Section 4.5 Let's say that you want to use the class java.awt.Color in a program that you are writing. One way to do this is to use the full name of the class. For example, you could say java.awt.Color rectColor; to declare a variable named rectColor whose type is java.awt.Color. Of course, this can get tiresome, so Java makes it possible to avoid using the full names of classes. If you put import java.awt.Color; at the beginning of a Java source code file, then, in the rest of the file, you can abbreviate the full name java.awt.Color to just the name of the class, Color. This would allow you to say just Color rectColor; to declare the variable rectColor. (The only effect of the import statement is to allow you to use simple class names instead of full "package.class" names; you aren't really importing anything substantial. If you leave out the import statement, you can still access the class -- you just have to use its full name.) There is a shortcut for importing all the classes from a given package. You can import all the classes from java.awt by saying import java.awt.*; and you can import all the classes from javax.swing with the line import javax.swing.*; In fact, any Java program that uses a graphical user interface is likely to begin with one or both of these lines. A program might also include lines such as "import java.net.*;" or "import java.io.*;" to get easy access to networking and input/output classes. (When you start importing lots of packages in this way, you have to be careful about one thing: It's possible for two classes that are in different packages to have the same name. For example, both the java.awt package and the java.util package contain classes named List. If you import both java.awt.* and http://math.hws.edu/eck/cs124/javanotes4/c4/s5.html (3 of 4) [6/11/2004 11:09:22 AM] Java Programming: Section 4.5 java.util.*, the simple name List will be ambiguous. If you try to declare a variable of type List, you will get a compiler error message about an ambiguous class name. The solution is simple: use the full name of the class, either java.awt.List or java.util.List. Another solution is to use import to import the individual classes you need, instead of importing entire packages.) Because the package java.lang is so fundamental, all the classes in java.lang are automatically imported into every program. It's as if every program began with the statement "import java.lang.*;". This is why we have been able to use the class name String instead of java.lang.String, and Math.sqrt() instead of java.lang.Math.sqrt(). It would still, however, be perfectly legal to use the longer forms of the names. Programmers can create new packages. Suppose that you want some classes that you are writing to be in a package named utilities. Then the source code file that defines those classes must begin with the line "package utilities;". Any program that uses the classes should include the directive "import utilities.*;" to obtain access to all the classes in the utilities package. Unfortunately, things are a little more complicated than this. Remember that if a program uses a class, then the class must be "available" when the program is compiled and when it is executed. Exactly what this means depends on which Java environment you are using. Most commonly, classes in a package named utilities should be in a directory with the name utilities, and that directory should be located in the same place as the program that uses the classes. In projects that define large numbers of classes, it makes sense to organize those classes into one or more packages. It also makes sense for programmers to create new packages as toolboxes that provide functionality and API's for dealing with areas not covered in the standard Java API. (And in fact such "toolmaking" programmers often have more prestige than the applications programmers who use their tools.) However, I will not be creating any packages in this textbook. You need to know about packages mainly so that you will be able to import the standard packages. These packages are always available to the programs that you write. You might wonder where the standard classes are actually located. Again, that depends to some extent on the version of Java that you are using. But they are likely to be collected together into a large file named rt.jar or classes.zip, which is located in some place where the Java compiler and the Java interpreter will know to look for it. Although we won't be creating packages explicitly, every class is actually part of a package. If a class is not specifically placed in a package, then it is put in something called the default package, which has no name. All the examples that you see in these notes are in the default package. [ Next Section | Previous Section | Chapter Index | Main Index ] http://math.hws.edu/eck/cs124/javanotes4/c4/s5.html (4 of 4) [6/11/2004 11:09:22 AM] Java Programming: Section 4.6 Section 4.6 More on Program Design UNDERSTANDING HOW PROGRAMS WORK IS ONE THING. Designing a program to perform some particular task is another thing altogether. In Section 3.2, I discussed how stepwise refinement can be used to methodically develop an algorithm. We can now see how subroutines can fit into the process. Stepwise refinement is inherently a top-down process, but the process does have a "bottom," that is, a point at which you stop refining the pseudocode algorithm and translate what you have directly into proper programming language. In the absence of subroutines, the process would not bottom out until you get down to the level of assignment statements and very primitive input/output operations. But if you have subroutines lying around to perform certain useful tasks, you can stop refining as soon as you've managed to express your algorithm in terms of those tasks. This allows you to add a bottom-up element to the top-down approach of stepwise refinement. Given a problem, you might start by writing some subroutines that perform tasks relevant to the problem domain. The subroutines become a toolbox of ready-made tools that you can integrate into your algorithm as you develop it. (Alternatively, you might be able to buy or find a software toolbox written by someone else, containing subroutines that you can use in your project as black boxes.) Subroutines can also be helpful even in a strict top-down approach. As you refine your algorithm, you are free at any point to take any sub-task in the algorithm and make it into a subroutine. Developing that subroutine then becomes a separate problem, which you can work on separately. Your main algorithm will merely call the subroutine. This, of course, is just a way of breaking your problem down into separate, smaller problems. It is still a top-down approach because the top-down analysis of the problem tells you what subroutines to write. In the bottom-up approach, you start by writing or obtaining subroutines that are relevant to the problem domain, and you build your solution to the problem on top of that foundation of subroutines. Preconditions and Postconditions When working with subroutines as building blocks, it is important to be clear about how a subroutine interacts with the rest of the program. This interaction is specified by the contract of the subroutine, as discussed in Section 1. A convenient way to express the contract of a subroutine is in terms of preconditions and postconditions. The precondition of a subroutine is something that must be true when the subroutine is called, if the subroutine is to work correctly. For example, for the built-in function Math.sqrt(x), a precondition is that the parameter, x, is greater than or equal to zero, since it is not possible to take the square root of a negative number. In terms of a contract, a precondition represents an obligation of the caller of the subroutine. If you call a subroutine without meeting its precondition, then there is no reason to expect it to work properly. The program might crash or give incorrect results, but you can only blame yourself, not the subroutine. A postcondition of a subroutine represents the other side of the contract. It is something that will be true after the subroutine has run (assuming that its preconditions were met -- and that there are no bugs in the subroutine). The postcondition of the function Math.sqrt() is that the square of the value that is returned by this function is equal to the parameter that is provided when the subroutine is called. Of course, this will only be true if the preconditiion -- that the parameter is greater than or equal to zero -- is met. A postcondition of the built-in subroutine System.out.print() is that the value of the parameter has been displayed on the screen. Preconditions most often give restrictions on the acceptable values of parameters, as in the example of Math.sqrt(x). However, they can also refer to global variables that are used in the subroutine. The postcondition of a subroutine specifies the task that it performs. For a function, the postcondition should specify the value that the function returns. Subroutines are often described by comments that explicitly specify their preconditions and postconditions. When you are given a pre-written subroutine, a statement of its preconditions and postcondtions tells you how to use it and what it does. When you are assigned to write a subroutine, the preconditions and postconditions give you an exact http://math.hws.edu/eck/cs124/javanotes4/c4/s6.html (1 of 6) [6/11/2004 11:09:23 AM] Java Programming: Section 4.6 specification of what the subroutine is expected to do. I will use this approach in the example that constitutes the rest of this section. I will also use it occasionally later in the book, although I will generally be less formal in my commenting style. Preconditions and postconditions will be discussed more thoroughly in Chapter 9, which deals with techniques for writing correct and robust programs. A Design Example Let's work through an example of program design using subroutines. In this example, we will both use prewritten subroutines as building blocks and design new subroutines that we need to complete the project. Suppose that I have found an already-written class called Mosaic. This class allows a program to work with a window that displays little colored rectangles arranged in rows and columns. The window can be opened, closed, and otherwise manipulated with static member subroutines defined in the Mosaic class. Here are some of the available routines: void Mosaic.open(int rows, int cols, int w, int h); Precondition: The parameters rows, cols, w, and h are greater than zero. Postcondition: A "mosaic" window is opened on the screen that can display rows and columns of colored rectangles. Each rectangle is w pixels wide and h pixels high. The number of rows is given by the first parameter and the number of columns by the second. Initially, all the rectangles are black. Note: The rows are numbered from 0 to rows - 1, and the columns are numbered from 0 to cols - 1. void Mosaic.setColor(int row, int col, int r, int g, int b); Precondition: row and col are in the valid ranges of row numbers and column numbers. r, g, and b are in the range 0 to 255. Also, the mosaic window should be open. Postcondition: The color of the rectangle in row number row and column number col has been set to the color specified by r, g, and b. r gives the amount of red in the color with 0 representing no red and 255 representing the maximum possible amount of red. The larger the value of r, the more red in the color. g and b work similarly for the green and blue color components. int Mosaic.getRed(int row, int col); int Mosaic.getBlue(int row, int col); int Mosaic.getGreen(int row, int col); Precondition: row and col are in the valid ranges of row numbers and column numbers. Also, the mosaic window should be open. Postcondition: Returns an int value that represents one of the three color components of the rectangle in row number row and column number col. The return value is in the range 0 to 255. (Mosaic.getRed() returns the red component of the rectangle, Mosaic.getGreen() the green component, and Mosaic.getBlue() the blue component.) void Mosaic.delay(int milliseconds); Precondition: milliseconds is a positive integer. Postcondition: The program has paused for at least the number http://math.hws.edu/eck/cs124/javanotes4/c4/s6.html (2 of 6) [6/11/2004 11:09:23 AM] Java Programming: Section 4.6 of milliseconds given by the parameter, where one second is equal to 1000 milliseconds. Note: This can be used to insert a time delay in the program (to regulate the speed at which the colors are changed, for example). boolean Mosaic.isOpen(); Precondition: None. Postcondition: The return value is true if the mosaic window is open on the screen, and is false otherwise. Note: The window will be closed if the user clicks its close box. It can also be closed programmatically by calling the subroutine Mosaic.close(). My idea is to use the Mosaic class as the basis for a neat animation. I want to fill the window with randomly colored squares, and then randomly change the colors in a loop that continues as long as the window is open. "Randomly change the colors" could mean a lot of different things, but after thinking for a while, I decide it would be interesting to have a "disturbance" that wanders randomly around the window, changing the color of each square that it encounters. Here's an applet that shows what the program will do: (Applet "RandomMosaicWalkApplet" would be displayed here if Java were available.) With basic routines for manipulating the window as a foundation, I can turn to the specific problem at hand. A basic outline for my program is Open a Mosaic window Fill window with random colors; Move around, changing squares at random. Filling the window with random colors seems like a nice coherent task that I can work on separately, so let's decide to write a separate subroutine to do it. The third step can be expanded a bit more, into the steps: Start in the middle of the window, then keep moving to a new square and changing the color of that square. This should continue as long as the mosaic window is still open. Thus we can refine the algorithm to: Open a Mosaic window Fill window with random colors; Set the current position to the middle square in the window; As long as the mosaic window is open: Randomly change color of current square; Move current position up, down, left, or right, at random; I need to represent the current position in some way. That can be done with two int variables named currentRow and currentColumn. I'll use 10 rows and 20 columns of squares in my mosaic, so setting the current position to be in the center means setting currentRow to 5 and currentColumn to 10. I already have a subroutine, Mosaic.open(), to open the window, and I have a function, Mosaic.isOpen(), to test whether the window is open. To keep the main routine simple, I decide that I will write two more subroutines of my own to carry out the two tasks in the while loop. The algorithm can then be written in Java as: Mosaic.open(10,20,10,10) fillWithRandomColors(); currentRow = 5; // Middle row, halfway down the window. currentColumn = 10; // Middle column. while ( Mosaic.isOpen() ) { changeToRandomColor(currentRow, currentColumn); randomMove(); } With the proper wrapper, this is essentially the main() routine of my program. It turns out I have to make one small modification: To prevent the animation from running too fast, the line "Mosaic.delay(20);" is added to http://math.hws.edu/eck/cs124/javanotes4/c4/s6.html (3 of 6) [6/11/2004 11:09:23 AM] Java Programming: Section 4.6 the while loop. The main() routine is taken care of, but to complete the program, I still have to write the subroutines fillWithRandomColors(), changeToRandomColor(int,int), and randomMove(). Writing each of these subroutines is a separate, small task. The fillWithRandomColors() routine is defined by the postcondition that "each of the rectangles in the mosaic has been changed to a random color." Pseudocode for an algorithm to accomplish this task can be given as: For each row: For each column: set the square in that row and column to a random color "For each row" and "for each column" can be implemented as for loops. We've already planned to write a subroutine changeToRandomColor that can be used to set the color. (The possibility of reusing subroutines in several places is one of the big payoffs of using them!) So, fillWithRandomColors() can be written in proper Java as: static void fillWithRandomColors() { for (int row = 0; row < 10; row++) for (int column = 0; column < 20; column++) changeToRandomColor(row,column); } Turning to the changeToRandomColor subroutine, we already have a method, Mosaic.setColor(row,col,r,g,b), that can be used to change the color of a square. If we want a random color, we just have to choose random values for r, g, and b. According to the precondition of the Mosaic.setColor() subroutine, these random values must be integers in the range from 0 to 255. A formula for randomly selecting such an integer is "(int)(256*Math.random())". So the random color subroutine becomes: static void changeToRandomColor(int rowNum, int colNum) { int red = (int)(256*Math.random()); int green = (int)(256*Math.random()); int blue = (int)(256*Math.random()); mosaic.setColor(rowNum,colNum,red,green,blue); } Finally, consider the randomMove subroutine, which is supposed to randomly move the disturbance up, down, left, or right. To make a random choice among four directions, we can choose a random integer in the range 0 to 3. If the integer is 0, move in one direction; if it is 1, move in another direction; and so on. The position of the disturbance is given by the variables currentRow and currentColumn. To "move up" means to subtract 1 from currentRow. This leaves open the question of what to do if currentRow becomes -1, which would put the disturbance above the window. Rather than let this happen, I decide to move the disturbance to the opposite edge of the applet by setting currentRow to 9. (Remember that the 10 rows are numbered from 0 to 9.) Moving the disturbance down, left, or right is handled similarly. If we use a switch statement to decide which direction to move, the code for randomMove becomes: int directionNum; directoinNum = (int)(4*Math.random()); switch (directionNum) { case 0: // move up currentRow--; if (currentRow < 0) // CurrentRow is outside the mosaic; currentRow = 9; // move it to the opposite edge. break; case 1: // move right currentColumn++; if (currentColumn >= 20) currentColumn = 0; break; case 2: // move down http://math.hws.edu/eck/cs124/javanotes4/c4/s6.html (4 of 6) [6/11/2004 11:09:23 AM] Java Programming: Section 4.6 currentRow++; if (currentRow >= 10) currentRow = 0; break; case 3: // move left currentColumn--; if (currentColumn < 0) currentColumn = 19; break; } Putting this all together, we get the following complete program. The variables currentRow and currentColumn are defined as static members of the class, rather than local variables, because each of them is used in several different subroutines. This program actually depends on two other classes, Mosaic and another class called MosaicCanvas that is used by Mosaic. If you want to compile and run this program, both of these classes must be available to the program. public class RandomMosaicWalk { /* This program shows a window full of randomly colored squares. A "disturbance" moves randomly around in the window, randomly changing the color of each square that it visits. The program runs until the user closes the window. */ static int currentRow; // row currently containing the disturbance static int currentColumn; // column currently containing disturbance public static void main(String[] args) { // Main program creates the window, fills it with // random colors, then moves the disturbance in // a random walk around the window for as long as // the window is open. Mosaic.open(10,20,10,10); fillWithRandomColors(); currentRow = 5; // start at center of window currentColumn = 10; while (Mosaic.isOpen()) { changeToRandomColor(currentRow, currentColumn); randomMove(); Mosaic.delay(20); } } // end of main() static void fillWithRandomColors() { // Precondition: The mosaic window is open. // Postcondition: Each rectangle has been set to a // random color for (int row=0; row < 10; row++) { for (int column=0; column < 20; column++) { changeToRandomColor(row, column); } } } // end of fillWithRandomColors() static void changeToRandomColor(int rowNum, int colNum) { // Precondition: rowNum and colNum are in the valid range http://math.hws.edu/eck/cs124/javanotes4/c4/s6.html (5 of 6) [6/11/2004 11:09:23 AM] Java Programming: Section 4.6 // of row and column numbers. // Postcondition: The rectangle in the specified row and // column has been changed to a random color. int red = (int)(256*Math.random()); // choose random levels in range int green = (int)(256*Math.random()); // 0 to 255 for red, green, int blue = (int)(256*Math.random()); // and blue color components Mosaic.setColor(rowNum,colNum,red,green,blue); } // end of changeToRandomColor() static void randomMove() { // Precondition: The global variables currentRow and currentColumn // specify a valid position in the grid. // Postcondition: currentRow or currentColumn is changed to // one of the neighboring positions in the grid, // up, down, left, or right from the previous // position. If this moves the position outside // the grid, then it is moved to the opposite edge // of the window. int directionNum; // Randomly set to 0, 1, 2, or 3 to choose direction. directionNum = (int)(4*Math.random()); switch (directionNum) { case 0: // move up currentRow--; if (currentRow < 0) currentRow = 9; break; case 1: // move right currentColumn++; if (currentColumn >= 20) currentColumn = 0; break; case 2: // move down currentRow ++; if (currentRow >= 10) currentRow = 0; break; case 3: // move left currentColumn--; if (currentColumn < 0) currentColumn = 19; break; } } // end of randomMove() } // end of class RandomMosaicWalk [ Next Section | Previous Section | Chapter Index | Main Index ] http://math.hws.edu/eck/cs124/javanotes4/c4/s6.html (6 of 6) [6/11/2004 11:09:23 AM] Java Programming: Section 4.7 Section 4.7 The Truth about Declarations NAMES ARE FUNDAMENTAL TO PROGRAMMING, as I said a few chapters ago. There are a lot of details involved in declaring and using names. I have been avoiding some of those details. In this section, I'll reveal most of the truth (although still not the full truth) about declaring and using variables in Java. The material under the headings "Combining Initialization with Declaration" and "Named Constants and the final Modifier" is particularly important, since I will be using it regularly in future chapters. Combining Initialization with Declaration When a variable declaration is executed, memory is allocated for the variable. This memory must be initialized to contain some definite value before the variable can be used in an expression. In the case of a local variable, the declaration is often followed closely by an assignment statement that does the initialization. For example, int count; // Declare a variable named count. count = 0; // Give count its initial value. However, the truth about declaration statements is that it is legal to include the initialization of the variable in the declaration statement. The two statements above can therefore be abbreviated as int count = 0; // Declare count and give it an initial value. The computer still executes this statement in two steps: Declare the variable count, then assign the value 0 to the newly created variable. The initial value does not have to be a constant. It can be any expression. It is legal to initialize several variables in one declaration statement. For example, char firstInitial = 'D', secondInitial = 'E'; int x, y = 1; // OK, but only y has been initialized! int N = 3, M = N+2; // OK, N is initialized // before its value is used. This feature is especially common in for loops, since it makes it possible to declare a loop control variable at the same point in the loop where it is initialized. Since the loop control variable generally has nothing to do with the rest of the program outside the loop, it's reasonable to have its declaration in the part of the program where it's actually used. For example: for ( int i = 0; i < 10; i++ ) { System.out.println(i); } Again, you should remember that this is simply an abbreviation for the following, where I've added an extra pair of braces to show that i is considered to be local to the for statement and no longer exists after the for loop ends: { int i; for ( i = 0; i < 10; i++ ) { System.out.println(i); } } A member variable can also be initialized at the point where it is declared. For example: public class Bank { static double interestRate = 0.05; static int maxWithdrawal = 200; . http://math.hws.edu/eck/cs124/javanotes4/c4/s7.html (1 of 7) [6/11/2004 11:09:24 AM] Java Programming: Section 4.7 . // More variables and subroutines. . } A static member variable is created as soon as the class is loaded by the Java interpreter, and the initialization is also done at that time. In the case of member variables, this is not simply an abbreviation for a declaration followed by an assignment statement. Declaration statements are the only type of statement that can occur outside of a subroutine. Assignment statements cannot, so the following is illegal: public class Bank { static double interestRate; interestRate = 0.05; // ILLEGAL: . // Can't be outside a subroutine! . . Because of this, declarations of member variables often include initial values. As mentioned in Section 2, if no initial value is provided for a member variable, then a default initial value is used. For example, "static int count;" is equivalent to "static int count = 0;". Named Constants and the final Modifier Sometimes, the value of a variable is not supposed to change after it is initialized. For example, in the above example where interestRate is initialized to the value 0.05, it's quite possible that that is meant to be the value throughout the entire program. In this case, the programmer is probably defining the variable, interestRate, to give a meaningful name to the otherwise meaningless number, 0.05. It's easier to understand what's going on when a program says "principal += principal*interestRate;" rather than "principal += principal*0.05;". In Java, the modifier "final" can be applied to a variable declaration to ensure that the value of the variable cannot be changed after the variable has been initialized. For example, if the member variable interestRate is declared with final static double interestRate = 0.05; then it would be impossible for the value of interestRate to change anywhere else in the program. Any assignment statement that tries to assign a value to interestRate will be rejected by the computer as a syntax error when the program is compiled. It is legal to apply the final modifier to local variables and even to formal parameters, but it is most useful for member variables. I will often refer to a static member variable that is declared to be final as a named constant, since its value remains constant for the whole time that the program is running. The readability of a program can be greatly enhanced by using named constants to give meaningful names to important quantities in the program. A recommended style rule for named constants is to give them names that consist entirely of upper case letters, with underscore characters to separate words if necessary. For example, the preferred style for the interest rate constant would be final static double INTEREST_RATE = 0.05; This is the style that is generally used in Java's standard classes, which define many named constants. For example, the Math class defines a named constant PI to represent the mathematical constant of that name. Since it is a member of the Math class, you would have to refer to it as Math.PI in your own programs. Many constants are provided to give meaningful names to be used as parameters in subroutine calls. For example, a standard class named Font contains named constants Font.PLAIN, Font.BOLD, and Font.ITALIC. These constants are used for specifying different styles of text when calling various subroutines in the Font class. Curiously enough, one of the major reasons to use named constants is that it's easy to change the value of a named constant. Of course, the value can't change while the program is running. But between runs of the program, it's easy to change the value in the source code and recompile the program. Consider the interest rate example. It's quite possible that the value of the interest rate is used many times throughout the program. Suppose that the bank changes the interest rate and the program has to be modified. If the literal number 0.05 were used throughout the http://math.hws.edu/eck/cs124/javanotes4/c4/s7.html (2 of 7) [6/11/2004 11:09:24 AM] Java Programming: Section 4.7 program, the programmer would have to track down each place where the interest rate is used in the program and change the rate to the new value. (This is made even harder by the fact that the number 0.05 might occur in the program with other meanings besides the interest rate, as well as by the fact that someone might have used 0.025 to represent half the interest rate.) On the other hand, if the named constant INTEREST_RATE is declared and used consistently throughout the program, then only the single line where the constant is initialized needs to be changed. As an extended example, I will give a new version of the RandomMosaicWalk program from the previous section. This version uses named constants to represent the number of rows in the mosaic, the number of columns, and the size of each little square. The three constants are declared as final static member variables with the lines: final static int ROWS = 30; // Number of rows in mosaic. final static int COLUMNS = 30; // Number of columns in mosaic. final static int SQUARE_SIZE = 15; // Size of each square in mosaic. The rest of the program is carefully modified to use the named constants. For example, in the new version of the program, the Mosaic window is opened with the statement Mosaic.open(ROWS, COLUMNS, SQUARE_SIZE, SQUARE_SIZE); Sometimes, it's not easy to find all the places where a named constants needs to be used. It's always a good idea to run a program using several different values for any named constants, to test that it works properly in all cases. Here is the complete new program, RandomMosaicWalk2, with all modifications from the previous version shown in red. public class RandomMosaicWalk2 { /* This program shows a window full of randomly colored squares. A "disturbance" moves randomly around in the window, randomly changing the color of each square that it visits. The program runs until the user closes the window. */ final static int ROWS = 30; // Number of rows in mosaic. final static int COLUMNS = 30; // Number of columns in mosaic. final static int SQUARE_SIZE = 15; // Size of each square in mosaic. static int currentRow; // row currently containing the disturbance static int currentColumn; // column currently containing disturbance public static void main(String[] args) { // Main program creates the window, fills it with // random colors, then moves the disturbance in // a random walk around the window. Mosaic.open(ROWS, COLUMNS, SQUARE_SIZE, SQUARE_SIZE); fillWithRandomColors(); currentRow = ROWS / 2; // start at center of window currentColumn = COLUMNS / 2; while (Mosaic.isOpen()) { changeToRandomColor(currentRow, currentColumn); randomMove(); Mosaic.delay(20); } } // end of main() static void fillWithRandomColors() { // Precondition: The mosaic window is open. http://math.hws.edu/eck/cs124/javanotes4/c4/s7.html (3 of 7) [6/11/2004 11:09:24 AM] Java Programming: Section 4.7 // Postcondition: Each rectangle has been set to a // random color for (int row=0; row < ROWS; row++) { for (int column=0; column < COLUMNS; column++) { changeToRandomColor(row, column); } } } // end of fillWithRandomColors() static void changeToRandomColor(int rowNum, int colNum) { // Precondition: rowNum and colNum are in the valid range // of row and column numbers. // Postcondition: The rectangle in the specified row and // column has been changed to a random color. int red = (int)(256*Math.random()); int green = (int)(256*Math.random()); int blue = (int)(256*Math.random()); Mosaic.setColor(rowNum,colNum,red,green,blue); } // end of changeToRandomColor() static void randomMove() { // Precondition: The global variables currentRow and currentColumn // specify a valid position in the grid. // Postcondition: currentRow or currentColumn is changed to // one of the neighboring positions in the grid, // up, down, left, or right from the previous // position. (If this moves the position outside // the grid, then it is moved to the opposite edge // of the window.) int directionNum; // Randomly set to 0, 1, 2, or 3 // to choose direction. directionNum = (int)(4*Math.random()); switch (directionNum) { case 0: // move up currentRow--; if (currentRow < 0) currentRow = ROWS - 1; break; case 1: // move right currentColumn++; if (currentColumn >= COLUMNS) currentColumn = 0; break; case 2: // move down currentRow ++; if (currentRow >= ROWS) currentRow = 0; break; case 3: // move left currentColumn--; if (currentColumn < 0) currentColumn = COLUMNS - 1; break; } } // end of randomMove() } // end of class RandomMosaicWalk2 http://math.hws.edu/eck/cs124/javanotes4/c4/s7.html (4 of 7) [6/11/2004 11:09:24 AM] Java Programming: Section 4.7 Java 1.5 Note: Constants are often used to represent a small set of related values. For example, you might declare int constants named RECT, OVAL, and ROUNDRECT to represent shapes. Note that the values of these constants are not really important; they exist only to name the different types of shapes. Java 1.5 introduces enumerated types to represent such sets of constants. Enumerated types have long been available in other programming languages. In Java 1.5, an enumerated type for representing shapes could be defined as "enum ShapeName { RECT, OVAL, ROUNDRECT }" (or more likely "public static enum ShapeName { RECT, OVAL, ROUNDRECT }"). This defines a type named ShapeName that can be used to define variables and parameters in that same way as any other type. The values of this type are denoted ShapeName.RECT, ShapeName.OVAL, and ShapeName.ROUNDRECT. In fact, ShapeName is really a class (except that it must be defined inside another class). An enum is preferable to a bunch of int-valued constants because the enum is "type-safe": A variable of type ShapeName can only have one of the three specified values of type ShapeName. There is no way to guarantee that a variable of type int has one of the three values that represent shapes. Naming and Scope Rules When a variable declaration is executed, memory is allocated for that variable. The variable name can be used in at least some part of the program source code to refer to that memory or to the data that is stored in the memory. The portion of the program source code where the variable name is valid is called the scope of the variable. Similarly, we can refer to the scope of subroutine names and formal parameter names. For static member subroutines, scope is straightforward. The scope of a static subroutine is the entire source code of the class in which it is defined. That is, it is possible to call the subroutine from any point in the class. It is even possible to call a subroutine from within itself. This is an example of something called "recursion," a fairly advanced topic that we will return to later. For a variable that is declared as a static member variable in a class, the situation is similar, but with one complication. It is legal to have a local variable or a formal parameter that has the same name as a member variable. In that case, within the scope of the local variable or parameter, the member variable is hidden. Consider, for example, a class named Game that has the form: public class Game { static int count; // member variable static void playGame() { int count; // local variable . . // Some statements to define playGame() . } . . // More variables and subroutines. . } // end Game In the statements that make up the body of the playGame() subroutine, the name "count" refers to the local variable. In the rest of the Game class, "count" refers to the member variable, unless hidden by other local variables or parameters named count. However, there is one further complication. The member variable named count can also be referred to by the full name Game.count. Usually, the full name is only used outside the class where count is defined. However, there is no rule against using it inside the class. The full name, Game.count, can be used inside the playGame() subroutine to refer to the member variable. So, the full scope rule for static member variables is that the scope of a member variable includes the entire class in which it is defined, but where the simple name of the member variable is hidden by a local variable or formal parameter name, the member variable must be referred to by its full name of the form className.variableName. (Scope http://math.hws.edu/eck/cs124/javanotes4/c4/s7.html (5 of 7) [6/11/2004 11:09:24 AM] Java Programming: Section 4.7 rules for non-static members are similar to those for static members, except that, as we shall see, non-static members cannot be used in static subroutines.) The scope of a formal parameter of a subroutine is the block that makes up the body of the subroutine. The scope of a local variable extends from the declaration statement that defines the variable to the end of the block in which the declaration occurs. As noted above, it is possible to declare a loop control variable of a for loop in the for statement, as in "for (int i=0; i < 10; i++)". The scope of such a declaration is considered as a special case: It is valid only within the for statement and does not extend to the remainder of the block that contains the for statement. It is not legal to redefine the name of a formal parameter or local variable within its scope, even in a nested block. For example, this is not allowed: void badSub(int y) { int x; while (y > 0) { int x; // ERROR: x is already defined. . . . } } In many languages, this would be legal. The declaration of x in the while loop would hide the original declaration. It is not legal in Java. However, once the block in which a variable is declared ends, its name does become available for reuse in Java. For example: void goodSub(int y) { while (y > 10) { int x; . . . // The scope of x ends here. } while (y > 0) { int x; // OK: Previous declaration of x has expired. . . . } } You might wonder whether local variable names can hide subroutine names. This can't happen, for a reason that might be surprising. There is no rule that variables and subroutines have to have different names. The computer can always tell whether a name refers to a variable or to a subroutine, because a subroutine name is always followed by a left parenthesis. It's perfectly legal to have a variable called count and a subroutine called count in the same class. (This is one reason why I often write subroutine names with parentheses, as when I talk about the main() routine. It's a good idea to think of the parentheses as part of the name.) Even more is true: It's legal to reuse class names to name variables and subroutines. The syntax rules of Java guarantee that the computer can always tell when a name is being used as a class name. A class name is a type, and so it can be used to declare variables and to specify the return type of a function. This means that you could legally have a class called Insanity in which you declare a function static Insanity Insanity( Insanity Insanity ) { ... } The first Insanity is the return type of the function. The second is the function name, the third is the type of the formal parameter, and the fourth is a formal parameter name. However, please remember that not everything that is possible is a good idea! http://math.hws.edu/eck/cs124/javanotes4/c4/s7.html (6 of 7) [6/11/2004 11:09:24 AM] Java Programming: Section 4.7 End of Chapter 4 [ Next Chapter | Previous Section | Chapter Index | Main Index ] http://math.hws.edu/eck/cs124/javanotes4/c4/s7.html (7 of 7) [6/11/2004 11:09:24 AM] Java Programming: Chapter 4 Exercises Programming Exercises For Chapter 4 THIS PAGE CONTAINS programming exercises based on material from Chapter 4 of this on-line Java textbook. Each exercise has a link to a discussion of one possible solution of that exercise. Exercise 4.1: To "capitalize" a string means to change the first letter of each word in the string to upper case (if it is not already upper case). For example, a capitalized version of "Now is the time to act!" is "Now Is The Time To Act!". Write a subroutine named printCapitalized that will print a capitalized version of a string to standard output. The string to be printed should be a parameter to the subroutine. Test your subroutine with a main() routine that gets a line of input from the user and applies the subroutine to it. Note that a letter is the first letter of a word if it is not immediately preceded in the string by another letter. Recall that there is a standard boolean-valued function Character.isLetter(char) that can be used to test whether its parameter is a letter. There is another standard char-valued function, Character.toUpperCase(char), that returns a capitalized version of the single character passed to it as a parameter. That is, if the parameter is a letter, it returns the upper-case version. If the parameter is not a letter, it just returns a copy of the parameter. See the solution! Exercise 4.2: The hexadecimal digits are the ordinary, base-10 digits '0' through '9' plus the letters 'A' through 'F'. In the hexadecimal system, these digits represent the values 0 through 15, respectively. Write a function named hexValue that uses a switch statement to find the hexadecimal value of a given character. The character is a parameter to the function, and its hexadecimal value is the return value of the function. You should count lower case letters 'a' through 'f' as having the same value as the corresponding upper case letters. If the parameter is not one of the legal hexadecimal digits, return -1 as the value of the function. A hexadecimal integer is a sequence of hexadecimal digits, such as 34A7, FF8, 174204, or FADE. If str is a string containing a hexadecimal integer, then the corresponding base-10 integer can be computed as follows: value = 0; for ( i = 0; i < str.length(); i++ ) value = value*16 + hexValue( str.charAt(i) ); Of course, this is not valid if str contains any characters that are not hexadecimal digits. Write a program that reads a string from the user. If all the characters in the string are hexadecimal digits, print out the corresponding base-10 value. If not, print out an error message. See the solution! Exercise 4.3: Write a function that simulates rolling a pair of dice until the total on the dice comes up to be a given number. The number that you are rolling for is a parameter to the function. The number of times you have to roll the dice is the return value of the function. You can assume that the parameter is one of the possible totals: 2, 3, ..., 12. Use your function in a program that computes and prints the number of rolls it takes to get snake eyes. (Snake eyes means that the total showing on the dice is 2.) http://math.hws.edu/eck/cs124/javanotes4/c4/exercises.html (1 of 3) [6/11/2004 11:09:24 AM] Java Programming: Chapter 4 Exercises See the solution! Exercise 4.4: This exercise builds on Exercise 4.3. Every time you roll the dice repeatedly, trying to get a given total, the number of rolls it takes can be different. The question naturally arises, what's the average number of rolls? Write a function that performs the experiment of rolling to get a given total 10000 times. The desired total is a parameter to the subroutine. The average number of rolls is the return value. Each individual experiment should be done by calling the function you wrote for exercise 4.3. Now, write a main program that will call your function once for each of the possible totals (2, 3, ..., 12). It should make a table of the results, something like: Total On Dice Average Number of Rolls ------------- ----------------------- 2 35.8382 3 18.0607 . . . . See the solution! Exercise 5: The sample program RandomMosaicWalk.java from Section 4.6 shows a "disturbance" that wanders around a grid of colored squares. When the disturbance visits a square, the color of that square is changed. The applet at the bottom of Section 4.7 shows a variation on this idea. In this applet, all the squares start out with the default color, black. Every time the disturbance visits a square, a small amount is added to the red component of the color of that square. Write a subroutine that will add 25 to the red component of one of the squares in the mosaic. The row and column numbers of the square should be passed as parameters to the subroutine. Recall that you can discover the current red component of the square in row r and column c with the function call Mosaic.getRed(r,c). Use your subroutine as a substitute for the changeToRandomColor() subroutine in the program RandomMosaicWalk2.java. (This is the improved version of the program from Section 4.7 that uses named constants for the number of rows, number of columns, and square size.) Set the number of rows and the number of columns to 80. Set the square size to 5. See the solution! Exercise 6: For this exercise, you will write a program that has the same behavior as the following applet. Your program will be based on the non-standard Mosaic class, which was described in Section 4.6. (Unfortunately, the applet doesn't look too good on many versions of Java.) The applet shows a rectangle that grows from the center of the applet to the edges, getting brighter as it grows. The rectangle is made up of the little squares of the mosaic. You should first write a subroutine that draws a rectangle on a Mosaic window. More specifically, write a subroutine named rectangle such that the subroutine call statement rectangle(top,left,height,width,r,g,b); will call Mosaic.setColor(row,col,r,g,b) for each little square that lies on the outline of a rectangle. The topmost row of the rectangle is specified by top. The number of rows in the rectangle is specified by height (so the bottommost row is top+height-1). The leftmost column of the rectangle is specifed by left. The number of columns in the rectangle is specified by width (so the rightmost column is left+width-1.) The animation loops through the same sequence of steps over and over. In one step, a rectangle is drawn in gray (that is, with all three color components having the same value). There is a pause of 200 milliseconds http://math.hws.edu/eck/cs124/javanotes4/c4/exercises.html (2 of 3) [6/11/2004 11:09:24 AM] Java Programming: Chapter 4 Exercises so the user can see the rectangle. Then the very same rectangle is drawn in black, effectively erasing the gray rectangle. Finally, the variables giving the top row, left column, size, and color level of the rectangle are adjusted to get ready for the next step. In the applet, the color level starts at 50 and increases by 10 after each step. You might want to make a subroutine that does one loop through all the steps of the animation. The main() routine simply opens a Mosaic window and then does the animation loop over and over until the user closes the window. There is a 1000 millisecond delay between one animation loop and the next. Use a Mosaic window that has 41 rows and 41 columns. (I advise you not to used named constants for the numbers of rows and columns, since the problem is complicated enough already.) See the solution! [ Chapter Index | Main Index ] http://math.hws.edu/eck/cs124/javanotes4/c4/exercises.html (3 of 3) [6/11/2004 11:09:24 AM] Java Programming: Chapter 4 Quiz Quiz Questions For Chapter 4 THIS PAGE CONTAINS A SAMPLE quiz on material from Chapter 4 of this on-line Java textbook. You should be able to answer these questions after studying that chapter. Sample answers to all the quiz questions can be found here. Question 1: A "black box" has an interface and an implementation. Explain what is meant by the terms interface and implementation. Question 2: A subroutine is said to have a contract. What is meant by the contract of a subroutine? When you want to use a subroutine, why is it important to understand its contract? The contract has both "syntactic" and "semantic" aspects. What is the syntactic aspect? What is the semantic aspect? Question 3: Briefly explain how subroutines can be a useful tool in the top-down design of programs. Question 4: Discuss the concept of parameters. What are parameters for? What is the difference between formal parameters and actual parameters? Question 5: Give two different reasons for using named constants (declared with the final modifier). Question 6: What is an API? Give an example. Question 7: Write a subroutine named "stars" that will output a line of stars to standard output. (A star is the character "*".) The number of stars should be given as a parameter to the subroutine. Use a for loop. For example, the command "stars(20)" would output ******************** Question 8: Write a main() routine that uses the subroutine that you wrote for Question 7 to output 10 lines of stars with 1 star in the first line, 2 stars in the second line, and so on, as shown below. * ** *** **** ***** ****** ******* ******** ********* ********** Question 9: Write a function named countChars that has a String and a char as parameters. The function should count the number of times the character occurs in the string, and it should return the result as the value of the function. Question 10: Write a subroutine with three parameters of type int. The subroutine should determine which of its parameters is smallest. The value of the smallest parameter should be returned as the value of the subroutine. [ Answers | Chapter Index | Main Index ] http://math.hws.edu/eck/cs124/javanotes4/c4/quiz.html [6/11/2004 11:09:25 AM] Java Programming: Chapter 5 Index Chapter 5 Programming in the Large II Objects and Classes WHEREAS A SUBROUTINE represents a single task, an object can encapsulate both data (in the form of instance variables) and a number of different tasks or "behaviors" related to that data (in the form of instance methods). Therefore objects provide another, more sophisticated type of structure that can be used to help manage the complexity of large programs. This chapter covers the creation and use of objects in Java. Section 4 covers the central ideas of object-oriented programming: inheritance and polymorphism. However, in this textbook, we will generally use these ideas in a limited form, by creating independent classes and building on existing classes rather than by designing entire hierarchies of classes from scratch. Sections 5 and 6 cover some of the many details of object oriented programming in Java. Although these details are used occasionally later in the book, you might want to skim through them now and return to them later when they are actually needed. Contents of Chapter 5: ● Section 1: Objects, Instance Methods, and Instance Variables ● Section 2: Constructors and Object Initialization ● Section 3: Programming with Objects ● Section 4: Inheritance, Polymorphism, and Abstract Classes ● Section 5: this and super ● Section 6: Interfaces, Nested Classes and Other Details ● Programming Exercises ● Quiz on this Chapter [ First Section | Next Chapter | Previous Chapter | Main Index ] http://math.hws.edu/eck/cs124/javanotes4/c5/ [6/11/2004 11:09:53 AM] Java Programming: Section 5.1 Section 5.1 Objects, Instance Methods, and Instance Variables OBJECT-ORIENTED PROGRAMMING (OOP) represents an attempt to make programs more closely model the way people think about and deal with the world. In the older styles of programming, a programmer who is faced with some problem must identify a computing task that needs to be performed in order to solve the problem. Programming then consists of finding a sequence of instructions that will accomplish that task. But at the heart of object-oriented programming, instead of tasks we find objects -- entities that have behaviors, that hold information, and that can interact with one another. Programming consists of designing a set of objects that somehow model the problem at hand. Software objects in the program can represent real or abstract entities in the problem domain. This is supposed to make the design of the program more natural and hence easier to get right and easier to understand. To some extent, OOP is just a change in point of view. We can think of an object in standard programming terms as nothing more than a set of variables together with some subroutines for manipulating those variables. In fact, it is possible to use object-oriented techniques in any programming language. However, there is a big difference between a language that makes OOP possible and one that actively supports it. An object-oriented programming language such as Java includes a number of features that make it very different from a standard language. In order to make effective use of those features, you have to "orient" your thinking correctly. Objects are closely related to classes. We have already been working with classes for several chapters, and we have seen that a class can contain variables and subroutines. If an object is also a collection of variables and subroutines, how do they differ from classes? And why does it require a different type of thinking to understand and use them effectively? In the one section where we worked with objects rather than classes, Section 3.7, it didn't seem to make much difference: We just left the word "static" out of the subroutine definitions! I have said that classes "describe" objects, or more exactly that the non-static portions of classes describe objects. But it's probably not very clear what this means. The more usual terminology is to say that objects belong to classes, but this might not be much clearer. (There is a real shortage of English words to properly distinguish all the concepts involved. An object certainly doesn't "belong" to a class in the same way that a member variable "belongs" to a class.) From the point of view of programming, it is more exact to say that classes are used to create objects. A class is a kind of factory for constructing objects. The non-static parts of the class specify, or describe, what variables and subroutines the objects will contain. This is part of the explanation of how objects differ from classes: Objects are created and destroyed as the program runs, and there can be many objects with the same structure, if they are created using the same class. Consider a simple class whose job is to group together a few static member variables. For example, the following class could be used to store information about the person who is using the program: class UserData { static String name; static int age; } In a program that uses this class, there is only one copy of each of the variables UserData.name and UserData.age. There can only be one "user," since we only have memory space to store data about one user. The class, UserData, and the variables it contains exist as long as the program runs. Now, consider a similar class that includes non-static variables: class PlayerData { String name; http://math.hws.edu/eck/cs124/javanotes4/c5/s1.html (1 of 6) [6/11/2004 11:09:54 AM] Java Programming: Section 5.1 int age; } In this case, there is no such variable as PlayerData.name or PlayerData.age, since name and age are not static members of PlayerData. So, there is nothing much in the class at all -- except the potential to create objects. But, it's a lot of potential, since it can be used to create any number of objects! Each object will have its own variables called name and age. There can be many "players" because we can make new objects to represent new players on demand. A program might use this class to store information about multiple players in a game. Each player has a name and an age. When a player joins the game, a new PlayerData object can be created to represent that player. If a player leaves the game, the PlayerData object that represents that player can be destroyed. A system of objects in the program is being used to dynamically model what is happening in the game. You can't do this with "static" variables! In Section 3.7, we worked with applets, which are objects. The reason they didn't seem to be any different from classes is because we were only working with one applet in each class that we looked at. But one class can be used to make many applets. Think of an applet that scrolls a message across a Web page. There could be several such applets on the same page, all created from the same class. If the scrolling message in the applet is stored in a non-static variable, then each applet will have its own variable, and each applet can show a different message. The situation is even clearer if you think about windows, which, like applets, are objects. As a program runs, many windows might be opened and closed, but all those windows can belong to the same class. Here again, we have a dynamic situation where multiple objects are created and destroyed as a program runs. An object that belongs to a class is said to be an instance of that class. The variables that the object contains are called instance variables. The subroutines that the object contains are called instance methods. (Recall that in the context of object-oriented programming, "method" is a synonym for "subroutine". From now on, for subroutines in objects, I will prefer the term "method.") For example, if the PlayerData class, as defined above, is used to create an object, then that object is an instance of the PlayerData class, and name and age are instance variables in the object. It is important to remember that the class of an object determines the types of the instance variables; however, the actual data is contained inside the individual objects, not the class. Thus, each object has its own set of data. An applet that scrolls a message across a Web page might include a subroutine named scroll(). Since the applet is an object, this subroutine is an instance method of the applet. The source code for the method is in the class that is used to create the applet. Still, it's better to think of the instance method as belonging to the object, not to the class. The non-static subroutines in the class merely specify the instance methods that every object created from the class will contain. The scroll() methods in two different applets do the same thing in the sense that they both scroll messages across the screen. But there is a real difference between the two scroll() methods. The messages that they scroll can be different. (You might say that the subroutine definition in the class specifies what type of behavior the objects will have, but the specific behavior can vary from object to object, depending on the values of their instance variables.) As you can see, the static and the non-static portions of a class are very different things and serve very different purposes. Many classes contain only static members, or only non-static. However, it is possible to mix static and non-static members in a single class, and we'll see a few examples later in this chapter where it is reasonable to do so. By the way, static member variables and static member subroutines in a class are sometimes called class variables and class methods, since they belong to the class itself, rather than to instances of that class. This terminology is most useful when the class contains both static and non-static members. So far, I've been talking mostly in generalities, and I haven't given you much idea what you have to put in a program if you want to work with objects. Let's look at a specific example to see how it works. Consider this extremely simplified version of a Student class, which could be used to store information about students taking a course: http://math.hws.edu/eck/cs124/javanotes4/c5/s1.html (2 of 6) [6/11/2004 11:09:54 AM] Java Programming: Section 5.1 class Student { String name; // Student's name. double test1, test2, test3; // Grades on three tests. double getAverage() { // compute average test grade return (test1 + test2 + test3) / 3; } } // end of class Student None of the members of this class are declared to be static, so the class exists only for creating objects. This class definition says that any object that is an instance of the Student class will include instance variables named name, test1, test2, and test3, and it will include an instance method named getAverage(). The names and tests in different objects will generally have different values. When called for a particular student, the method getAverage() will compute an average using that student's test grades. Different students can have different averages. (Again, this is what it means to say that an instance method belongs to an individual object, not to the class.) In Java, a class is a type, similar to the built-in types such as int and boolean. So, a class name can be used to specify the type of a variable in a declaration statement, the type of a formal parameter, or the return type of a function. For example, a program could define a variable named std of type Student with the statement Student std; However, declaring a variable does not create an object! This is an important point, which is related to this Very Important Fact: In Java, no variable can ever hold an object. A variable can only hold a reference to an object. You should think of objects as floating around independently in the computer's memory. In fact, there is a special portion of memory called the heap where objects live. Instead of holding an object itself, a variable holds the information necessary to find the object in memory. This information is called a reference or pointer to the object. In effect, a reference to an object is the address of the memory location where the object is stored. When you use a variable of class type, the computer uses the reference in the variable to find the actual object. Objects are actually created by an operator called new, which creates an object and returns a reference to that object. For example, assuming that std is a variable of type Student, declared as above, the assignment statement std = new Student(); would create a new object which is an instance of the class Student, and it would store a reference to that object in the variable std. The value of the variable is a reference to the object, not the object itself. It is not quite true, then, to say that the object is the "value of the variable std" (though sometimes it is hard to avoid using this terminology). It is certainly not at all true to say that the object is "stored in the variable std." The proper terminology is that "the variable std refers to the object," and I will try to stick to that terminology as much as possible. So, suppose that the variable std refers to an object belonging to the class Student. That object has instance variables name, test1, test2, and test3. These instance variables can be referred to as std.name, std.test1, std.test2, and std.test3. This follows the usual naming convention that when B is part of A, then the full name of B is A.B. For example, a program might include the lines http://math.hws.edu/eck/cs124/javanotes4/c5/s1.html (3 of 6) [6/11/2004 11:09:54 AM] Java Programming: Section 5.1 System.out.println("Hello, " + std.name + ". Your test grades are:"); System.out.println(std.test1); System.out.println(std.test2); System.out.println(std.test3); This would output the name and test grades from the object to which std refers. Similarly, std can be used to call the getAverage() instance method in the object by saying std.getAverage(). To print out the student's average, you could say: System.out.println( "Your average is " + std.getAverage() ); More generally, you could use std.name any place where a variable of type String is legal. You can use it in expressions. You can assign a value to it. You can even use it to call subroutines from the String class. For example, std.name.length() is the number of characters in the student's name. It is possible for a variable like std, whose type is given by a class, to refer to no object at all. We say in this case that std holds a null reference. The null reference is written in Java as "null". You can store a null reference in the variable std by saying std = null; and you could test whether the value of std is null by testing if (std == null) . . . If the value of a variable is null, then it is, of course, illegal to refer to instance variables or instance methods through that variable -- since there is no object, and hence no instance variables to refer to. For example, if the value of the variable std is null, then it would be illegal to refer to std.test1. If your program attempts to use a null reference illegally like this, the result is an error called a null pointer exception. Let's look at a sequence of statements that work with objects: Student std, std1, // Declare four variables of std2, std3; // type Student. std = new Student(); // Create a new object belonging // to the class Student, and // store a reference to that // object in the variable std. std1 = new Student(); // Create a second Student object // and store a reference to // it in the variable std1. std2 = std1; // Copy the reference value in std1 // into the variable std2. std3 = null; // Store a null reference in the // variable std3. std.name = "John Smith"; // Set values of some instance variables. std1.name = "Mary Jones"; // (Other instance variables have default // initial values of zero.) After the computer executes these statements, the situation in the computer's memory looks like this: http://math.hws.edu/eck/cs124/javanotes4/c5/s1.html (4 of 6) [6/11/2004 11:09:54 AM] Java Programming: Section 5.1 This picture shows variables as little boxes, labeled with the names of the variables. Objects are shown as boxes with round corners. When a variable contains a reference to an object, the value of that variable is shown as an arrow pointing to the object. The variable std3, with a value of null, doesn't point anywhere. The arrows from std1 and std2 both point to the same object. This illustrates a Very Important Point: When one object variable is assigned to another, only a reference is copied. The object referred to is not copied. When the assignment "std2 = std1;" was executed, no new object was created. Instead, std2 was set to refer to the very same object that std1 refers to. This has some consequences that might be surprising. For example, std1.name and std2.name refer to exactly the same variable, namely the instance variable in the object that both std1 and std2 refer to. After the string "Mary Jones" is assigned to the variable std1.name, it is also be true that the value of std2.name is "Mary Jones". There is a potential for a lot of confusion here, but you can help protect yourself from it if you keep telling yourself, "The object is not in the variable. The variable just holds a pointer to the object." You can test objects for equality and inequality using the operators == and !=, but here again, the semantics are different from what you are used to. When you make a test "if (std1 == std2)", you are testing whether the values stored in std1 and std2 are the same. But the values are references to objects, not objects. So, you are testing whether std1 and std2 refer to the same object, that is, whether they point to the same location in memory. This is fine, if its what you want to do. But sometimes, what you want to check is whether the instance variables in the objects have the same values. To do that, you would need to ask whether "std1.test1 == std2.test1 && std1.test2 == std2.test2 && std1.test3 == std2.test3 && std1.name.equals(std2.name)" I've remarked previously that Strings are objects, and I've shown the strings "Mary Jones" and "John Smith" as objects in the above illustration. A variable of type String can only hold a reference http://math.hws.edu/eck/cs124/javanotes4/c5/s1.html (5 of 6) [6/11/2004 11:09:54 AM] Java Programming: Section 5.1 to a string, not the string itself. It could also hold the value null, meaning that it does not refer to any string at all. This explains why using the == operator to test strings for equality is not a good idea. Suppose that greeting is a variable of type String, and that the string it refers to is "Hello". Then would the test greeting == "Hello" be true? Well, maybe, maybe not. The variable greeting and the String literal "Hello" each refer to a string that contains the characters H-e-l-l-o. But the strings could still be different objects, that just happen to contain the same characters. The function greeting.equals("Hello") tests whether greeting and "Hello" contain the same characters, which is almost certainly the question you want to ask. The expression greeting == "Hello" tests whether greeting and "Hello" contain the same characters stored in the same memory location. The fact that variables hold references to objects, not objects themselves, has a couple of other consequences that you should be aware of. They follow logically, if you just keep in mind the basic fact that the object is not stored in the variable. The object is somewhere else; the variable points to it. Suppose that a variable that refers to an object is declared to be final. This means that the value stored in the variable can never be changed, once the variable has been initialized. The value stored in the variable is a reference to the object. So the variable will continue to refer to the same object as long as the variable exists. However, this does not prevent the data in the object from changing. The variable is final, not the object. It's perfectly legal to say final Student stu = new Student(); stu.name = "John Doe"; // Change data in the object; // The value stored in stu is not changed. Next, suppose that obj is a variable that refers to an object. Let's consider what happens when obj is passed as an actual parameter to a subroutine. The value of obj is assigned to a formal parameter in the subroutine, and the subroutine is executed. The subroutine has no power to change the value stored in the variable, obj. It only has a copy of that value. However, that value is a reference to an object. Since the subroutine has a reference to the object, it can change the data stored in the object. After the subroutine ends, obj still points to the same object, but the data stored in the object might have changed. Suppose x is a variable of type int and stu is a variable of type Student. Compare: void dontChange(int z) { void change(Student s) { z = 42; s.name = "Fred"; } } The lines: The lines: x = 17; stu.name = "Jane"; dontChange(x); change(stu); System.out.println(x); System.out.println(stu.name); output the value 17. output the value "Fred". The value of x is not The value of stu is not changed by the subroutine, changed, but stu.name is. which is equivalent to This is equivalent to z = x; s = stu; z = 42; s.name = "Fred"; [ Next Section | Previous Chapter | Chapter Index | Main Index ] http://math.hws.edu/eck/cs124/javanotes4/c5/s1.html (6 of 6) [6/11/2004 11:09:54 AM] Java Programming: Section 5.2 Section 5.2 Constructors and Object Initialization OBJECT TYPES IN JAVA ARE VERY DIFFERENT from the primitive types. Simply declaring a variable whose type is given as a class does not automatically create an object of that class. Objects must be explicitly constructed. For the computer, the process of constructing an object means, first, finding some unused memory in the heap that can be used to hold the object and, second, filling in the object's instance variables. As a programmer, you don't care where in memory the object is stored, but you will usually want to exercise some control over what initial values are stored in a new object's instance variables. In many cases, you will also want to do more complicated initialization or bookkeeping every time an object is created. An instance variable can be assigned an initial value in its declaration, just like any other variable. For example, consider a class named PairOfDice. An object of this class will represent a pair of dice. It will contain two instance variables to represent the numbers showing on the dice and an instance method for rolling the dice: public class PairOfDice { public int die1 = 3; // Number showing on the first die. public int die2 = 4; // Number showing on the second die. public void roll() { // Roll the dice by setting each of the dice to be // a random number between 1 and 6. die1 = (int)(Math.random()*6) + 1; die2 = (int)(Math.random()*6) + 1; } } // end class PairOfDice The instance variables die1 and die2 are initialized to the values 3 and 4 respectively. These initializations are executed whenever a PairOfDice object is constructed. It's important to understand when and how this happens. There can be many PairOfDice objects. Each time one is created, it gets its own instance variables, and the assignments "die1 = 3" and "die2 = 4" are executed to fill in the values of those variables. To make this clearer, consider a variation of the PairOfDice class: public class PairOfDice { public int die1 = (int)(Math.random()*6) + 1; public int die2 = (int)(Math.random()*6) + 1; public void roll() { die1 = (int)(Math.random()*6) + 1; die2 = (int)(Math.random()*6) + 1; } } // end class PairOfDice Here, the dice are initialized to random values, as if a new pair of dice were being thrown onto the gaming table. Since the initialization is executed for each new object, a set of random initial values will be computed for each new pair of dice. Different pairs of dice can have different initial values. For initialization of static member variables, of course, the situation is quite different. There is only one copy of a static variable, and initialization of that variable is executed just once, when the class is first loaded. http://math.hws.edu/eck/cs124/javanotes4/c5/s2.html (1 of 7) [6/11/2004 11:09:55 AM] Java Programming: Section 5.2 If you don't provide any initial value for an instance variable, a default initial value is provided automatically. Instance variables of numerical type (int, double, etc.) are automatically initialized to zero if you provide no other values; boolean variables are initialized to false; and char variables, to the Unicode character with code number zero. An instance variable can also be a variable of object type. For such variables, the default initial value is null. (In particular, since Strings are objects, the default initial value for String variables is null.) Objects are created with the operator, new. For example, a program that wants to use a PairOfDice object could say: PairOfDice dice; // Declare a variable of type PairOfDice. dice = new PairOfDice(); // Construct a new object and store a // reference to it in the variable. In this example, "new PairOfDice()" is an expression that allocates memory for the object, initializes the object's instance variables, and then returns a reference to the object. This reference is the value of the expression, and that value is stored by the assignment statement in the variable, dice. Part of this expression, "PairOfDice()", looks like a subroutine call, and that is no accident. It is, in fact, a call to a special type of subroutine called a constructor. This might puzzle you, since there is no such subroutine in the class definition. However, every class has a constructor. If the programmer doesn't provide one, then the system will provide a default constructor. This default constructor does nothing beyond the basics: allocate memory and initialize instance variables. If you want more than that to happen when an object is created, you can include one or more constructors in the class definition. The definition of a constructor looks much like the definition of any other subroutine, with three exceptions. A constructor does not have any return type (not even void). The name of the constructor must be the same as the name of the class in which it is defined. The only modifiers that can be used on a constructor definition are the access modifiers public, private, and protected. (In particular, a constructor can't be declared static.) However, a constructor does have a subroutine body of the usual form, a block of statements. There are no restrictions on what statements can be used. And it can have a list of formal parameters. In fact, the ability to include parameters is one of the main reasons for using constructors. The parameters can provide data to be used in the construction of the object. For example, a constructor for the PairOfDice class could provide the values that are initially showing on the dice. Here is what the class would look like in that case: public class PairOfDice { public int die1; // Number showing on the first die. public int die2; // Number showing on the second die. public PairOfDice(int val1, int val2) { // Constructor. Creates a pair of dice that // are initially showing the values val1 and val2. die1 = val1; // Assign specified values die2 = val2; // to the instance variables. } public void roll() { // Roll the dice by setting each of the dice to be // a random number between 1 and 6. die1 = (int)(Math.random()*6) + 1; die2 = (int)(Math.random()*6) + 1; } } // end class PairOfDice http://math.hws.edu/eck/cs124/javanotes4/c5/s2.html (2 of 7) [6/11/2004 11:09:55 AM] Java Programming: Section 5.2 The constructor is declared as "public PairOfDice(int val1, int val2)...", with no return type and with the same name as the name of the class. This is how the Java compiler recognizes a constructor. The constructor has two parameters, and values for these parameters must be provided when the constructor is called. For example, the expression "new PairOfDice(3,4)" would create a PairOfDice object in which the values of the instance variables die1 and die2 are initially 3 and 4. Of course, in a program, the value returned by the constructor should be used in some way, as in PairOfDice dice; // Declare a variable of type PairOfDice. dice = new PairOfDice(1,1); // Let dice refer to a new PairOfDice // object that initially shows 1, 1. Now that we've added a constructor to the PairOfDice class, we can no longer create an object by saying "new PairOfDice()"! The system provides a default constructor for a class only if the class definition does not already include a constructor. However, this is not a big problem, since we can add a second constructor to the class, one that has no parameters. In fact, you can have as many different constructors as you want, as long as their signatures are different, that is, as long as they have different numbers or types of formal parameters. In the PairOfDice class, we might have a constructor with no parameters which produces a pair of dice showing random numbers: public class PairOfDice { public int die1; // Number showing on the first die. public int die2; // Number showing on the second die. public PairOfDice() { // Constructor. Rolls the dice, so that they initially // show some random values. roll(); // Call the roll() method to roll the dice. } public PairOfDice(int val1, int val2) { // Constructor. Creates a pair of dice that // are initially showing the values val1 and val2. die1 = val1; // Assign specified values die2 = val2; // to the instance variables. } public void roll() { // Roll the dice by setting each of the dice to be // a random number between 1 and 6. die1 = (int)(Math.random()*6) + 1; die2 = (int)(Math.random()*6) + 1; } } // end class PairOfDice Now we have the option of constructing a PairOfDice object either with "new PairOfDice()" or with "new PairOfDice(x,y)", where x and y are int-valued expressions. This class, once it is written, can be used in any program that needs to work with one or more pairs of dice. None of those programs will ever have to use the obscure incantation "(int)(Math.random()*6)+1", because it's done inside the PairOfDice class. And the programmer, having once gotten the dice-rolling thing straight will never have to worry about it again. Here, for example, is a main program that uses the PairOfDice class to count how many times two pairs of dice are rolled before the two pairs come up showing the same value: http://math.hws.edu/eck/cs124/javanotes4/c5/s2.html (3 of 7) [6/11/2004 11:09:55 AM] Java Programming: Section 5.2 public class RollTwoPairs { public static void main(String[] args) { PairOfDice firstDice; // Refers to the first pair of dice. firstDice = new PairOfDice(); PairOfDice secondDice; // Refers to the second pair of dice. secondDice = new PairOfDice(); int countRolls; // Counts how many times the two pairs of // dice have been rolled. int total1; // Total showing on first pair of dice. int total2; // Total showing on second pair of dice. countRolls = 0; do { // Roll the two pairs of dice until totals are the same. firstDice.roll(); // Roll the first pair of dice. total1 = firstDice.die1 + firstDice.die2; // Get total. System.out.println("First pair comes up " + total1); secondDice.roll(); // Roll the second pair of dice. total2 = secondDice.die1 + secondDice.die2; // Get total. System.out.println("Second pair comes up " + total2); countRolls++; // Count this roll. System.out.println(); // Blank line. } while (total1 != total2); System.out.println("It took " + countRolls + " rolls until the totals were the same."); } // end main() } // end class RollTwoPairs This applet simulates this program: (Applet "RollTwoPairsConsole" would be displayed here if Java were available.) Constructors are subroutines, but they are subroutines of a special type. They are certainly not instance methods, since they don't belong to objects. Since they are responsible for creating objects, they exist before any objects have been created. They are more like static member subroutines, but they are not and cannot be declared to be static. In fact, according to the Java language specification, they are technically not members of the class at all! Unlike other subroutines, a constructor can only be called using the new operator, in an expression that has the form http://math.hws.edu/eck/cs124/javanotes4/c5/s2.html (4 of 7) [6/11/2004 11:09:55 AM] Java Programming: Section 5.2 new class-name ( parameter-list ) where the parameter-list is possibly empty. I call this an expression because it computes and returns a value, namely a reference to the object that is constructed. Most often, you will store the returned reference in a variable, but it is also legal to use a constructor call in other ways, for example as a parameter in a subroutine call or as part of a more complex expression. Of course, if you don't save the reference in a variable, you won't have any way of referring to the object that was just created. A constructor call is more complicated than an ordinary subroutine or function call. It is helpful to understand the exact steps that the computer goes through to execute a constructor call: 1. First, the computer gets a block of unused memory in the heap, large enough to hold an object of the specified type. 2. It initializes the instance variables of the object. If the declaration of an instance variable specifies an initial value, then that value is computed and stored in the instance variable. Otherwise, the default initial value is used. 3. The actual parameters in the constructor, if any, are evaluated, and the values are assigned to the formal parameters of the constructor. 4. The statements in the body of the constructor, if any, are executed. 5. A reference to the object is returned as the value of the constructor call. The end result of this is that you have a reference to a newly constructed object. You can use this reference to get at the instance variables in that object or to call its instance methods. For another example, let's rewrite the Student class that was used in Section 1. I'll add a constructor, and I'll also take the opportunity to make the instance variable, name, private. public class Student { private String name; // Student's name. public double test1, test2, test3; // Grades on three tests. Student(String theName) { // Constructor for Student objects; // provides a name for the Student. name = theName; } public String getName() { // Accessor method for reading value of private // instance variable, name. return name; } public double getAverage() { // Compute average test grade. return (test1 + test2 + test3) / 3; } } // end of class Student An object of type Student contains information about some particular student. The constructor in this class has a parameter of type String, which specifies the name of that student. Objects of type Student can be created with statements such as: std = new Student("John Smith"); http://math.hws.edu/eck/cs124/javanotes4/c5/s2.html (5 of 7) [6/11/2004 11:09:55 AM] Java Programming: Section 5.2 std1 = new Student("Mary Jones"); In the original version of this class, the value of name had to be assigned by a program after it created the object of type Student. There was no guarantee that the programmer would always remember to set the name properly. In the new version of the class, there is no way to create a Student object except by calling the constructor, and that constructor automatically sets the name. The programmer's life is made easier, and whole hordes of frustrating bugs are squashed before they even have a chance to be born. Another type of guarantee is provided by the private modifier. Since the instance variable, name, is private, there is no way for any part of the program outside the Student class to get at the name directly. The program sets the value of name, indirectly, when it calls the constructor. I've provided a function, getName(), that can be used from outside the class to find out the name of the student. But I haven't provided any way to change the name. Once a student object is created, it keeps the same name as long as it exists. Garbage Collection So far, this section has been about creating objects. What about destroying them? In Java, the destruction of objects takes place automatically. An object exists in the heap, and it can be accessed only through variables that hold references to the object. What should be done with an object if there are no variables that refer to it? Such things can happen. Consider the following two statements (though in reality, you'd never do anything like this): Student std = new Student("John Smith"); std = null; In the first line, a reference to a newly created Student object is stored in the variable std. But in the next line, the value of std is changed, and the reference to the Student object is gone. In fact, there are now no references whatsoever to that object stored in any variable. So there is no way for the program ever to use the object again. It might as well not exist. In fact, the memory occupied by the object should be reclaimed to be used for another purpose. Java uses a procedure called garbage collection to reclaim memory occupied by objects that are no longer accessible to a program. It is the responsibility of the system, not the programmer, to keep track of which objects are "garbage". In the above example, it was very easy to see that the Student object had become garbage. Usually, it's much harder. If an object has been used for a while, there might be several references to the object stored in several variables. The object doesn't become garbage until all those references have been dropped. In many other programming languages, it's the programmer's responsibility to delete the garbage. Unfortunately, keeping track of memory usage is very error-prone, and many serious program bugs are caused by such errors. A programmer might accidently delete an object even though there are still references to that object. This is called a dangling pointer error, and it leads to problems when the program tries to access an object that is no longer there. Another type of error is a memory leak, where a programmer neglects to delete objects that are no longer in use. This can lead to filling memory with objects that are completely inaccessible, and the program might run out of memory even though, in fact, large amounts of memory are being wasted. Because Java uses garbage collection, such errors are simply impossible. Garbage collection is an old idea and has been used in some programming languages since the 1960s. You might wonder why all languages don't use garbage collection. In the past, it was considered too slow and wasteful. However, research into garbage collection techniques combined with the incredible speed of modern computers have combined to make garbage collection feasible. Programmers should rejoice. http://math.hws.edu/eck/cs124/javanotes4/c5/s2.html (6 of 7) [6/11/2004 11:09:55 AM] Java Programming: Section 5.2 [ Next Section | Previous Section | Chapter Index | Main Index ] http://math.hws.edu/eck/cs124/javanotes4/c5/s2.html (7 of 7) [6/11/2004 11:09:55 AM] Java Programming: Section 5.3 Section 5.3 Programming with Objects THERE ARE SEVERAL WAYS in which object-oriented concepts can be applied to the process of designing and writing programs. The broadest of these is object-oriented analysis and design which applies an object-oriented methodology to the earliest stages of program development, during which the overall design of a program is created. Here, the idea is to identify things in the problem domain that can be modeled as objects. On another level, object-oriented programming encourages programmers to produce generalized software components that can be used in a wide variety of programming projects. Built-in Classes Although the focus of object-oriented programming is generally on the design and implementation of new classes, it's important not to forget that the designers of Java have already provided a large number of reusable classes. Some of these classes are meant to be extended to produce new classes, while others can be used directly to create useful objects. A true mastery of Java requires familiarity with the full range of built-in classes -- something that takes a lot of time and experience to develop. In the next chapter, we will begin the study of Java's GUI classes, and you will encounter other built-in classes throughout the remainder of this book. But let's take a moment to look at a few built-in classes that you might find useful. A string can be built up from smaller pieces using the + operator, but this is not very efficient. If str is a String and ch is a character, then executing the command "str = str + ch;" involves creating a whole new string that is a copy of str, with the value of ch appended onto the end. Copying the string takes some time. Building up a long string letter by letter would require a surprising amount of processing. The class java.lang.StringBuffer makes it possible to be efficient about building up a long string from a number of smaller pieces. Like a String, a StringBuffer contains a sequence of characters. However, it is possible to add new characters onto the end of a StringBuffer without making a copy of the data that it already contains. If buffer is a variable of type StringBuffer and x is a value of any type, then the command buffer.append(x) will add x, converted into a string representation, onto the end of the data that was already in the buffer. This command actually modifies the buffer, rather than making a copy, and that can be done efficiently. A long string can be built up in a StringBuffer using a sequence of append() commands. When the string is complete, the function buffer.toString() will return a copy of the string in the buffer as an ordinary value of type String. A number of useful classes are collected in the package java.util. For example, this package contains classes for working with collections of objects (one of the contexts in which wrapper classes for primitive types are useful). We will study these collection classes in Chapter 12. The class java.util.Date is used to represent times. When a Date object is constructed without parameters, the result represents the current date and time, so an easy way to display this information is: System.out.println( new Date() ); Of course, to use the Date class in this way, you must make it available by importing it with one of the statements "import java.util.Date;" or "import java.util.*;" at the beginning of your program. (See Section 4.5 for a discussion of packages and import.) Finally, I will mention the class java.util.Random. An object belonging to this class is a source of random numbers. (The standard function Math.random() uses one of these objects behind the scenes to generate its random numbers.) An object of type Random can generate random integers, as well as random real numbers. If randGen is created with the command: http://math.hws.edu/eck/cs124/javanotes4/c5/s3.html (1 of 11) [6/11/2004 11:09:56 AM] Java Programming: Section 5.3 Random randGen = new Random(); and if N is a positive integer, then randGen.nextInt(N) generates a random integer in the range from 0 to N-1. For example, this makes it a little easier to roll a pair of dice. Instead of saying "die1 = (int)(6*Math.random())+1;", one can say "die1 = randGen.nextInt(6)+1;". (Since you also have to import the class java.util.Random and create the Random object, you might not agree that it is actually easier.) The main point here, again, is that many problems have already been solved, and the solutions are available in Java's standard classes. If you are faced with a task that looks like it should be fairly common, it might be worth looking through a Java reference to see whether someone has already written a subroutine that you can use. Generalized Software Components Every programmer builds up a stock of techniques and expertise expressed as snippets of code that can be reused in new programs using the tried-and-true method of cut-and-paste: The old code is physically copied into the new program and then edited to customize it as necessary. The problem is that the editing is error-prone and time-consuming, and the whole enterprise is dependent on the programmer's ability to pull out that particular piece of code from last year's project that looks like it might be made to fit. (On the level of a corporation that wants to save money by not reinventing the wheel for each new project, just keeping track of all the old wheels becomes a major task.) Well-designed classes are software components that can be reused without editing. A well-designed class is not carefully crafted to do a particular job in a particular program. Instead, it is crafted to model some particular type of object or a single coherent concept. Since objects and concepts can recur in many problems, a well-designed class is likely to be reusable without modification in a variety of projects. Furthermore, in an object-oriented programming language, it is possible to make subclasses of an existing class. This makes classes even more reusable. If a class needs to be customized, a subclass can be created, and additions or modifications can be made in the subclass without making any changes to the original class. This can be done even if the programmer doesn't have access to the source code of the class and doesn't know any details of its internal, hidden implementation. We will discuss subclasses in the next section. Object-oriented Analysis and Design A large programming project goes through a number of stages, starting with specification of the problem to be solved, followed by analysis of the problem and design of a program to solve it. Then comes coding, in which the program's design is expressed in some actual programming language. This is followed by testing and debugging of the program. After that comes a long period of maintenance, which means fixing any new problems that are found in the program and modifying it to adapt it to changing requirements. Together, these stages form what is called the software life cycle. (In the real world, the ideal of consecutive stages is seldom if ever achieved. During the analysis stage, it might turn out that the specifications are incomplete or inconsistent. A problem found during testing requires at least a brief return to the coding stage. If the problem is serious enough, it might even require a new design. Maintenance usually involves redoing some of the work from previous stages....) Large, complex programming projects are only likely to succeed if a careful, systematic approach is adopted during all stages of the software life cycle. The systematic approach to programming, using accepted principles of good design, is called software engineering. The software engineer tries to efficiently construct programs that verifyably meet their specifications and that are easy to modify if necessary. There is a wide range of "methodologies" that can be applied to help in the systematic design of programs. (Most http://math.hws.edu/eck/cs124/javanotes4/c5/s3.html (2 of 11) [6/11/2004 11:09:56 AM] Java Programming: Section 5.3 of these methodologies seem to involve drawing little boxes to represent program components, with labeled arrows to represent relationships among the boxes.) We have been discussing object orientation in programming languages, which is relevant to the coding stage of program development. But there are also object-oriented methodologies for analysis and design. The question in this stage of the software life cycle is, How can one discover or invent the overall structure of a program? As an example of a rather simple object-oriented approach to analysis and design, consider this advice: Write down a description of the problem. Underline all the nouns in that description. The nouns should be considered as candidates for becoming classes or objects in the program design. Similarly, underline all the verbs. These are candidates for methods. This is your starting point. Further analysis might uncover the need for more classes and methods, and it might reveal that subclassing can be used to take advantage of similarities among classes. This is perhaps a bit simple-minded, but the idea is clear and the general approach can be effective: Analyze the problem to discover the concepts that are involved, and create classes to represent those concepts. The design should arise from the problem itself, and you should end up with a program whose structure reflects the structure of the problem in a natural way. Programming Examples The PairOfDice class in the previous section is already an example of a generalized software component, although one that could certainly be improved. The class represents a single, coherent concept, "a pair of dice." The instance variables hold the data relevant to the state of the dice, that is, the number showing on each of the dice. The instance method represents the behaviour of a pair of dice, that is, the ability to be rolled. This class would be reusable in many different programming projects. On the other hand, the Student class from the previous section is not very reusable. It seems to be crafted to represent students in a particular course where the grade will be based on three tests. If there are more tests or quizzes or papers, it's useless. If there are two people in the class who have the same name, we are in trouble (one reason why numerical student ID's are often used). Admittedly, it's much more difficult to develop a general-purpose student class than a general-purpose pair-of-dice class. But this particular Student class is good mostly as an example in a programming textbook. Let's do another example in a domain that is simple enough that we have a chance of coming up with something reasonably reusable. Consider card games that are played with a standard deck of playing cards (a so-called "poker" deck, since it is used in the game of poker). In a typical card game, each player gets a hand of cards. The deck is shuffled and cards are dealt one at a time from the deck and added to the players' hands. In some games, cards can be removed from a hand, and new cards can be added. The game is won or lost depending on the value (ace, 2, ..., king) and suit (spades, diamonds, clubs, hearts) of the cards that a player receives. If we look for nouns in this description, there are several candidates for objects: game, player, hand, card, deck, value, and suit. Of these, the value and the suit of a card are simple values, and they will just be represented as instance variables in a Card object. In a complete program, the other five nouns might be represented by classes. But let's work on the ones that are most obviously reusable: card, hand, and deck. If we look for verbs in the description of a card game, we see that we can shuffle a deck and deal a card from a deck. This gives use us two candidates for instance methods in a Deck class. Cards can be added to and removed from hands. This gives two candidates for instance methods in a Hand class. Cards are relatively passive things, but we need to be able to determine their suits and values. We will discover more instance methods as we go along. First, we'll design the deck class in detail. When a deck of cards is first created, it contains 52 cards in some standard order. The Deck class will need a constructor to create a new deck. The constructor needs no parameters because any new deck is the same as any other. There will be an instance method called shuffle() that will rearrange the 52 cards into a random order. The dealCard() instance method will http://math.hws.edu/eck/cs124/javanotes4/c5/s3.html (3 of 11) [6/11/2004 11:09:56 AM] Java Programming: Section 5.3 get the next card from the deck. This will be a function with a return type of Card, since the caller needs to know what card is being dealt. It has no parameters. What will happen if there are no more cards in the deck when its dealCard() method is called? It should probably be considered an error to try to deal a card from an empty deck. But this raises another question: How will the rest of the program know whether the deck is empty? Of course, the program could keep track of how many cards it has used. But the deck itself should know how many cards it has left, so the program should just be able to ask the deck object. We can make this possible by specifying another instance method, cardsLeft(), that returns the number of cards remaining in the deck. This leads to a full specification of all the subroutines in the Deck class: Constructor and instance methods in class Deck: public Deck() // Constructor. Create an unshuffled deck of cards. public void shuffle() // Put all the used cards back into the deck, // and shuffle it into a random order. public int cardsLeft() // As cards are dealt from the deck, the number of // cards left decreases. This function returns the // number of cards that are still left in the deck. public Card dealCard() // Deals one card from the deck and returns it. This is everything you need to know in order to use the Deck class. Of course, it doesn't tell us how to write the class. This has been an exercise in design, not in programming. In fact, writing the class involves a programming technique, arrays, which will not be covered until Chapter 8. Nevertheless, you can look at the source code, Deck.java, if you want. And given the source code, you can use the class in your programs without understanding the implementation. We can do a similar analysis for the Hand class. When a hand object is first created, it has no cards in it. An addCard() instance method will add a card to the hand. This method needs a parameter of type Card to specify which card is being added. For the removeCard() method, a parameter is needed to specify which card to remove. But should we specify the card itself ("Remove the ace of spades"), or should we specify the card by its position in the hand ("Remove the third card in the hand")? Actually, we don't have to decide, since we can allow for both options. We'll have two removeCard() instance methods, one with a parameter of type Card specifying the card to be removed and one with a parameter of type int specifying the position of the card in the hand. (Remember that you can have two methods in a class with the same name, provided they have different types of parameters.) Since a hand can contain a variable number of cards, it's convenient to be able to ask a hand object how many cards it contains. So, we need an instance method getCardCount() that returns the number of cards in the hand. When I play cards, I like to arrange the cards in my hand so that cards of the same value are next to each other. Since this is a generally useful thing to be able to do, we can provide instance methods for sorting the cards in the hand. Here is a full specification for a reusable Hand class: Constructor and instance methods in class Hand: public Hand() { // Create a Hand object that is initially empty. public void clear() { // Discard all cards from the hand, making the hand empty. http://math.hws.edu/eck/cs124/javanotes4/c5/s3.html (4 of 11) [6/11/2004 11:09:56 AM] Java Programming: Section 5.3 public void addCard(Card c) { // Add the card c to the hand. c should be non-null. // (If c is null, nothing is added to the hand.) public void removeCard(Card c) { // If the specified card is in the hand, it is removed. public void removeCard(int position) { // If the specified position is a valid position in the // hand, then the card in that position is removed. public int getCardCount() { // Return the number of cards in the hand. public Card getCard(int position) { // Get the card from the hand in given position, where // positions are numbered starting from 0. If the // specified position is not the position number of // a card in the hand, then null is returned. public void sortBySuit() { // Sorts the cards in the hand so that cards of the same // suit are grouped together, and within a suit the cards // are sorted by value. Note that aces are considered // to have the lowest value, 1. public void sortByValue() { // Sorts the cards in the hand so that cards of the same // value are grouped together. Cards with the same value // are sorted by suit. Note that aces are considered // to have the lowest value, 1. Again, you don't yet know enough to implement this class. But given the source code, Hand.java, you can use the class in your own programming projects. We have covered enough material to write a Card class. The class will have a constructor that specifies the value and suit of the card that is being created. There are four suits, which can be represented by the integers 0, 1, 2, and 3. It would be tough to remember which number represents which suit, so I've defined named constants in the Card class to represent the four possibilities. For example, Card.SPADES is a constant that represents the suit, spades. (These constants are declared to be public final static ints. This is one case in which it makes sense to have static members in a class that otherwise has only instance variables and instance methods.) The possible values of a card are the numbers 1, 2, ..., 13, with 1 standing for an ace, 11 for a jack, 12 for a queen, and 13 for a king. Again, I've defined some named constants to represent the values of aces and face cards. So, cards can be constructed by statements such as: card1 = new Card( Card.ACE, Card.SPADES ); // Construct ace of spades. card2 = new Card( 10, Card.DIAMONDS ); // Construct 10 of diamonds. card3 = new Card( v, s ); // This is OK, as long as v and s // are integer expressions. A Card object needs instance variables to represent its value and suit. I've made these private so that they cannot be changed from outside the class, and I've provided instance methods getSuit() and getValue() so that it will be possible to discover the suit and value from outside the class. The instance variables are initialized in the constructor, and are never changed after that. In fact, I've declared the http://math.hws.edu/eck/cs124/javanotes4/c5/s3.html (5 of 11) [6/11/2004 11:09:56 AM] Java Programming: Section 5.3 instance variables suit and value to be final, since they are never changed after they are initialized. (An instance variable can be declared final provided it is either given an initial value in its declaration or is initialized in every constructor in the class.) Finally, I've added a few convenience methods to the class to make it easier to print out cards in a human-readable form. For example, I want to be able to print out the suit of a card as the word "Diamonds", rather than as the meaningless code number 2, which is used in the class to represent diamonds. Since this is something that I'll probably have to do in many programs, it makes sense to include support for it in the class. So, I've provided instance methods getSuitAsString() and getValueAsString() to return string representations of the suit and value of a card. Finally, there is an instance method toString() that returns a string with both the value and suit, such as "Queen of Hearts". There is a good reason for calling this method toString(). When any object is output with System.out.print(), the object's toString() method is called to produce the string representation of the object. For example, if card refers to an object of type Card, then System.out.println(card) is equivalent to System.out.println(card.toString()). Similarly, if an object is appended to a string using the + operator, the object's toSring() method is used. Thus, System.out.println( "Your card is the " + card ); is equivalent to System.out.println( "Your card is the " + card.toString() ); If the card is the queen of hearts, either of these will print out "Your card is the Queen of Hearts". Here is the complete Card class. It is general enough to be highly reusable, so the work that went into designing, writing, and testing it pays off handsomely in the long run. /* An object of class card represents one of the 52 cards in a standard deck of playing cards. Each card has a suit and a value. */ public class Card { public final static int SPADES = 0, // Codes for the 4 suits. HEARTS = 1, DIAMONDS = 2, CLUBS = 3; public final static int ACE = 1, // Codes for non-numeric cards. JACK = 11, // Cards 2 through 10 have QUEEN = 12, // their numerical values KING = 13; // for their codes. private final int suit; // The suit of this card, one of the // four constants: SPADES, HEARTS, // DIAMONDS, CLUBS. private final int value; // The value of this card, from 1 to 13. public Card(int theValue, int theSuit) { // Construct a card with the specified value and suit. // Value must be between 1 and 13. Suit must be between // 0 and 3. If the parameters are outside these ranges, http://math.hws.edu/eck/cs124/javanotes4/c5/s3.html (6 of 11) [6/11/2004 11:09:56 AM] Java Programming: Section 5.3 // the constructed card object will be invalid. value = theValue; suit = theSuit; } public int getSuit() { // Return the int that codes for this card's suit. return suit; } public int getValue() { // Return the int that codes for this card's value. return value; } public String getSuitAsString() { // Return a String representing the card's suit. // (If the card's suit is invalid, "??" is returned.) switch ( suit ) { case SPADES: return "Spades"; case HEARTS: return "Hearts"; case DIAMONDS: return "Diamonds"; case CLUBS: return "Clubs"; default: return "??"; } } public String getValueAsString() { // Return a String representing the card's value. // If the card's value is invalid, "??" is returned. switch ( value ) { case 1: return "Ace"; case 2: return "2"; case 3: return "3"; case 4: return "4"; case 5: return "5"; case 6: return "6"; case 7: return "7"; case 8: return "8"; case 9: return "9"; case 10: return "10"; case 11: return "Jack"; case 12: return "Queen"; case 13: return "King"; default: return "??"; } } public String toString() { // Return a String representation of this card, such as // "10 of Hearts" or "Queen of Spades". return getValueAsString() + " of " + getSuitAsString(); } } // end class Card http://math.hws.edu/eck/cs124/javanotes4/c5/s3.html (7 of 11) [6/11/2004 11:09:57 AM] Java Programming: Section 5.3 I will finish this section by presenting a complete program that uses the Card and Deck classes. The program lets the user play a very simple card game called HighLow. A deck of cards is shuffled, and one card is dealt from the deck and shown to the user. The user predicts whether the next card from the deck will be higher or lower than the current card. If the user predicts correctly, then the next card from the deck becomes the current card, and the user makes another prediction. This continues until the user makes an incorrect prediction. The number of correct predictions is the user's score. My program has a subroutine that plays one game of HighLow. This subroutine has a return value that represents the user's score in the game. The main() routine lets the user play several games of HighLow. At the end, it reports the user's average score. I won't go through the development of the algorithms used in this program, but I encourage you to read it carefully and make sure that you understand how it works. Here is the program: /* This program lets the user play HighLow, a simple card game that is described in the output statements at the beginning of the main() routine. After the user plays several games, the user's average score is reported. */ public class HighLow { public static void main(String[] args) { TextIO.putln("This program lets you play the simple card game,"); TextIO.putln("HighLow. A card is dealt from a deck of cards."); TextIO.putln("You have to predict whether the next card will be"); TextIO.putln("higher or lower. Your score in the game is the"); TextIO.putln("number of correct predictions you make before"); TextIO.putln("you guess wrong."); TextIO.putln(); int gamesPlayed = 0; // Number of games user has played. int sumOfScores = 0; // The sum of all the scores from // all the games played. double averageScore; // Average score, computed by dividing // sumOfScores by gamesPlayed. boolean playAgain; // Record user's response when user is // asked whether he wants to play // another game. do { int scoreThisGame; // Score for one game. scoreThisGame = play(); // Play the game and get the score. sumOfScores += scoreThisGame; gamesPlayed++; TextIO.put("Play again? "); playAgain = TextIO.getlnBoolean(); } while (playAgain); http://math.hws.edu/eck/cs124/javanotes4/c5/s3.html (8 of 11) [6/11/2004 11:09:57 AM] Java Programming: Section 5.3 averageScore = ((double)sumOfScores) / gamesPlayed; TextIO.putln(); TextIO.putln("You played " + gamesPlayed + " games."); TextIO.putln("Your average score was " + averageScore); } // end main() static int play() { // Lets the user play one game of HighLow, and returns the // user's score in the game. Deck deck = new Deck(); // Get a new deck of cards, and // store a reference to it in // the variable, Deck. Card currentCard; // The current card, which the user sees. Card nextCard; // The next card in the deck. The user tries // to predict whether this is higher or lower // than the current card. int correctGuesses ; // The number of correct predictions the // user has made. At the end of the game, // this will be the user's score. char guess; // The user's guess. 'H' if the user predicts that // the next card will be higher, 'L' if the user // predicts that it will be lower. deck.shuffle(); correctGuesses = 0; currentCard = deck.dealCard(); TextIO.putln("The first card is the " + currentCard); while (true) { // Loop ends when user's prediction is wrong. /* Get the user's prediction, 'H' or 'L'. */ TextIO.put("Will the next card be higher (H) or lower (L)? "); do { guess = TextIO.getlnChar(); guess = Character.toUpperCase(guess); if (guess != 'H' && guess != 'L') TextIO.put("Please respond with H or L: "); } while (guess != 'H' && guess != 'L'); /* Get the next card and show it to the user. */ nextCard = deck.dealCard(); TextIO.putln("The next card is " + nextCard); /* Check the user's prediction. */ http://math.hws.edu/eck/cs124/javanotes4/c5/s3.html (9 of 11) [6/11/2004 11:09:57 AM] Java Programming: Section 5.3 if (nextCard.getValue() == currentCard.getValue()) { TextIO.putln("The value is the same as the previous card."); TextIO.putln("You lose on ties. Sorry!"); break; // End the game. } else if (nextCard.getValue() > currentCard.getValue()) { if (guess == 'H') { TextIO.putln("Your prediction was correct."); correctGuesses++; } else { TextIO.putln("Your prediction was incorrect."); break; // End the game. } } else { // nextCard is lower if (guess == 'L') { TextIO.putln("Your prediction was correct."); correctGuesses++; } else { TextIO.putln("Your prediction was incorrect."); break; // End the game. } } /* To set up for the next iteration of the loop, the nextCard becomes the currentCard, since the currentCard has to be the card that the user sees, and the nextCard will be set to the next card in the deck after the user makes his prediction. */ currentCard = nextCard; TextIO.putln(); TextIO.putln("The card is " + currentCard); } // end of while loop TextIO.putln(); TextIO.putln("The game is over."); TextIO.putln("You made " + correctGuesses + " correct predictions."); TextIO.putln(); return correctGuesses; } // end play() } // end class HighLow Here is an applet that simulates the program: (Applet "HighlLowConsole" would be displayed here if Java were available.) http://math.hws.edu/eck/cs124/javanotes4/c5/s3.html (10 of 11) [6/11/2004 11:09:57 AM] Java Programming: Section 5.3 [ Next Section | Previous Section | Chapter Index | Main Index ] http://math.hws.edu/eck/cs124/javanotes4/c5/s3.html (11 of 11) [6/11/2004 11:09:57 AM] Java Programming: Section 5.4 Section 5.4 Inheritance, Polymorphism, and Abstract Classes A CLASS REPRESENTS A SET OF OBJECTS which share the same structure and behaviors. The class determines the structure of objects by specifying variables that are contained in each instance of the class, and it determines behavior by providing the instance methods that express the behavior of the objects. This is a powerful idea. However, something like this can be done in most programming languages. The central new idea in object-oriented programming -- the idea that really distinguishes it from traditional programming -- is to allow classes to express the similarities among objects that share some, but not all, of their structure and behavior. Such similarities can be expressed using inheritance and polymorphism. The topics covered in this section are relatively advanced aspects of object-oriented programming. Any programmer should know what is meant by subclass, inheritance, and polymorphism. However, it will probably be a while before you actually do anything with inheritance except for extending classes that already exist. The term inheritance refers to the fact that one class can inherit part or all of its structure and behavior from another class. The class that does the inheriting is said to be a subclass of the class from which it inherits. If class B is a subclass of class A, we also say that class A is a superclass of class B. (Sometimes the terms derived class and base class are used instead of subclass and superclass.) A subclass can add to the structure and behavior that it inherits. It can also replace or modify inherited behavior (though not inherited structure). The relationship between subclass and superclass is sometimes shown by a diagram in which the subclass is shown below, and connected to, its superclass. In Java, when you create a new class, you can declare that it is a subclass of an existing class. If you are defining a class named "B" and you want it to be a subclass of a class named "A", you would write class B extends A { . . // additions to, and modifications of, . // stuff inherited from class A . } Several classes can be declared as subclasses of the same superclass. The subclasses, which might be referred to as "sibling classes," share some structures and behaviors -- namely, the ones they inherit from their common superclass. The superclass expresses these shared structures and behaviors. In the diagram to the left, classes B, C, and D are sibling classes. Inheritance can also extend over several "generations" of classes. This is shown in the diagram, where class E is a subclass of class D which is itself a subclass of class A. In this case, class E is considered to be a subclass of class A, even though it is not a direct subclass. http://math.hws.edu/eck/cs124/javanotes4/c5/s4.html (1 of 10) [6/11/2004 11:09:58 AM] Java Programming: Section 5.4 Let's look at an example. Suppose that a program has to deal with motor vehicles, including cars, trucks, and motorcycles. (This might be a program used by a Department of Motor Vehicles to keep track of registrations.) The program could use a class named Vehicle to represent all types of vehicles. The Vehicle class could include instance variables such as registrationNumber and owner and instance methods such as transferOwnership(). These are variables and methods common to all vehicles. Three subclasses of Vehicle -- Car, Truck, and Motorcycle -- could then be used to hold variables and methods specific to particular types of vehicles. The Car class might add an instance variable numberOfDoors, the Truck class might have numberOfAxels, and the Motorcycle class could have a boolean variable hasSidecar. (Well, it could in theory at least, even if it might give a chuckle to the people at the Department of Motor Vehicles.) The declarations of these classes in Java program would look, in outline, like this: class Vehicle { int registrationNumber; Person owner; // (assuming that a Person class has been defined) void transferOwnership(Person newOwner) { . . . } . . . } class Car extends Vehicle { int numberOfDoors; . . . } class Truck extends Vehicle { int numberOfAxels; . . . } class Motorcycle extends Vehicle { boolean hasSidecar; . . . } Suppose that myCar is a variable of type Car that has been declared and initialized with the statement Car myCar = new Car(); (Note that, as with any variable, it is OK to declare a variable and initialize it in a single statement. This is equivalent to the declaration "Car myCar;" followed by the assignment statement "myCar = new Car();".) Given this declaration, a program could refer to myCar.numberOfDoors, since numberOfDoors is an instance variable in the class Car. But since class Car extends class Vehicle, a car also has all the structure and behavior of a vehicle. This means that myCar.registrationNumber, myCar.owner, and myCar.transferOwnership() also exist. Now, in the real world, cars, trucks, and motorcycles are in fact vehicles. The same is true in a program. That is, an object of type Car or Truck or Motorcycle is automatically an object of type Vehicle. This brings us to the following Important Fact: A variable that can hold a reference to an object of class A can also hold a reference to an object belonging to any subclass of A. The practical effect of this in our example is that an object of type Car can be assigned to a variable of type http://math.hws.edu/eck/cs124/javanotes4/c5/s4.html (2 of 10) [6/11/2004 11:09:58 AM] Java Programming: Section 5.4 Vehicle. That is, it would be legal to say Vehicle myVehicle = myCar; or even Vehicle myVehicle = new Car(); After either of these statements, the variable myVehicle holds a reference to a Vehicle object that happens to be an instance of the subclass, Car. The object "remembers" that it is in fact a Car, and not just a Vehicle. Information about the actual class of an object is stored as part of that object. It is even possible to test whether a given object belongs to a given class, using the instanceof operator. The test: if (myVehicle instanceof Car) ... determines whether the object referred to by myVehicle is in fact a car. On the other hand, the assignment statement myCar = myVehicle; would be illegal because myVehicle could potentially refer to other types of vehicles that are not cars. This is similar to a problem we saw previously in Section 2.5: The computer will not allow you to assign an int value to a variable of type short, because not every int is a short. Similarly, it will not allow you to assign a value of type Vehicle to a variable of type Car because not every vehicle is a car. As in the case of ints and shorts, the solution here is to use type-casting. If, for some reason, you happen to know that myVehicle does in fact refer to a Car, you can use the type cast (Car)myVehicle to tell the computer to treat myVehicle as if it were actually of type Car. So, you could say myCar = (Car)myVehicle; and you could even refer to ((Car)myVehicle).numberOfDoors. As an example of how this could be used in a program, suppose that you want to print out relevant data about a vehicle. You could say: System.out.println("Vehicle Data:"); System.out.println("Registration number: " + myVehicle.registrationNumber); if (myVehicle instanceof Car) { System.out.println("Type of vehicle: Car"); Car c; c = (Car)myVehicle; System.out.println("Number of doors: " + c.numberOfDoors); } else if (myVehicle instanceof Truck) { System.out.println("Type of vehicle: Truck"); Truck t; t = (Truck)myVehicle; System.out.println("Number of axels: " + t.numberOfAxels); } else if (myVehicle instanceof Motorcycle) { System.out.println("Type of vehicle: Motorcycle"); Motorcycle m; m = (Motorcycle)myVehicle; System.out.println("Has a sidecar: " + m.hasSidecar); } Note that for object types, when the computer executes a program, it checks whether type-casts are valid. So, for example, if myVehicle refers to an object of type Truck, then the type cast (Car)myVehicle http://math.hws.edu/eck/cs124/javanotes4/c5/s4.html (3 of 10) [6/11/2004 11:09:58 AM] Java Programming: Section 5.4 will produce an error. As another example, consider a program that deals with shapes drawn on the screen. Let's say that the shapes include rectangles, ovals, and roundrects of various colors. Three classes, Rectangle, Oval, and RoundRect, could be used to represent the three types of shapes. These three classes would have a common superclass, Shape, to represent features that all three shapes have in common. The Shape class could include instance variables to represent the color, position, and size of a shape. It could include instance methods for changing the color, position, and size of a shape. Changing the color, for example, might involve changing the value of an instance variable, and then redrawing the shape in its new color: class Shape { Color color; // Color of the shape. (Recall that class Color // is defined in package java.awt. Assume // that this class has been imported.) void setColor(Color newColor) { // Method to change the color of the shape. color = newColor; // change value of instance variable redraw(); // redraw shape, which will appear in new color } void redraw() { // method for drawing the shape ? ? ? // what commands should go here? } . . . // more instance variables and methods } // end of class Shape Now, you might see a problem here with the method redraw(). The problem is that each different type of shape is drawn differently. The method setColor() can be called for any type of shape. How does the computer know which shape to draw when it executes the redraw()? Informally, we can answer the question like this: The computer executes redraw() by asking the shape to redraw itself. Every shape object knows what it has to do to redraw itself. In practice, this means that each of the specific shape classes has its own redraw() method: class Rectangle extends Shape { void redraw() { . . . // commands for drawing a rectangle } . . . // possibly, more methods and variables } http://math.hws.edu/eck/cs124/javanotes4/c5/s4.html (4 of 10) [6/11/2004 11:09:58 AM] Java Programming: Section 5.4 class Oval extends Shape { void redraw() { . . . // commands for drawing an oval } . . . // possibly, more methods and variables } class RoundRect extends Shape { void redraw() { . . . // commands for drawing a rounded rectangle } . . . // possibly, more methods and variables } If oneShape is a variable of type Shape, it could refer to an object of any of the types, Rectangle, Oval, or RoundRect. As a program executes, and the value of oneShape changes, it could even refer to objects of different types at different times! Whenever the statement oneShape.redraw(); is executed, the redraw method that is actually called is the one appropriate for the type of object to which oneShape actually refers. There may be no way of telling, from looking at the text of the program, what shape this statement will draw, since it depends on the value that oneShape happens to have when the program is executed. Even more is true. Suppose the statement is in a loop and gets executed many times. If the value of oneShape changes as the loop is executed, it is possible that the very same statement "oneShape.redraw();" will call different methods and draw different shapes as it is executed over and over. We say that the redraw() method is polymorphic. A method is polymorphic if the action performed by the method depends on the actual type of the object to which the method is applied. Polymorphism is one of the major distinguishing features of object-oriented programming. Perhaps this becomes more understandable if we change our terminology a bit: In object-oriented programming, calling a method is often referred to as sending a message to an object. The object responds to the message by executing the appropriate method. The statement "oneShape.redraw();" is a message to the object referred to by oneShape. Since that object knows what type of object it is, it knows how it should respond to the message. From this point of view, the computer always executes "oneShape.redraw();" in the same way: by sending a message. The response to the message depends, naturally, on who receives it. From this point of view, objects are active entities that send and receive messages, and polymorphism is a natural, even necessary, part of this view. Polymorphism just means that different objects can respond to the same message in different ways. One of the most beautiful things about polymorphism is that it lets code that you write do things that you didn't even conceive of, at the time you wrote it. If for some reason, I decide that I want to add beveled rectangles to the types of shapes my program can deal with, I can write a new subclass, BeveledRect, of class Shape and give it its own redraw() method. Automatically, code that I wrote previously -- such as the statement oneShape.redraw() -- can now suddenly start drawing beveled rectangles, even though the beveled rectangle class didn't exist when I wrote the statement! In the statement "oneShape.redraw();", the redraw message is sent to the object oneShape. Look back at the method from the Shape class for changing the color of a shape: void setColor(Color newColor) { color = newColor; // change value of instance variable redraw(); // redraw shape, which will appear in new color http://math.hws.edu/eck/cs124/javanotes4/c5/s4.html (5 of 10) [6/11/2004 11:09:58 AM] Java Programming: Section 5.4 } A redraw message is sent here, but which object is it sent to? Well, the setColor method is itself a message that was sent to some object. The answer is that the redraw message is sent to that same object, the one that received the setColor message. If that object is a rectangle, then it is the redraw() method from the Rectangle class that is executed. If the object is an oval, then it is the redraw() method from the Oval class. This is what you should expect, but it means that the redraw(); statement in the setColor() method does not necessarily call the redraw() method in the Shape class! The redraw() method that is executed could be in any subclass of Shape. Again, this is not a real surprise if you think about it in the right way. Remember that an instance method is always contained in an object. The class only contains the source code for the method. When a Rectangle object is created, it contains a redraw() method. The source code for that method is in the Rectangle class. The object also contains a setColor() method. Since the Rectangle class does not define a setColor() method, the source code for the rectangle's setColor() method comes from the superclass, Shape. But even though the source codes for the two methods are in different classes, the methods themselves are part of the same object. When the rectangle's setColor() method is executed and calls redraw(), the redraw() method that is executed is the one in the same object. Whenever a Rectangle, Oval, or RoundRect object has to draw itself, it is the redraw() method in the appropriate class that is executed. This leaves open the question, What does the redraw() method in the Shape class do? How should it be defined? The answer may be surprising: We should leave it blank! The fact is that the class Shape represents the abstract idea of a shape, and there is no way to draw such a thing. Only particular, concrete shapes like rectangles and ovals can be drawn. So, why should there even be a redraw() method in the Shape class? Well, it has to be there, or it would be illegal to call it in the setColor() method of the Shape class, and it would be illegal to write "oneShape.redraw();", where oneShape is a variable of type Shape. The compiler would complain that oneShape is a variable of type Shape and there's no redraw() method in the Shape class. Nevertheless the version of redraw() in the Shape class will never be called. In fact, if you think about it, there can never be any reason to construct an actual object of type Shape! You can have variables of type Shape, but the objects they refer to will always belong to one of the subclasses of Shape. We say that Shape is an abstract class. An abstract class is one that is not used to construct objects, but only as a basis for making subclasses. An abstract class exists only to express the common properties of all its subclasses. Similarly, we say that the redraw() method in class Shape is an abstract method, since it is never meant to be called. In fact, there is nothing for it to do -- any actual redrawing is done by redraw() methods in the subclasses of Shape. The redraw() method in Shape has to be there. But it is there only to tell the computer that all Shapes understand the redraw message. As an abstract method, it exists merely to specify the common interface of all the actual, concrete versions of redraw() in the subclasses of Shape. There is no reason for the abstract redraw() in class Shape to contain any code at all. Shape and its redraw() method are semantically abstract. You can also tell the computer, syntactically, that they are abstract by adding the modifier "abstract" to their definitions. For an abstract method, the block of code that gives the implementation of an ordinary method is replaced by a semicolon. An implementation must be provided for the abstract method in any concrete subclass of the abstract class. Here's what the Shape class would look like as an abstract class: abstract class Shape { Color color; // color of shape. http://math.hws.edu/eck/cs124/javanotes4/c5/s4.html (6 of 10) [6/11/2004 11:09:58 AM] Java Programming: Section 5.4 void setColor(Color newColor) { // method to change the color of the shape color = newColor; // change value of instance variable redraw(); // redraw shape, which will appear in new color } abstract void redraw(); // abstract method -- must be defined in // concrete subclasses . . . // more instance variables and methods } // end of class Shape Once you have done this, it becomes illegal to try to create actual objects of type Shape, and the computer will report an error if you try to do so. In Java, every class that you declare has a superclass. If you don't specify a superclass, then the superclass is automatically taken to be Object, a predefined class that is part of the package java.lang. (The class Object itself has no superclass, but it is the only class that has this property.) Thus, class myClass { . . . is exactly equivalent to class myClass extends Object { . . . Every other class is, directly or indirectly, a subclass of Object. This means that any object, belonging to any class whatsoever, can be assigned to a variable of type Object. The class Object represents very general properties that are shared by all objects, belonging to any class. Object is the most abstract class of all! The Object class actually finds a use in some cases where objects of a very general sort are being manipulated. For example, java has a standard class, java.util.ArrayList, that represents a list of Objects. (The ArrayList class is in the package java.util. If you want to use this class in a program you write, you would ordinarily use an import statement to make it possible to use the short name, ArrayList, instead of the full name, java.util.ArrayList. ArrayList is discussed more fully in Section 8.3.) The ArrayList class is very convenient, because an ArrayList can hold any number of objects, and it will grow, when necessary, as objects are added to it. Since the items in the list are of type Object, the list can actually hold objects of any type. A program that wants to keep track of various Shapes that have been drawn on the screen can store those shapes in an ArrayList. Suppose that the ArrayList is named listOfShapes. A shape, oneShape, can be added to the end of the list by calling the instance method "listOfShapes.add(oneShape);". The shape could be removed from the list with "listOfShapes.remove(oneShape);". The number of shapes in the list is given by the function "listOfShapes.size()". And it is possible to retrieve the i-th object from the list with the function call "listOfShapes.get(i)". (Items in the list are numbered from 0 to listOfShapes.size() - 1.) However, note that this method returns an Object, not a Shape. (Of course, the people who wrote the ArrayList class didn't even know about Shapes, so the method they wrote could hardly have a return type of Shape!) Since you know that the items in the list are, in fact, Shapes and not just Objects, you can type-cast the Object returned by listOfShapes.get(i) to be a value of type Shape: oneShape = (Shape)listOfShapes.get(i); http://math.hws.edu/eck/cs124/javanotes4/c5/s4.html (7 of 10) [6/11/2004 11:09:59 AM] Java Programming: Section 5.4 Let's say, for example, that you want to redraw all the shapes in the list. You could do this with a simple for loop, which is lovely example of object-oriented programming and polymorphism: for (int i = 0; i < listOfShapes.size(); i++) { Shape s; // i-th element of the list, considered as a Shape s = (Shape)listOfShapes.get(i); s.redraw(); } It might be worthwhile to look at an applet that actually uses an abstract Shape class and an ArrayList to hold a list of shapes: (Applet "ShapeDraw" would be displayed here if Java were available.) If you click one of the buttons along the bottom of this applet, a shape will be added to the screen in the upper left corner of the applet. The color of the shape is given by the "pop-up menu" in the lower right. Once a shape is on the screen, you can drag it around with the mouse. A shape will maintain the same front-to-back order with respect to other shapes on the screen, even while you are dragging it. However, you can move a shape out in front of all the other shapes if you hold down the shift key as you click on it. In this applet the only time when the actual class of a shape is used is when that shape is added to the screen. Once the shape has been created, it is manipulated entirely as an abstract shape. The routine that implements dragging, for example, works only with variables of type Shape. As the Shape is being dragged, the dragging routine just calls the Shape's draw method each time the shape has to be drawn, so it doesn't have to know how to draw the shape or even what type of shape it is. The object is responsible for drawing itself. If I wanted to add a new type of shape to the program, I would define a new subclass of Shape, add another button to the applet, and program the button to add the correct type of shape to the screen. No other changes in the programming would be necessary. You might want to look at the source code for this applet, ShapeDraw.java, even though you won't be able to understand all of it at this time. Even the definitions of the shape classes are somewhat different from those I described in this section. (For example, the draw() method used in the applet has a parameter of type Graphics. This parameter is required because of the way Java handles all drawing.) I'll return to this example in later chapters when you know more about applets. However, it would still be worthwhile to look at the definition of the Shape class and its subclasses in the source code for the applet. You might also check how an ArrayList is used to hold a list of shapes. Extending Existing Classes We have been discussing subclasses, but so far we have dealt mostly with the theory. In the remainder of this section, I want to emphasize the practical matter of Java syntax by giving an example. In day-to-day programming, especially for programmers who are just beginning to work with objects, subclassing is used mainly in one situation. There is an existing class that can be adapted with a few changes or additions. This is much more common than designing groups of classes and subclasses from scratch. The existing class can be extended to make a subclass. The syntax for this is class subclass-name extends existing-class-name { . . // Changes and additions. . } http://math.hws.edu/eck/cs124/javanotes4/c5/s4.html (8 of 10) [6/11/2004 11:09:59 AM] Java Programming: Section 5.4 (Of course, the class can optionally be declared to be public.) As an example, suppose you want to write a program that plays the card game, Blackjack. You can use the Card, Hand, and Deck classes developed in Section 3. However, a hand in the game of Blackjack is a little different from a hand of cards in general, since it must be possible to compute the "value" of a Blackjack hand according to the rules of the game. The rules are as follows: The value of a hand is obtained by adding up the values of the cards in the hand. The value of a numeric card such as a three or a ten is its numerical value. The value of a Jack, Queen, or King is 10. The value of an Ace can be either 1 or 11. An Ace should be counted as 11 unless doing so would put the total value of the hand over 21. Note that this means that the second, third, or fourth Ace in the hand will always be counted as 1. One way to handle this is to extend the existing Hand class by adding a method that computes the Blackjack value of the hand. Here's the definition of such a class: public class BlackjackHand extends Hand { public int getBlackjackValue() { // Returns the value of this hand for the // game of Blackjack. int val; // The value computed for the hand. boolean ace; // This will be set to true if the // hand contains an ace. int cards; // Number of cards in the hand. val = 0; ace = false; cards = getCardCount(); for ( int i = 0; i < cards; i++ ) { // Add the value of the i-th card in the hand. Card card; // The i-th card; int cardVal; // The blackjack value of the i-th card. card = getCard(i); cardVal = card.getValue(); // The normal value, 1 to 13. if (cardVal > 10) { cardVal = 10; // For a Jack, Queen, or King. } if (cardVal == 1) { ace = true; // There is at least one ace. } val = val + cardVal; } // Now, val is the value of the hand, counting any ace as 1. // If there is an ace, and if changing its value from 1 to // 11 would leave the score less than or equal to 21, // then do so by adding the extra 10 points to val. if ( ace == true && val + 10 <= 21 ) val = val + 10; return val; } // end getBlackjackValue() http://math.hws.edu/eck/cs124/javanotes4/c5/s4.html (9 of 10) [6/11/2004 11:09:59 AM] Java Programming: Section 5.4 } // end class BlackjackHand Since BlackjackHand is a subclass of Hand, an object of type BlackjackHand contains all the instance variables and instance methods defined in Hand, plus the new instance method getBlackjackValue(). For example, if bHand is a variable of type BlackjackHand, then the following are all legal method calls: bHand.getCardCount(), bHand.removeCard(0), and bHand.getBlackjackValue(). Inherited variables and methods from the Hand class can also be used in the definition of BlackjackHand (except for any that are declared to be private). The statement "cards = getCardCount();" in the above definition of getBlackjackValue() calls the instance method getCardCount(), which was defined in the Hand class. Extending existing classes is an easy way to build on previous work. We'll see that many standard classes have been written specifically to be used as the basis for making subclasses. [ Next Section | Previous Section | Chapter Index | Main Index ] http://math.hws.edu/eck/cs124/javanotes4/c5/s4.html (10 of 10) [6/11/2004 11:09:59 AM] Java Programming: Section 5.5 Section 5.5 this and super ALTHOUGH THE BASIC IDEAS of object-oriented programming are reasonably simple and clear, they are subtle, and they take time to get used to. And unfortunately, beyond the basic ideas there are a lot of details. This section and the next cover more of those annoying details. You should not necessarily master everything in these two sections the first time through, but you should read it to be aware of what is possible. For the most part, when I need to use this material later in the text, I will explain it again briefly, or I will refer you back to it. In this section, we'll look at two variables, this and super that are automatically defined in any instance method. The Special Variables this and super A static member of a class has a simple name, which can only be used inside the class definition. For use outside the class, it has a full name of the form class-name.simple-name. For example, "System.out" is a static member variable with simple name "out" in the class "System". It's always legal to use the full name of a static member, even within the class where it's defined. Sometimes it's even necessary, as when the simple name of a static member variable is hidden by a local variable of the same name. Instance variables and instance methods also have simple names. The simple name of such an instance member can be used in instance methods in the class where the instance member is defined. Instance members also have full names, but remember that instance variables and methods are actually contained in objects, not classes. The full name of an instance member has to contain a reference to the object that contains the instance member. To get at an instance variable or method from outside the class definition, you need a variable that refers to the object. Then the full name is of the form variable-name.simple-name. But suppose you are writing the definition of an instance method in some class. How can you get a reference to the object that contains that instance method? You might need such a reference, for example, if you want to use the full name of an instance variable, because the simple name of the instance variable is hidden by a local variable or parameter. Java provides a special, predefined variable named "this" that you can use for such purposes. The variable, this, is used in the source code of an instance method to refer to the object that contains the method. This intent of the name, this, is to refer to "this object," the one right here that this very method is in. If x is an instance variable in the same object, then this.x can be used as a full name for that variable. If otherMethod() is an instance method in the same object, then this.otherMethod() could be used to call that method. Whenever the computer executes an instance method, it automatically sets the variable, this, to refer to the object that contains the method. One common use of this is in constructors. For example: public class Student { private String name; // Name of the student. public Student(String name) { // Constructor. Create a student with specified name. this.name = name; } . . // More variables and methods. . http://math.hws.edu/eck/cs124/javanotes4/c5/s5.html (1 of 4) [6/11/2004 11:10:00 AM] Java Programming: Section 5.5 } In the constructor, the instance variable called name is hidden by a formal parameter. However, the instance variable can still be referred to by its full name, this.name. In the assignment statement, the value of the formal parameter, name, is assigned to the instance variable, this.name. This is considered to be acceptable style: There is no need to dream up cute new names for formal parameters that are just used to initialize instance variables. You can use the same name for the parameter as for the instance variable. There are other uses for this. Sometimes, when you are writing an instance method, you need to pass the object that contains the method to a subroutine, as an actual parameter. In that case, you can use this as the actual parameter. For example, if you wanted to print out a string representation of the object, you could say "System.out.println(this);". Or you could assign the value of this to another variable in an assignment statement. In fact, you can do anything with this that you could do with any other variable, except change its value. Java also defines another special variable, named "super", for use in the definitions of instance methods. The variable super is for use in a subclass. Like this, super refers to the object that contains the method. But it's forgetful. It forgets that the object belongs to the class you are writing, and it remembers only that it belongs to the superclass of that class. The point is that the class can contain additions and modifications to the superclass. super doesn't know about any of those additions and modifications; it can only be used to refer to methods and variables in the superclass. Let's say that the class that you are writing contains an instance method named doSomething(). Consider the subroutine call statement super.doSomething(). Now, super doesn't know anything about the doSomething() method in the subclass. It only knows about things in the superclass, so it tries to execute a method named doSomething() from the superclass. If there is none -- if the doSomething() method was an addition rather than a modification -- you'll get a syntax error. The reason super exists is so you can get access to things in the superclass that are hidden by things in the subclass. For example, super.x always refers to an instance variable named x in the superclass. This can be useful for the following reason: If a class contains an instance variable with the same name as an instance variable in its superclass, then an object of that class will actually contain two variables with the same name: one defined as part of the class itself and one defined as part of the superclass. The variable in the subclass does not replace the variable of the same name in the superclass; it merely hides it. The variable from the superclass can still be accessed, using super. When you write a method in a subclass that has the same signature as a method in its superclass, the method from the superclass is hidden in the same way. We say that the method in the subclass overrides the method from the superclass. Again, however, super can be used to access the method from the superclass. The major use of super is to override a method with a new method that extends the behavior of the inherited method, instead of replacing that behavior entirely. The new method can use super to call the method from the superclass, and then it can add additional code to provide additional behavior. As an example, suppose you have a PairOfDice class that includes a roll() method. Suppose that you want a subclass, GraphicalDice, to represent a pair of dice drawn on the computer screen. The roll() method in the GraphicalDice class should do everything that the roll() method in the PairOfDice class does. We can express this with a call to super.roll(). But in addition to that, the roll() method for a GraphicalDice object has to redraw the dice to show the new values. The GraphicalDice class might look something like this: public class GraphicalDice extends PairOfDice { public void roll() { // Roll the dice, and redraw them. http://math.hws.edu/eck/cs124/javanotes4/c5/s5.html (2 of 4) [6/11/2004 11:10:00 AM] Java Programming: Section 5.5 super.roll(); // Call the roll method from PairOfDice. redraw(); // Call a method to draw the dice. } . . // More stuff, including definition of redraw(). . } Note that this allows you to extend the behavior of the roll() method even if you don't know how the method is implemented in the superclass! Here is a more complete example. The applet at the end of Section 4.7 shows a disturbance that moves around in a mosaic of little squares. As it moves, the squares it visits become a brighter red. The result looks interesting, but I think it would be prettier if the pattern were symmetric. A symmetric version of the applet is shown at the bottom of the next section. The symmetric applet can be programmed as an easy extension of the original applet. In the symmetric version, each time a square is brightened, the squares that can be obtained from that one by horizontal and vertical reflection through the center of the mosaic are also brightened. The four red squares in the picture, for example, form a set of such symmetrically placed squares, as do the purple squares and the green squares. (The blue square is at the center of the mosaic, so reflecting it doesn't produce any other squares; it's its own reflection.) The original applet is defined by the class RandomBrighten. This class uses features of Java that you won't learn about for a while yet, but the actual task of brightening a square is done by a single method called brighten(). If row and col are the row and column numbers of a square, then "brighten(row,col);" increases the brightness of that square. All we need is a subclass of RandomBrighten with a modified brighten() routine. Instead of just brightening one square, the modified routine will also brighten the horizontal and vertical reflections of that square. But how will it brighten each of the four individual squares? By calling the brighten() method from the original class. It can do this by calling super.brighten(). There is still the problem of computing the row and column numbers of the horizontal and vertical reflections. To do this, you need to know the number of rows and the number of columns. The RandomBrighten class has instance variables named ROWS and COLUMNS to represent these quantities. Using these variables, it's possible to come up with formulas for the reflections, as shown in the definition of the brighten() method below. Here's the complete definition of the new class: public class SymmetricBrighten extends RandomBrighten { void brighten(int row, int col) { // Brighten the specified square and its horizontal // and vertical reflections. This overrides the brighten // method from the RandomBrighten class, which just // brightens one square. super.brighten(row, col); super.brighten(ROWS - 1 - row, col); super.brighten(row, COLUMNS - 1 - col); super.brighten(ROWS - 1 - row, COLUMNS - 1 - col); } http://math.hws.edu/eck/cs124/javanotes4/c5/s5.html (3 of 4) [6/11/2004 11:10:00 AM] Java Programming: Section 5.5 } // end class SymmetricBrighten This is the entire source code for the applet! Constructors in Subclasses Constructors are not inherited. That is, if you extend an existing class to make a subclass, the constructors in the superclass do not become part of the subclass. If you want constructors in the subclass, you have to define new ones from scratch. If you don't define any constructors in the subclass, then the computer will make up a default constructor, with no parameters, for you. This could be a problem, if there is a constructor in the superclass that does a lot of necessary work. It looks like you might have to repeat all that work in the subclass! This could be a real problem if you don't have the source code to the superclass, and don't know how it works, or if the constructor in the superclass initializes private member variables that you don't even have access to in the subclass! Obviously, there has to be some fix for this, and there is. It involves the special variable, super. As the very first statement in a constructor, you can use super to call a constructor from the superclass. The notation for this is a bit ugly and misleading, and it can only be used in this one particular circumstance: It looks like you are calling super as a subroutine (even though super is not a subroutine and you can't call constructors the same way you call other subroutines anyway). As an example, assume that the PairOfDice class has a constructor that takes two integers as parameters. Consider a subclass: public class GraphicalDice extends PairOfDice { public GraphicalDice() { // Constructor for this class. super(3,4); // Call the constructor from the // PairOfDice class, with parameters 3, 4. initializeGraphics(); // Do some initialization specific // to the GraphicalDice class. } . . // More constructors, methods, variables... . } This might seem rather technical, but unfortunately it is sometimes necessary. By the way, you can use the special variable this in exactly the same way to call another constructor in the same class. This can be useful since it can save you from repeating the same code in several constructors. [ Next Section | Previous Section | Chapter Index | Main Index ] http://math.hws.edu/eck/cs124/javanotes4/c5/s5.html (4 of 4) [6/11/2004 11:10:00 AM] Java Programming: Section 5.6 Section 5.6 Interfaces, Nested Classes, and Other Details THIS SECTION simply pulls together a few more miscellaneous features of object oriented progrmming in Java. Read it now, or just look through it and refer back to it later when you need this material. Interfaces Some object-oriented programming languages, such as C++, allow a class to extend two or more superclasses. This is called multiple inheritance. In the illustration below, for example, class E is shown as having both class A and class B as direct superclasses, while class F has three direct superclasses. Such multiple inheritance is not allowed in Java. The designers of Java wanted to keep the language reasonably simple, and felt that the benefits of multiple inheritance were not worth the cost in increased complexity. However, Java does have a feature that can be used to accomplish many of the same goals as multiple inheritance: interfaces. We've encountered the term "interface" before, in connection with black boxes in general and subroutines in particular. The interface of a subroutine consists of the name of the subroutine, its return type, and the number and types of its parameters. This is the information you need to know if you want to call the subroutine. A subroutine also has an implementation: the block of code which defines it and which is executed when the subroutine is called. In Java, interface is a reserved word with an additional, technical meaning. An "interface" in this sense consists of a set of subroutine interfaces, without any associated implementations. A class can implement an interface by providing an implementation for each of the subroutines specified by the interface. Here is an example of a very simple Java interface: public interface Drawable { public void draw(Graphics g); } This looks much like a class definition, except that the implementation of the method draw() is omitted. A class that implements the interface, Drawable, must provide an implementation for this method. Of course, the class can also include other methods and variables. For example, class Line implements Drawable { public void draw(Graphics g) { . . . // do something -- presumably, draw a line } http://math.hws.edu/eck/cs124/javanotes4/c5/s6.html (1 of 8) [6/11/2004 11:10:01 AM] Java Programming: Section 5.6 . . . // other methods and variables } Any class that implements the Drawable interface defines a draw() instance method. Any object created from such a class includes a draw() method. We say that an object implements an interface if it belongs to a class that implements the interface. For example, any object of type Line implements the Drawable interface. Note that it is not enough for the object to include a draw() method. The class that it belongs to has to say that it "implements Drawable. While a class can extend only one other class, it can implement any number of interfaces. In fact, a class can both extend another class and implement one or more interfaces. So, we can have things like class FilledCircle extends Circle implements Drawable, Fillable { . . . } The point of all this is that, although interfaces are not classes, they are something very similar. An interface is very much like an abstract class, that is, a class that can never be used for constructing objects, but can be used as a basis for making subclasses. The subroutines in an interface are abstract methods, which must be implemented in any concrete class that implements the interface. And as with abstract classes, even though you can't construct an object from an interface, you can declare a variable whose type is given by the interface. For example, if Drawable is an interface, and if Line and FilledCircle are classes that implement Drawable, then you could say: Drawable figure; // Declare a variable of type Drawable. It can // refer to any object that implements the // Drawable interface. figure = new Line(); // figure now refers to an object of class Line figure.draw(g); // calls draw() method from class Line figure = new FilledCircle(); // Now, figure refers to an object // of class FilledCircle. figure.draw(g); // calls draw() method from class FilledCircle A variable of type Drawable can refer to any object of any class that implements the Drawable interface. A statement like figure.draw(g), above, is legal because figure is of type Drawable, and any Drawable object has a draw() method. Note that a type is something that can be used to declare variables. A type can also be used to specify the type of a parameter in a subroutine, or the return type of a function. In Java, a type can be either a class, an interface, or one of the eight built-in primitive types. These are the only possibilities. Of these, however, only classes can be used to construct new objects. You are not likely to need to write your own interfaces until you get to the point of writing fairly complex programs. However, there are a few interfaces that are used in important ways in Java's standard packages. You'll learn about some of these standard interfaces in the next few chapters. Nested Classes A class seems like it should be a pretty important thing. A class is a high-level building block of a program, representing a potentially complex idea and its associated data and behaviors. I've always felt a bit silly writing tiny little classes that exist only to group a few scraps of data together. However, such trivial classes http://math.hws.edu/eck/cs124/javanotes4/c5/s6.html (2 of 8) [6/11/2004 11:10:01 AM] Java Programming: Section 5.6 are often useful and even essential. Fortunately, in Java, I can ease the embarrassment, because one class can be nested inside another class. My trivial little class doesn't have to stand on its own. It becomes part of a larger more respectable class. This is particularly useful when you want to create a little class specifically to support the work of a larger class. And, more seriously, there are other good reasons for nesting the definition of one class inside another class. In Java, a nested class or inner class is any class whose definition is inside the definition of another class. Inner classes can be either named or anonymous. I will come back to the topic of anonymous classes later in this section. A named inner class looks just like any other class, except that it is nested inside another class. (It can even contain further levels of nested classes, but you shouldn't carry these things too far.) Like any other item in a class, a named inner class can be either static or non-static. A static nested class is part of the static structure of the containing class. It can be used inside that class to create objects in the usual way. If it has not been declared private, then it can also be used outside the containing class, but when it is used outside the class, its name must indicate its membership in the containing class. This is similar to other static components of a class: A static nested class is part of the class itself in the same way that static member variables are parts of the class itself. For example, suppose a class named WireFrameModel represents a set of lines in three-dimensional space. (Such models are used to represent three-dimensional objects in graphics programs.) Suppose that the WireFrameModel class contains a static nested class, Line, that represents a single line. Then, outside of the class WireFrameModel, the Line class would be referred to as WireFrameModel.Line. Of course, this just follows the normal naming convention for static members of a class. The definition of the WireFrameModel class with its nested Line class would look, in outline, like this: public class WireFrameModel { . . . // other members of the WireFrameModel class static public class Line { // Represents a line from the point (x1,y1,z1) // to the point (x2,y2,z2) in 3-dimensional space. double x1, y1, z1; double x2, y2, z2; } // end class Line . . . // other members of the WireFrameModel class } // end WireFrameModel Inside the WireframeModel class, a Line object would be created with the constructor "new Line()". Outside the class, "new WireFrameModel.Line()" would be used. A static nested class has full access to the members of the containing class, even to the private members. This can be another motivation for declaring a nested class, since it lets you give one class access to the private members of another class without making those members generally available to other classes. When you compile the above class definition, two class files will be created. Even though the definition of Line is nested inside WireFrameModel, the compiled Line class is stored in a separate file. The name of the class file for Line will be WireFrameModel$Line.class. Non-static nested classes are not, in practice, very different from static nested classes, but a non-static nested class is actually associated to an object rather than to the class in which it is nested. This can get some getting used to. Any non-static member of a class is not really part of the class itself (although its source code is contained in the class definition). This is true for non-static nested classes, just as it is for any other non-static part of a http://math.hws.edu/eck/cs124/javanotes4/c5/s6.html (3 of 8) [6/11/2004 11:10:01 AM] Java Programming: Section 5.6 class. The non-static members of a class specify what will be contained in objects that are created from that class. The same is true -- at least logically -- for non-static nested classes. It's as if each object that belongs to the containing class has its own copy of the nested class. This copy has access to all the instance methods and instance variables of the object. Two copies of the nested class in different objects differ because the instance variables and methods they refer to are in different objects. In fact, the rule for deciding whether a nested class should be static or non-static is simple: If the class needs to use any instance variable or instance method, make it non-static. Otherwise, it might as well be static. From outside the containing class, a non-static nested class has to be referred to as variableName.NestedClassName, where variableName is a variable that refers to the object that contains the class. This is actually rather rare, however. A non-static nested class is generally used only inside the class in which it is nested, and there it can be referred to by its simple name. In order to create an object that belongs to a non-static nested class, you must first have an object that belongs to the containing class. (When working inside the class, the object "this" is used implicitly.) The nested class object is permanently associated with the containing class object, and it has complete access to the members of the containing class object. Looking at an example will help, and will hopefully convince you that non-static nested classes are really very natural. Consider a class that represents poker games. This class might include a nested class to represent the players of the game. This structure of the PokerGame class could be: class PokerGame { // Represents a game of poker. class Player { // Represents one of the players in this game. . . . } // end class Player private Deck deck; // A deck of cards for playing the game. private int pot; // The amount of money that has been bet. . . . } // end class PokerGame If game is a variable of type PokerGame, then, conceptually, game contains its own copy of the Player class. In an an instance method of a PokerGame object, a new Player object would be created by saying "new Player()", just as for any other class. (A Player object could be created outside the PokerGame class with an expression such as "new game.Player()". Again, however, this is rather rare.) The Player object will have access to the deck and pot instance variables in the PokerGame object. Each PokerGame object has its own deck and pot and Players. Players of that poker game use the deck and pot for that game; playes of another poker game use the other game's deck and pot. That's the effect of making the Player class non-static. This is the most natural way for players to behave. A Player object represents a player of one particular poker game. If Player were a static nested class, on the other hand, it would represent the general idea of a poker player, independent of a particular poker game. In some cases, you might find yourself writing a nested class and then using that class in just a single line of your program. Is it worth creating such a class? Indeed, it can be, but for cases like this you have the option of using an anonymous nested class. An anonymous class is created with a variation of the new operator that has the form new superclass-or-interface () { methods-and-variables http://math.hws.edu/eck/cs124/javanotes4/c5/s6.html (4 of 8) [6/11/2004 11:10:01 AM] Java Programming: Section 5.6 } This constructor defines a new class, without giving it a name, and it simultaneously creates an object that belongs to that class. This form of the new operator can be used in any statement where a regular "new" could be used. The intention of this expression is to create: "a new object belonging to a class that is the same as superclass-or-interface but with these methods-and-variables added." The effect is to create a uniquely customized object, just at the point in the program where you need it. Note that it is possible to base an anonymous class on an interface, rather than a class. In this case, the anonymous class must implement the interface by defining all the methods that are declared in the interface. Anonymous classes are most often used for handling events in graphical user interfaces, and we will encounter them several times in the next two chapters. For now, we will look at one not-very-plausible example. Consider the Drawable interface, which is defined earlier in this section. Suppose that we want a Drawable object that draws a filled, red, 100-pixel square. Rather than defining a separate class and then using that class to create the object, we can use an anonymous class to create the object in one statement: Drawable redSquare = new Drawable() { void draw(Graphics g) { g.setColor(Color.red); g.fillRect(10,10,100,100); } }; The semicolon at the end of this statement is not part of the class definition. It's the semicolon that is required at the end of every declaration statement. When a Java class is compiled, each anonymous nested class will produce a separate class file. If the name of the main class is MainClass, for example, then the names of the class files for the anonymous nested classes will be MainClass$1.class, MainClass$2.class, MainClass$3.class, and so on. More about Access Modifiers A class can be declared to be public. A public class can be accessed from anywhere. Certain classes have to be public. A class that defines a stand-alone application must be public, so that the system will be able to get at its main() routine. A class that defines an applet must be public so that it can be used by a Web browser. If a class is not declared to be public, then it can only be used by other classes in the same "package" as the class. Packages are discussed in Section 4.5. Classes that are not explicitly declared to be in any package are put into something called the default package. All the examples in this textbook are in the default package, so they are all accessible to one another whether or not they are declared public. So, except for applications and applets, which must be public, it makes no practical difference whether our classes are declared to be public or not. However, once you start writing packages, it does make a difference. A package should contain a set of related classes. Some of those classes are meant to be public, for access from outside the package. Others can be part of the internal workings of the package, and they should not be made public. A package is a kind of black box. The public classes in the package are the interface. (More exactly, the public variables and subroutines in the public classes are the interface). The non-public classes are part of the non-public implementation. Of course, all the classes in the package have unrestricted access to one another. Following this model, I will tend to declare a class public if it seems like it might have some general applicability. If it is written just to play some sort of auxiliary role in a larger project, I am more likely not to make it public. A member variable or subroutine in a class can also be declared to be public, which means that it is accessible from anywhere. It can be declared to be private, which means that it accessible only from inside the class where it is defined. Making a variable private gives you complete control over that http://math.hws.edu/eck/cs124/javanotes4/c5/s6.html (5 of 8) [6/11/2004 11:10:01 AM] Java Programming: Section 5.6 variable. The only code that will ever manipulate it is the code you write in your class. This is an important kind of protection. If no access modifier is specified for a variable or subroutine, then it is accessible from any class in the same package as the class. As with classes, in this textbook there is no practical difference between declaring a member public and using no access modifier at all. However, there might be stylistic reasons for preferring one over the other. And a real difference does arise once you start writing your own packages. There is a third access modifier that can be applied to a member variable or subroutine. If it is declared to be protected, then it can be used in the class where it is defined and in any subclass of that class. This is obviously less restrictive than private and more restrictive than public. Classes that are written specifically to be used as a basis for making subclasses often have protected members. The protected members are there to provide a foundation for the subclasses to build on. But they are still invisible to the public at large. Mixing Static and Non-static Classes, as I've said, have two very distinct purposes. A class can be used to group together a set of static member variables and static member subroutines. Or it can be used as a factory for making objects. The non-static variables and subroutine definintions in the class definition specify the instance variables and methods of the objects. In most cases, a class performs one or the other of these roles, not both. Sometimes, however, static and non-static members are mixed in a single class. In this case, the class plays a dual role. Sometimes, these roles are completely separate. It is also possible for the static and non-static parts of a class to interact. This happens when instance methods use static member variables or call static member subroutines. An instance method belongs to an object, not to the class itself, and there can be many objects with their own versions of the instance method. But there is only one copy of a static member variable. So, effectively, we have many objects sharing that one variable. Suppose, for example, that we want to write a PairOfDice class that uses the Random class mentioned in Section 3 for rolling the dice. To do this, a PairOfDice object needs access to an object of type Random. But there is no need for each PairOfDice object to have a separate Random object. (In fact, it would not even be a good idea: Because of the way random number generators work, a program should, in general, use only one source of random numbers.) A nice solution is to have a single Random variable as a static member of the PairOfDice class, so that it can be shared by all PairOfDice objects. For example: class PairOfDice { private static Random randGen = new Random(); // (Note: Assumes that java.util.Random has been imported.) public int die1; // Number showing on the first die. public int die2; // Number showing on the second die. public PairOfDice() { // Constructor. Creates a pair of dice that // initially shows random values. roll(); } public void roll() { // Roll the dice by setting each of the dice to be // a random number between 1 and 6. die1 = randGen.nextInt(6) + 1; die2 = randGen.nextInt(6) + 1; } http://math.hws.edu/eck/cs124/javanotes4/c5/s6.html (6 of 8) [6/11/2004 11:10:01 AM] Java Programming: Section 5.6 } // end class PairOfDice As another example, let's rewrite the Student class that was used in the Section 2. I've added an ID for each student and a static member called nextUniqueID. Although there is an ID variable in each student object, there is only one nextUniqueID variable. public class Student { private String name; // Student's name. private int ID; // Unique ID number for this student. public double test1, test2, test3; // Grades on three tests. private static int nextUniqueID = 0; // keep track of next available unique ID number Student(String theName) { // Constructor for Student objects; // provides a name for the Student, // and assigns the student a unique // ID number. name = theName; nextUniqueID++; ID = nextUniqueID; } public String getName() { // Accessor method for reading value of private // instance variable, name. return name; } public int getID() { // Accessor method for reading value of ID. return ID; } public double getAverage() { // Compute average test grade. return (test1 + test2 + test3) / 3; } } // end of class Student The initialization "nextUniqueID = 0" is done only once, when the class is first loaded. Whenever a Student object is constructed and the constructor says "nextUniqueID++;", it's always the same static member variable that is being incremented. When the very first Student object is created, nextUniqueID becomes 1. When the second object is created, nextUniqueID becomes 2. After the third object, it becomes 3. And so on. The constructor stores the new value of nextUniqueID in the ID variable of the object that is being created. Of course, ID is an instance variable, so every object has its own individual ID variable. The class is constructed so that each student will automatically get a different value for its ID variable. Furthermore, the ID variable is private, so there is no way for this variable to be tampered with after the object has been created. You are guaranteed, just by the way the class is designed, that every student object will have its own permanent, unique identification number. Which is kind of cool if you think about it. http://math.hws.edu/eck/cs124/javanotes4/c5/s6.html (7 of 8) [6/11/2004 11:10:01 AM] Java Programming: Section 5.6 End of Chapter 5 [ Next Chapter | Previous Section | Chapter Index | Main Index ] http://math.hws.edu/eck/cs124/javanotes4/c5/s6.html (8 of 8) [6/11/2004 11:10:01 AM] Java Programming: Chapter 5 Exercises Programming Exercises For Chapter 5 THIS PAGE CONTAINS programming exercises based on material from Chapter 5 of this on-line Java textbook. Each exercise has a link to a discussion of one possible solution of that exercise. Exercise 5.1: In all versions of the PairOfDice class in Section 2, the instance variables die1 and die2 are declared to be public. They really should be private, so that they are protected from being changed from outside the class. Write another version of the PairOfDice class in which the instance variables die1 and die2 are private. Your class will need methods that can be used to find out the values of die1 and die2. (The idea is to protect their values from being changed from outside the class, but still to allow the values to be read.) Include other improvements in the class, if you can think of any. Test your class with a short program that counts how many times a pair of dice is rolled, before the total of the two dice is equal to two. See the solution! Exercise 5.2: A common programming task is computing statistics of a set of numbers. (A statistic is a number that summarizes some property of a set of data.) Common statistics include the mean (also known as the average) and the standard deviation (which tells how spread out the data are from the mean). I have written a little class called StatCalc that can be used to compute these statistics, as well as the sum of the items in the dataset and the number of items in the dataset. You can read the source code for this class in the file StatCalc.java. If calc is a variable of type StatCalc, then the following methods are defined: ● calc.enter(item); where item is a number, adds the item to the dataset. ● calc.getCount() is a function that returns the number of items that have been added to the dataset. ● calc.getSum() is a function that returns the sum of all the items that have been added to the dataset. ● calc.getMean() is a function that returns the average of all the items. ● calc.getStandardDeviation() is a function that returns the standard deviation of the items. Typically, all the data are added one after the other calling the enter() method over and over, as the data become available. After all the data have been entered, any of the other methods can be called to get statistical information about the data. The methods getMean() and getStandardDeviation() should only be called if the number of items is greater than zero. Modify the current source code, StatCalc.java, to add instance methods getMax() and getMin(). The getMax() method should return the largest of all the items that have been added to the dataset, and getMin() should return the smallest. You will need to add two new instance variables to keep track of the largest and smallest items that have been seen so far. Test your new class by using it in a program to compute statistics for a set of non-zero numbers entered by the user. Start by creating an object of type StatCalc: StatCalc calc; // Object to be used to process the data. calc = new StatCalc(); http://math.hws.edu/eck/cs124/javanotes4/c5/exercises.html (1 of 3) [6/11/2004 11:10:01 AM] Java Programming: Chapter 5 Exercises Read numbers from the user and add them to the dataset. Use 0 as a sentinel value (that is, stop reading numbers when the user enters 0). After all the user's non-zero numbers have been entered, print out each of the six statistics that available from calc. See the solution! Exercise 5.3: This problem uses the PairOfDice class from Exercise 5.1 and the StatCalc class from Exercise 5.2. The program in Exercise 4.4 performs the experiment of counting how many times a pair of dice is rolled before a given total comes up. It repeats this experiment 10000 times and then reports the average number of rolls. It does this whole process for each possible total (2, 3, ..., 12). Redo that exercise. But instead of just reporting the average number of rolls, you should also report the standard deviation and the maximum number of rolls. Use a PairOfDice object to represent the dice. Use a StatCalc object to compute the statistics. (You'll need a new StatCalc object for each possible total, 2, 3, ..., 12. You can use a new pair of dice if you want, but it's not necessary.) See the solution! Exercise 5.4: The BlackjackHand class from Section 5.5 is an extension of the Hand class from Section 5.3. The instance methods in the Hand class are discussed in Section 5.3. In addition to those methods, BlackjackHand includes an instance method, getBlackjackValue(), that returns the value of the hand for the game of Blackjack. For this exercise, you will also need the Deck and Card classes from Section 5.3. A Blackjack hand typically contains from two to six cards. Write a program to test the BlackjackHand class. You should create a BlackjackHand object and a Deck object. Pick a random number between 2 and 6. Deal that many cards from the deck and add them to the hand. Print out all the cards in the hand, and then print out the value computed for the hand by getBlackjackValue(). Repeat this as long as the user wants to continue. In addition to TextIO, your program will depend on Card.java, Deck.java, Hand.java, and BlackjackHand.java. See the solution! Exercise 5.5 Write a program that let's the user play Blackjack. The game will be a simplified version of Blackjack as it is played in a casino. The computer will act as the dealer. As in the previous exercise, your program will need the classes defined in Card.java, Deck.java, Hand.java, and BlackjackHand.java. (This is the longest and most complex program that has come up so far in the exercises.) You should first write a subroutine in which the user plays one game. The subroutine should return a boolean value to indicate whether the user wins the game or not. Return true if the user wins, false if the dealer wins. The program needs an object of class Deck and two objects of type BlackjackHand, one for the dealer and one for the user. The general object in Blackjack is to get a hand of cards whose value is as close to 21 as possible, without going over. The game goes like this. First, two cards are dealt into each player's hand. If the dealer's hand has a value of 21 at this point, then the dealer wins. Otherwise, if the user has 21, then the user wins. (This is called a "Blackjack".) Note that the dealer wins on a tie, so if both players have Blackjack, then the dealer wins. http://math.hws.edu/eck/cs124/javanotes4/c5/exercises.html (2 of 3) [6/11/2004 11:10:01 AM] Java Programming: Chapter 5 Exercises Now, if the game has not ended, the user gets a chance to add some cards to her hand. In this phase, the user sees her own cards and sees one of the dealer's two cards. (In a casino, the dealer deals himself one card face up and one card face down. All the user's cards are dealt face up.) The user makes a decision whether to "Hit", which means to add another card to her hand, or to "Stand", which means to stop taking cards. If the user Hits, there is a possibility that the user will go over 21. In that case, the game is over and the user loses. If not, then the process continues. The user gets to decide again whether to Hit or Stand. If the user Stands, the game will end, but first the dealer gets a chance to draw cards. The dealer only follows rules, without any choice. The rule is that as long as the value of the dealer's hand is less than or equal to 16, the dealer Hits (that is, takes another card). The user should see all the dealer's cards at this point. Now, the winner can be determined: If the dealer has gone over 21, the user wins. Otherwise, if the dealer's total is greater than or equal to the user's total, then the dealer wins. Otherwise, the user wins. Two notes on programming: At any point in the subroutine, as soon as you know who the winner is, you can say "return true;" or "return false;" to end the subroutine and return to the main program. To avoid having an overabundance of variables in your subroutine, remember that a function call such as userHand.getBlackjackValue() can be used anywhere that a number could be used, including in an output statement or in the condition of an if statement. Write a main program that lets the user play several games of Blackjack. To make things interesting, give the user 100 dollars, and let the user make bets on the game. If the user loses, subtract the bet from the user's money. If the user wins, add an amount equal to the bet to the user's money. End the program when the user wants to quit or when she runs out of money. Here is an applet that simulates the program you are supposed to write. It would probably be worthwhile to play it for a while to see how it works. Sorry, your browser doesn't support Java. See the solution! [ Chapter Index | Main Index ] http://math.hws.edu/eck/cs124/javanotes4/c5/exercises.html (3 of 3) [6/11/2004 11:10:01 AM] Java Programming: Chapter 5 Quiz Quiz Questions For Chapter 5 THIS PAGE CONTAINS A SAMPLE quiz on material from Chapter 5 of this on-line Java textbook. You should be able to answer these questions after studying that chapter. Sample answers to all the quiz questions can be found here. Question 1: Object-oriented programming uses classes and objects. What are classes and what are objects? What is the relationship between classes and objects? Question 2: Explain carefully what null means in Java, and why this special value is necessary. Question 3: What is a constructor? What is the purpose of a constructor in a class? Question 4: Suppose that Kumquat is the name of a class and that fruit is a variable of type Kumquat. What is the meaning of the statement "fruit = new Kumquat();"? That is, what does the computer do when it executes this statement? (Try to give a complete answer. The computer does several things.) Question 5: What is meant by the terms instance variable and instance method? Question 6: Explain what is meant by the terms subclass and superclass. Question 7: Explain the term polymorphism. Question 8: Java uses "garbage collection" for memory management. Explain what is meant here by garbage collection. What is the alternative to garbage collection? Question 9: For this problem, you should write a very simple but complete class. The class represents a counter that counts 0, 1, 2, 3, 4,.... The name of the class should be Counter. It has one private instance variable representing the value of the counter. It has two instance methods: increment() adds one to the counter value, and getValue() returns the current counter value. Write a complete definition for the class, Counter. Question 10: This problem uses the Counter class from Question 9. The following program segment is meant to simulate tossing a coin 100 times. It should use two Counter objects, headCount and tailCount, to count the number of heads and the number of tails. Fill in the blanks so that it will do so. Counter headCount, tailCount; tailCount = new Counter(); headCount = new Counter(); for ( int flip = 0; flip < 100; flip++ ) { if (Math.random() < 0.5) // There's a 50/50 chance that this is true. ______________________ ; // Count a "head". else ______________________ ; // Count a "tail". } System.out.println("There were " + ___________________ + " heads."); System.out.println("There were " + ___________________ + " tails."); [ Answers | Chapter Index | Main Index ] http://math.hws.edu/eck/cs124/javanotes4/c5/quiz.html [6/11/2004 11:10:02 AM] Java Programming: Chapter 6 Index Chapter 6 Applets, HTML, and GUI's JAVA IS A PROGRAMMING LANGUAGE DESIGNED for networked computers and the World Wide Web. Java applets are downloaded over a network to appear on a Web page. Part of learning Java is learning to program applets and other Graphical User Interface programs. GUI programs are event-driven. That is, user actions such as clicking on a button or pressing a key on the keyboard generate events, and the program must respond to these events as they occur. Event-driven programming builds on all the skills you have learned in the first five chapters of this text. You need to be able to write the subroutines that respond to events. Inside these subroutines, you are doing the kind of programming-in-the-small that was covered in Chapters 2 and 3. And of course, objects are everywhere. Events are objects. Applets and other GUI components are objects. Events are handled by instance methods contained in objects. In Java, event-oriented programming is object-oriented programming. This chapter covers the basics of applets, graphics, components, and events. There is also a section that covers HyperText Markup Language (HTML), the language used for writing Web pages. The discussion of applets and GUI's will continue in the next chapter with more details and with more advanced techniques. Contents Chapter 6: ● Section 1: The Basic Java Applet and JApplet ● Section 2: HTML Basics and the Web ● Section 3: Graphics and Painting ● Section 4: Mouse Events ● Section 5: Keyboard Events ● Section 6: Introduction to Layouts and Components ● Programming Exercises ● Quiz on this Chapter [ First Section | Next Chapter | Previous Chapter | Main Index ] http://math.hws.edu/eck/cs124/javanotes4/c6/ [6/11/2004 11:10:25 AM] Java Programming: Section 6.1 Section 6.1 The Basic Java Applet and JApplet JAVA APPLETS ARE SMALL PROGRAMS that are meant to run on a page in a Web browser. Very little of that statement is completely accurate, however. An applet is not a complete program. It doesn't have to be small. And while applets are generally meant to be used on Web pages, there are other ways to use them. A technically more correct, but not very useful, definition would say simply that an applet is an object that belongs to the class java.applet.Applet or to one of its subclasses. Either definition still leaves us a long way to go to really understand applets. An applet is inherently part of a graphical user interface. It is a type of graphical component that can be displayed in a window (whether belonging to a Web browser or to some other program). When shown in a window, an applet is a rectangular area that can contain other components, such as buttons and text boxes. It can display graphical elements such as images, rectangles, and lines. And it can respond to certain "events," such as when the user clicks on the applet with a mouse. The Applet class, defined in the package java.applet, is really only useful as a basis for making subclasses. An object of type Applet has certain basic behaviors, but doesn't actually do anything useful. It's just a blank area on the screen that doesn't respond to any events. To create a useful applet, a programmer must define a subclass that extends the Applet class. There are several methods in the Applet class that are defined to do nothing at all. The programmer must override at least some of these methods and give them something to do. Back in Section 2.1, when you first learned about Java programs, you encountered the idea of a main() routine, which is not meant to be called by the programmer. The main() routine of a program is there to be called by "the system" when it needs to execute the program. The programmer writes the main routine to say what happens when the system runs the program. An applet needs no main() routine, since it is not a stand-alone program. However, many of the methods in an applet are similar to main() in that they are meant to be called by the system, and the job of the programmer is to say what happens in response to the system's calls. In this section, we'll look at a few of the things that applets can do. We'll spend the rest of this chapter and the next filling in the details. One of the methods that is defined in the Applet class to do nothing is the paint() method. You've already encountered this method briefly in Section 3.7. The paint() method is called by the system when the applet needs to be drawn. In a subclass of Applet, the paint() method can be redefined to draw various graphical elements such as rectangles, lines, and text on the applet. The definition of this method must have the form: public void paint(Graphics g) { // draw some stuff } The parameter g, of type Graphics, is provided by the system when it calls the paint() method. In Java, all drawing of any kind is done using methods provided by a Graphics object. There are many such methods. I will discuss graphics in more detail in Section 3. As a first example of an applet, let's go the traditional route and look at an applet that displays the string "Hello World!". We'll use the paint() method to display this string. The import statements at the beginning make it possible to use the short names Applet and Graphics instead of the full names of the classes java.applet.Applet and java.awt.Graphics. (See Section 4.5 for a discussion of "packages," such as java.awt and java.applet.) http://math.hws.edu/eck/cs124/javanotes4/c6/s1.html (1 of 6) [6/11/2004 11:10:26 AM] Java Programming: Section 6.1 import java.awt.*; import java.applet.*; public class HelloWorldApplet extends Applet { // An applet that simply displays the string Hello World! public void paint(Graphics g) { g.drawString("Hello World!", 10, 30); } } // end of class HelloWorldApplet The drawString() method, defined in the Graphics class, actually does the drawing. The parameters of this method specify the string to be drawn and the point in the applet where the string is to be placed. More about this later. Now, an applet is an object, not a class. So far we have only defined a class. Where does an actual applet object come from? It is possible, of course, to create such objects: Applet hw = new HelloWorldApplet(); This might even be useful if you are writing a program and would like to add an applet to a window you've created. Most often, however, applet objects are created by "the system." For example, when an applet appears on a page in a Web browser, "the system" means the Web browser. It is up to the browser program to create the applet object and to add it to a Web page. The Web browser, in turn, gets instructions about what is to appear on a given Web page from the source document for that page. For an applet to appear on a Web page, the source document for that page must specify the name of the applet and its size. This specification, like the rest of the document, is written in a language called HTML. I will discuss HTML in more detail in Section 2. Here is some HTML code that instructs a Web browser to display a HelloWorldApplet: <center> <applet code="HelloWorldApplet.class" width=200 height=50> </applet> </center> and here is the applet that this code displays: (Applet "HelloWorldApplet" would be displayed here if Java were available.) If you are viewing this page with a web browser that supports Java, you should see the message "Hello world!". The message is displayed in a rectangle that is 200 pixels in width and 50 pixels in height. You shouldn't be able to see the rectangle as such, since by default, an applet has a background color that is the same as the color of the Web page on which it is displayed. (This might not actually be the case in your browser.) The Applet class defines another method that is essential for programming applets, the init() method. This method is called just after the applet object has been created and before it appears on the screen. Its purpose is to give the applet a chance to do any necessary initialization. Again, this method is called by the system, not by your program. Your job as a programmer is just to provide a definition of the init() method. The definition of the method must have the form: public void init() { // do initialization } http://math.hws.edu/eck/cs124/javanotes4/c6/s1.html (2 of 6) [6/11/2004 11:10:26 AM] Java Programming: Section 6.1 (You might wonder, by the way, why initialization is done in the init() method rather than in a constructor. In fact, it is possible to define a constructor for your applet class. To create the applet object, the system calls the constructor that has no parameters. You can write such a constructor for an applet class and can do initializations in the constructor as well as in the init() method. The most significant difference is that when the constructor is called, the size of the applet is not available. By the time init(), is called, the size is known and can be used to customize the initialization according to the size. In general, though, it is customary to do applet initialization in the init() method.) Suppose, for example, that we want to change the colors used by the HelloWorldApplet. An applet has a "background color" which is used to fill the entire area of the applet before any other drawing is done, and it has a "foreground color" which is used as the default color for drawing in the applet. It is convenient to set these colors in the init() method. Here is a version of the HelloWorldApplet that does this: (Applet "HelloWorldApplet2" would be displayed here if Java were available.) and here is the source code for this applet, including the init() method: import java.awt.*; import java.applet.*; public class HelloWorldApplet2 extends Applet { public void init() { // Initialize the applet by setting it to use blue // and yellow as background and foreground colors. setBackground(Color.blue); setForeground(Color.yellow); } public void paint(Graphics g) { g.drawString("Hello World!", 10, 30); } } // end of class HelloWorldApplet2 JApplets and Swing The AWT (Abstract Windowing Toolkit) has been part of Java from the beginning, but, almost from the beginning, it has been clear that the AWT was not powerful or flexible enough for writing complex, sophisticated applications. This does not prevent it from being useful -- especially for applets, which are generally not as complex as full-scale, independent applications. The Swing graphical user interface library was created to address the problems with the AWT. With the release of Java version 1.2, Swing became an official part of Java. (Versions of Java starting with 1.2 are also called, rather confusingly, "Java 2.") There are still good reasons to write applets based on the AWT, such as the lack of support in many Web browsers for Java 2. However, at this point, anyone writing a stand-alone graphical application in Java should almost certainly be using Swing, and it is Swing that I will concentrate on in this book. If you want to write applets using the AWT, you might want to look at the previous version of this book, which can be found on the web at http://math.hws.edu/eck/cs124/javanotes3/. The classes that make up the Swing library can be found in the package javax.swing. Swing includes the class javax.swing.JApplet to be used as a basis for writing applets. JApplet is actually a subclass of Applet, so JApplets are in fact Applets in the usual sense. However, JApplets have a lot of extra structure that plain Applets don't have. Because of this structure, the painting of a JApplet is a more http://math.hws.edu/eck/cs124/javanotes4/c6/s1.html (3 of 6) [6/11/2004 11:10:26 AM] Java Programming: Section 6.1 complex affair and is handled by the system. So, when you make a subclass of JApplet you should not write a paint() method for it. As we will see, if you want to draw on a JApplet, you should add a component to the applet to be used for that purpose. On the other hand, you can and generally should write an init() method for a subclass of JApplet. In this book, I will use a plain Applet in only a few examples. In almost all cases, I will use a JApplet even where a plain applet might make more sense (that is, when the applet is just being used as a simple drawing surface). Let's take a look at a simple JApplet that uses Swing. This applet demonstrates some of the basic ideas of GUI programming. Although you won't understand everything in it at this time, it will give you a preliminary idea of how things work. GUI programs use "components" such as buttons to allow interaction with the user. Our sample applet contains a button. In fact, the button is the only thing in the applet, and it fills the entire rather small applet. Here's our sample JApplet, which is named HelloSwing: (Applet "HelloSwing" would be displayed here if Java were available.) If you click this button, a new window will open with a message and an "OK" button. Click the "OK" button to dismiss the window. The button in this applet is an object that belongs to the class JButton (more properly, javax.swing.JButton). When the applet is created, the button must be created and added to the applet. This is part of the process of initializing the applet and is done in the applet's init() method. In this method, the button is created with the statement: JButton bttn = new JButton("Click Me!"); The parameter to the constructor specifies the text that is displayed on the button. The button does not automatically appear on the screen. It has to be added to the applet's "content pane." This is done with the statement: getContentPane().add(bttn); Once it has been added to the applet, a JButton object mostly takes care of itself. In particular, it draws itself, so you don't have to worry about drawing it. When the user clicks the button, it generates an event. The applet (or, in fact, any object) can be programmed to respond to this event. Event-handling is the major topic in GUI programming, and I will cover it in detail later. But in outline, it works like this: The type of event generated by a button is called an ActionEvent. For the applet to respond to an event of this type, it must define a method public void actionPerformed(ActionEvent evt) { . . . } Furthermore, the button must be told that the applet will be "listening" for action events from the button. This is done by calling one of the button object's instance methods, addActionListener(), in the applet's init() method. What should the applet do in its actionPerformed() method? When the user clicks the button, we want a message window to appear on the screen. Fortunately, Swing makes this easy. The class swing.javax.JOptionPane has a static method named showMessageDialog() that can be used for this purpose, so all we have to do in actionPerformed() is call that method. Given all this, you can understand a lot of what goes on in the source code for the HelloSwing applet. This example shows several aspects of applet programming: An init() method sets up the applet and adds components, the components generate events, and event-handling methods say what happens in response to those events. Here is the source code: http://math.hws.edu/eck/cs124/javanotes4/c6/s1.html (4 of 6) [6/11/2004 11:10:26 AM] Java Programming: Section 6.1 // An applet that appears on the page as a button that says // "Click Me!". When the button is clicked, an informational // dialog box appears to say Hello from Swing. import javax.swing.*; // Swing GUI classes are defined here. import java.awt.event.*; // Event handling class are defined here. public class HelloSwing extends JApplet implements ActionListener { public void init() { // This method is called by the system before the applet // appears. It is used here to create the button and add // it to the "content pane" of the JApplet. The applet // is also registered as an ActionListener for the button. JButton bttn = new JButton("Click Me!"); bttn.addActionListener(this); getContentPane().add(bttn); } // end init() public void actionPerformed(ActionEvent evt) { // This method is called when an action event occurs. // In this case, the only possible source of the event // is the button. So, when this method is called, we know // that the button has been clicked. Respond by showing // an informational dialog box. The dialog box will // contain an "OK" button which the user must click to // dismiss the dialog box. String title = "Greetings"; // Shown in title bar of dialog box. String message = "Hello from the Swing User Interface Library."; JOptionPane.showMessageDialog(null, message, title, JOptionPane.INFORMATION_MESSAGE); } // end actionPerformed() } // end class HelloSwing In this source code, I've set up the applet itself to listen for action events from the button. Some people don't consider this to be very good style. They prefer to create a separate object to listen for and respond to events. This is more "object-oriented" in the sense that each object has its own clearly defined area of responsibility. The most convenient way to make a separate event-handling object is to use a nested anonymous class. These classes were introduced in Section 5.6. We will see more examples of this in the future, but here, for the record, is a version of HelloSwing that uses an anonymous class for event handling. This applet has exactly the same behavior as the original version: import javax.swing.*; // Swing GUI classes are defined here. import java.awt.event.*; // Event handling class are defined here. public class HelloSwing2 extends JApplet { public void init() { // This method is called by the system before the applet // appears. It is used here to create the button and add http://math.hws.edu/eck/cs124/javanotes4/c6/s1.html (5 of 6) [6/11/2004 11:10:26 AM] Java Programming: Section 6.1 // it to the "content pane" of the JApplet. An anonymous // class is used to create an ActionListener for the button. JButton bttn = new JButton("Click Me!"); bttn.addActionListener( new ActionListener() { // The "action listener" for the button is defined // by this nested anonymous class. public void actionPerformed(ActionEvent evt) { // This method is called to respond when the user // presses the button. It displays a message in // a dialog box, along with an "OK" button which // the user can click to dismiss the dialog box. String title = "Greetings"; // Shown in box's title bar. String message = "Another hello from Swing."; JOptionPane.showMessageDialog(null, message, title, JOptionPane.INFORMATION_MESSAGE); } // end actionPerformed() }); getContentPane().add(bttn); } // end init() } // end class HelloSwing2 [ Next Section | Previous Chapter | Chapter Index | Main Index ] http://math.hws.edu/eck/cs124/javanotes4/c6/s1.html (6 of 6) [6/11/2004 11:10:26 AM] Java Programming: Section 6.2 Section 6.2 HTML Basics APPLETS GENERALLY APPEAR ON PAGES in a Web browser program. Such pages are themselves written in a language called HTML (HyperText Markup Language). An HTML document describes the contents of a page. A Web browser interprets the HTML code to determine what to display on the page. The HTML code doesn't look much like the resulting page that appears in the browser. The HTML document does contain all the text that appears on the page, but that text is "marked up" with commands that determine the structure and appearance of the text and determine what will appear on the page in addition to the text. HTML has developed rapidly in the last few years, and it has become a rather complicated language. In this section, I will cover just the basics of the language. While that leaves out all the fancy stuff, it does include just about everything I've used to make the Web pages in this on-line text. It is possible to write an HTML page using an ordinary text editor, typing in all the mark-up commands by hand. However, there are many Web-authoring programs that make it possible to create Web pages without ever looking at the underlying code. Using these tools, you can compose a Web page in much the same way that you would write a paper with a word processor. For example, Netscape Composer, which is part of Netscape Communicator, works in this way. However, my opinion is that making high-quality Web pages still requires some work with raw HTML, and serious Web authors still need to learn the HTML language. The mark-up commands used by HTML are called tags. An HTML tag takes the form <tag-name optional-modifiers> Where the tag-name is a word that specifies the command, and the optional-modifiers, if present, are used to provide additional information for the command (much like parameters in subroutines). A modifier takes the form modifier-name = value Usually, the value is enclosed in quotes, and it must be if it is more than one word long or if it contains certain special characters. There are a few modifiers which have no value, in which case only the name of the modifier is present. HTML is case insensitive, which means that you can use uppercase and lowercase letters interchangeably in tags and modifiers. A simple example of a tag is <HR>, which draws a line -- also called a "horizontal rule" -- across the page. The HR tag can take several possible modifiers such as WIDTH and ALIGN. For example, the short line just after the heading of this page was produced by the HTML command: <HR align=center width="33%"> The WIDTH here is specified as 33% of the available space. It could also be given as a fixed number of pixels. The value for ALIGN could be CENTER, LEFT, or RIGHT. A LEFT alignment would shove the line to the left side of the page, and a RIGHT alignment, to the right side. WIDTH and ALIGN are optional modifiers. If you leave them out, then their default values will be used. The default for WIDTH is 100%, and the default for ALIGN is LEFT. Many tags require matching closing tags, which take the form </tag-name> For example, the tag <PRE> must always have a matching closing tag </PRE> later in the document. The http://math.hws.edu/eck/cs124/javanotes4/c6/s2.html (1 of 7) [6/11/2004 11:10:27 AM] Java Programming: Section 6.2 tag applies to everything that comes between the opening tag and the closing tag. The <PRE> tag tells a Web browser to display everything between the <PRE> and the </PRE> just as it is formatted in the original HTML source code, including all the spaces and carriage returns. (But tags between <PRE> and </PRE> are still interpreted by the browser.) "PRE" stands for preformatted text. All of the sample programs in these notes are formatted using the <PRE> command. It is important for you to understand that when you don't use PRE, the computer will completely ignore the formatting of the text in the HTML source code. The only thing it pays attention to is the tags. Five blank lines in the source code have no more effect than one blank line or even a single blank space. Outside of <PRE>, if you want to force a new line on the Web page, you can use the tag <BR>, which stands for "break". For example, I might give my address as: David Eck<BR> Department of Mathematics and Computer Science<BR> Hobart and William Smith Colleges<BR> Geneva, NY 14456<BR> If you want extra vertical space in your web page, you can use several <BR>'s in a row. Similarly, you need a tag to indicate how the text should be broken up into paragraphs. This is done with the <P> tag, which should be placed at the beginning of every paragraph. The <P> tag has a matching </P>, which should be placed at the end of each paragraph. The closing </P> is technically optional, but it is considered good form to use it. If you want all the lines of the paragraph to be shoved over to the right, you can use <P ALIGN=RIGHT> instead of <P>. (This is mostly useful when used with one short line, or when used with <BR> to make several short lines.) You can also use <P ALIGN=CENTER> for centered lines. By the way, if tags like <P> and <HR> have special meanings in HTML, you might wonder how I can get them to appear here on this page. To get certain special characters to appear on the page, you have to use an entity name in the HTML source code. The entity name for < is <, and the entity name for > is >. Entity names begin with & and end with a semicolon. The character & is itself a special character whose entity name is &. There are also entity names for nonstandard characters such as the accented e, é, which has the entity name é. The rest of this page discusses several other basic HTML tags. This is not meant to be a complete discussion. But it is enough to produce interesting pages. Overall Document Structure HTML documents have a standard structure. They begin with <HTML> and end with </HTML>. Between these tags, there are two sections, the head, which is marked off by <HEAD> and </HEAD>, and the body, which -- as I'm sure you have guessed -- is surrounded by <BODY> and </BODY>. Often, the head contains only one item: a title for the document. This title might be shown, for example, in the title bar of a Web browser window. The title should not contain any HTML tags. The body contains the actual page contents that are displayed by the browser. So, an HTML document takes this form: <HTML> <HEAD> <TITLE>page-title</TITLE> </HEAD> <BODY> http://math.hws.edu/eck/cs124/javanotes4/c6/s2.html (2 of 7) [6/11/2004 11:10:27 AM] Java Programming: Section 6.2 page-contents </BODY> </HTML> Web browsers are not very picky about enforcing this structure; you can probably get away with leaving out everything but the actual page contents. But it is good form to follow this structure for your pages. The <BODY> tag can take a number of modifiers that affect the appearance of the page when it is displayed. The modifier named BGCOLOR can be used to set the background color of the page. For example, <BODY bgcolor=white> will ensure that the background color for the page is white. You can add modifiers to control the color of regular text (TEXT), hypertext links (LINK), and links to pages that have already been visited (VLINK). When the user clicks and holds the mouse button on a link, the link is said to be active; you can control the color of active links with the ALINK modifier. For example, how about a page with a black background, white text, blue links, red active links, and gray visited links: <BODY bgcolor=black text=white link=blue alink=red vlink=gray> There are several standard color names that you can use in this context, but if you want complete control, you'll have to learn how to specify colors using hexadecimal numbers. It is also possible to use an image for the background of the page, instead of a solid color. Look up the details if you are interested. Headings and Font Styles HTML has a number of tags that affect the size and style of displayed text. For a heading, which is meant to stand out on a line by itself, HTML offers the tags <H1>, <H2>, ..., <H6>. These tags are always used with matching closing tags such as </H1>. The <H1> tag is meant for the most important headings and produces the largest size text. I've found <H4> through <H6> to be too small to be useful. You can use <BR> tags in headings, if you want multi-line headings. You can also use links and images, which are described below. The heading tags can take ALIGN as a modifier, with the value LEFT, RIGHT, or CENTER. For example, the heading A Sample Heading was written as "<H1 align=center>A Sample Heading</H1>" in the HTML source code. There are a number of different style tags that you can apply to text. For example, bold text can be obtained by surrounding the text with <B> and </B>. You can use <i> for italic, <U> for underlined, and <TT> for typewriter style text. Most browsers support <SUB> for subscripted text and <SUP> for superscripted text. For example, "x<SUP>2</SUP>" will give: x2. Because HTML is meant to describe the logical structure of a document, rather than its exact appearance, it has a number of tags for displaying the logical style of the text. For example, the <EM> tag is meant to emphasize the text surrounded by <EM> and </EM>, while <STRONG> is for strong emphasis. And the <CITE> style tag is meant for titles of books. http://math.hws.edu/eck/cs124/javanotes4/c6/s2.html (3 of 7) [6/11/2004 11:10:27 AM] Java Programming: Section 6.2 You can get even more control over the style of the text by using the <FONT>...</FONT> tag. The <FONT> tag uses modifiers such as COLOR and SIZE to control the appearance of the font. For big blue text, you would say: <FONT color=blue size="+1">big blue text</FONT> The value "+1" for the SIZE modifier means "a little bigger than usual." You could use "+2" for an even bigger font, "-1" for a smaller font, and so on. However, only a limited number of different sizes are available. Lists There are several tags for producing lists of items. The most widely used of these are <UL> and <OL>. The <OL> tag gives an "ordered list", in which the items are numbered consecutively. The item numbers are provided by the browser. The <UL> tag gives an "unordered list", in which the items are all marked with the same special symbol. In the HTML source code, each list item is indicated by placing a <LI> tag at the beginning of the item. The end of the list is marked by the appropriate closing tag, </OL> or </UL>. For example, the following source code: <UL> <LI>Isaac Asimov <LI>Ursula Leguin <LI>Greg Bear <LI>C. J. Cherryh </UL> produces this list: ● Isaac Asimov ● Ursula Leguin ● Greg Bear ● C. J. Cherryh Links The most distinctive feature of HTML is that documents can contain links to other documents. The user can follow links from page to page and in the process visit pages from all over the Internet. The <A> tag is used to create a link. The text between the <A> and its matching </A> appears on the page. Usually, it is underlined and in a special color. The user can follow the link by clicking on this text. The <A> tag uses the modifier HREF to say which document the link should connect to. The value for HREF must be a URL (Uniform Resource Locator). A URL is a coded set of instructions for finding a document on the Internet. For example, the URL for my own "home page" is http://math.hws.edu/eck/ To make a link to this page, such as David's Home Page, I would use the HTML source code <A HREF="http://math.hws.edu/eck/">David's Home Page</A> The best place to find URLs is on existing Web pages. Most browsers display the URL for the page you are currently viewing, and they can display the URL of a link if you point to the link with the mouse. http://math.hws.edu/eck/cs124/javanotes4/c6/s2.html (4 of 7) [6/11/2004 11:10:27 AM] Java Programming: Section 6.2 If you are writing an HTML document and you want to make a link to another document that is in the same directory, you can use a relative URL. A relative URL consists of just the name of the file. For example, the page you are now viewing comes from a directory that also contains the other sections in this chapter. For a link to Section 1, which is in a file named s1.html, the relative URL would be just "s1.html", and the complete link would look like <A HREF="s1.html">Section 1</A> There are also relative URLs for linking to files that are in other directories. Using relative URLs is a good idea, since if you use them, you can move a whole collection of files without changing any of the links between them (as long as you don't change the relative locations of the files). When you type a URL into a Web browser, you can omit the "http://" at the beginning of the URL. However, in an <A> tag in an HTML document, the "http://" can only be omitted if the URL is a relative URL. For a normal URL, it is required. Images You can add images to a Web page with the <IMG> tag. (This is a tag that has no matching closing tag.) The actual image must be stored in a separate file from the HTML document. The <IMG> tag has a required modifier, named SRC, to specify the URL of the image file. For most browsers, the image should be in one of the formats GIF (with a file name ending in ".gif") or JPEG (with a file name ending in ".jpeg" or ".jpg"). A so-called animated gif file actually contains a series of images that the browser will display as an animation. Usually, the image is stored in the same place as the HTML document, and a relative URL is used to specify the image file. The <IMG> tag also has several optional modifiers. It's a good idea to always include the HEIGHT and WIDTH modifiers, which specify the size of the image in pixels. Some browsers, including Netscape, handle images better if they know in advance how big they are. For browsers that can't display images, you can use the ALT modifier to specify a string that will be displayed by the browser in place of the image. The ALIGN modifier can be used to affect the placement of the image. "ALIGN=RIGHT" will shove the image to the right edge of the page, and the text on the page will flow around the image. "ALIGN=LEFT" works similarly. (Unfortunately, "ALIGN=CENTER" doesn't have the meaning you would expect. Browsers treat images as if they are just big characters. Images can occur inside paragraphs, links, and headings, for example. Alignment values of CENTER, TOP, and BOTTOM are used to specify how the image should line up with other characters in a line of text: Should the baseline of the text be at the center, the top, or the bottom of the image? Alignment values of RIGHT and LEFT were added to HTML later, but they are the most useful values.) For example, here is HTML code that will place an image from a file named figure1.gif on the page. <IMG SRC="figure1.gif" ALIGN=RIGHT HEIGHT=150 WIDTH=100 ALT="Figure 1"> The image is 100 pixels wide and 150 pixels high. It will appear on the right edge of the page. If a browser can't display images, it will display the string "Figure 1" instead. There are many places on the Web where you can get graphics for use on your Web pages. For example, http://www.iconbazaar.com makes a large number of images available. You should, of course, check on the owner's copyright policy before using someone else's images on your pages. http://math.hws.edu/eck/cs124/javanotes4/c6/s2.html (5 of 7) [6/11/2004 11:10:27 AM] Java Programming: Section 6.2 The Applet tag and Applet Parameters The <APPLET> tag is used to add a Java applet to a Web page. This tag must have a matching </APPLET>. A required modifier named CODE gives the name of the compiled class file that contains the applet. HEIGHT and WIDTH modifiers are required to specify the size of the applet. If you want the applet to be centered on the page, you can put the applet in a paragraph with CENTER alignment So, an applet tag to display an applet named HelloWorldApplet centered on a Web page would look like this: <P ALIGN=CENTER> <APPLET CODE="HelloWorldApplet.class" HEIGHT=50 WIDTH=150> </APPLET> </P> This assumes that the file HelloWorldApplet.class is located in the same directory with the HTML document. If this is not the case, you can use another modifier, CODEBASE, to give the URL of the directory that contains the class file. The value of CODE itself is always just a file name, not a URL. If an applet uses a lot of .class files, it's a good idea to collect all the .class files into a single .zip or .jar file. Zip and jar files are archive files which hold a number of smaller files. Your Java development system is probably capable of creating them in some way. If your class files are in an archive, then you have to specify the name of the archive file in an ARCHIVE modifier in the <APPLET> tag. Archive files won't work on older browsers, but they should work for any browser that understands Java version 1.1 or later. Applets can use applet parameters to customize their behavior. Applet parameters are specified by using <PARAM> tags, which can only occur between an <APPLET> tag and the closing </APPLET>. The PARAM tag has required modifiers named NAME and VALUE, and it takes the form <PARAM NAME="param-name" VALUE="param-value"> The parameters are available to the applet when it runs. An applet can use the predefined method getParameter() to check for parameters specified in PARAM tags. The getParameter() method has the following interface: String getParameter(String paramName) The parameter paramName corresponds to the param-name in a PARAM tag. If the specified paramName actually occurs in one of the PARAM tags, then getParameter returns the associated param-value. If the specified paramName does not occur in any PARAM tag, then getParameter returns the value null. Parameter names are case-sensitive, so you can't use "size" in the PARAM tag and ask for "Size" in getParameter. By the way, if you put anything besides PARAM tags between <APPLET> and </APPLET>, it will be ignored by any browser that supports Java. On the other hand, a browser that does not support Java will ignore the APPLET and PARAM tags. This means that if you put a message such as "Your browser doesn't support Java" between <APPLET> and </APPLET>, then that message will only appear in browsers that don't support Java. Here is an example of an APPLET tag with PARAMs and some extra text for display in browsers that don't support Java: <APPLET code="ShowMessage.class" WIDTH=200 HEIGHT=50> <PARAM NAME="message" VALUE="Goodbye World!"> <PARAM NAME="font" VALUE="Serif"> <PARAM NAME="size" VALUE="36"> <p align=center>Sorry, but your browser doesn't support Java!</p> </APPLET> http://math.hws.edu/eck/cs124/javanotes4/c6/s2.html (6 of 7) [6/11/2004 11:10:27 AM] Java Programming: Section 6.2 The applet ShowMessage would presumably read these parameters in its init() method, which might go something like this: String display; // Instance variable: message to be displayed. String fontName; // Instance variable: font to use for display. public void init() { String value; value = getParameter("message"); // Get message PARAM, if any. if (value == null) display = "Hello World!"; // default value else display = value; // Value from PARAM tag. value = getParameter("font"); if (value == null) fontName = "SansSerif" else fontName = value; . . . Dealing with the size parameter would be just a little harder, since a parameter value is always a String, and the size is supposed to be an int. This means that the String value must somehow be converted to an int. We'll worry about how to do that later. [ Next Section | Previous Section | Chapter Index | Main Index ] http://math.hws.edu/eck/cs124/javanotes4/c6/s2.html (7 of 7) [6/11/2004 11:10:27 AM] Java Programming: Section 6.3 Section 6.3 Graphics and Painting EVERYTHING YOU SEE ON A COMPUTER SCREEN has to be drawn there, even the text. The Java API includes a range of classes and methods that are devoted to drawing. In this section, I'll look at some of the most basic of these. An applet is an example of a GUI component. The term component refers to a visual element in a GUI, including buttons, menus, text-input boxes, scroll bars, check boxes, and so on. In Java, GUI components are represented by objects belonging to subclasses of the class java.awt.Component. Most components in the Swing GUI -- although not top-level components like JApplet -- belong to subclasses of the class javax.swing.JComponent. Every component is responsible for drawing itself. For example, if you want to use a standard component, you only have to add it to your applet. You don't have to worry about painting it on the screen. That will happen automatically. Sometimes, however, you do want to draw on a component. You will have to do this whenever you want to display something that is not included among the standard, pre-defined component classes. When you want to do this, you have to define your own component class and provide a method in that class for drawing the component. As we have seen in Section 6.1 and in Section 3.7, when painting on a plain, non-Swing Applet, the drawing is done in a paint() method. To do custom drawing, you have to define a subclass of Applet and include a paint() method to do the drawing. However, when it comes to Swing and JApplets, things are a little more complicated. You should not draw directly on JApplets or on other top-level Swing components. Instead, you should make a separate component to use as a drawing surface, and you should add that component to the JApplet. You will have to write a class to represent the drawing surface, so programming a JApplet that does custom drawing will always involve writing at least two classes: a class for the applet itself and a class for the drawing surface. Typically, the class for the drawing surface will be defined as a subclass of javax.swing.JPanel, which by default is nothing but a blank area on the screen. A JPanel, like any JComponent, draws its content in the method public void paintComponent(Graphics g) To create a drawing surface, you should define a subclass of JPanel and provide a custom paintComponent() method. Create an object belonging to your new class, and add it to your JApplet. When the time comes for your component to be drawn on the screen, the system will call its paintComponent() to do the drawing. All this is not really as complicated as it might sound. We will go over this in more detail when the time comes. Note that the paintComponent() method has a parameter of type Graphics. The Graphics object will be provided by the system when it calls your method. You need this object to do the actual drawing. To do any drawing at all in Java, you need a graphics context. A graphics context is an object belonging to the class, java.awt.Graphics. Instance methods are provided in this class for drawing shapes, text, and images. Any given Graphics object can draw to only one location. In this chapter, that location will always be a GUI component belonging to some subclass of JComponent. The Graphics class is an abstract class, which means that it is impossible to create a graphics context directly, with a constructor. There are actually two ways to get a graphics context for drawing on a component: First of all, of course, when the paintComponent() method of a component is called by the system, the parameter to that method is a graphics context for drawing on the component. Second, each component has an instance method called getGraphics(). This method is a function that returns a graphics context that can be used for drawing on the component outside its paintComponent() method. The official line is that you should not do this, and I will avoid it for the most part. But I have found it convenient to use getGraphics() in some cases, since it can mean better performance for certain types of drawing. (Anyway, if the people who designed Java really didn't want us to use it, they shouldn't have made the getGraphics() method public!) http://math.hws.edu/eck/cs124/javanotes4/c6/s3.html (1 of 10) [6/11/2004 11:10:28 AM] Java Programming: Section 6.3 Most components do, in fact, do all drawing operations in their paintComponent() methods. What happens if, in the middle of some other method, you realize that the content of the component needs to be changed? You should not call paintComponent() directly to make the change; this method is meant to be called only by the system. Instead, you have to inform the system that the component needs to be redrawn, and let the system do its job by calling paintComponent(). You do this by calling the repaint() method. The method public void repaint(); is defined in the Component class, and so can be used with any component. You should call repaint() to inform the system that the component needs to be redrawn. The repaint() method returns immediately, without doing any painting itself. The system will call the component's paintComponent() method later, as soon as it gets a chance to do so, after processing other pending events if there are any. Note that the system can also call paintComponent() for other reasons. It is called when the component first appears on the screen. It will also be called if the component is covered up by another window and then uncovered. The system does not save a copy of the component's contents when it is covered. When it is uncovered, the component is responsible for redrawing itself. (As you will see, some of our early examples will not be able to do this correctly.) This means that, to work properly, the paintComponent() method must be smart enough to correctly redraw the component at any time. To make this possible, a program should store data about the state of the component in its instance variables. These variables should contain all the information necessary to redraw the component completely. The paintComponent() method should use the data in these variables to decide what to draw. When the program wants to change the content of the component, it should not simply draw the new content. It should change the values of the relevant variables and call repaint(). When the system calls paintComponent(), this method will use the new values of the variables and will draw the component with the desired modifications. This might seem a roundabout way of doing things. Why not just draw the modifications directly? There are at least two reasons. First of all, it really does turn out to be easier to get things right if all drawing is done in one method. Second, even if you did make modifications directly, you would still have to make the paintComponent() method aware of them in some way so that it will be able to redraw the component correctly when it is covered and uncovered. You will see how all this works in practice as we work through examples in the rest of this chapter. For now, we will spend the rest of this section looking at how to get some actual drawing done. Coordinates The screen of a computer is a grid of little squares called pixels. The color of each pixel can be set individually, and drawing on the screen just means setting the colors of individual pixels. A graphics context draws in a rectangle made up of pixels. A position in the rectangle is specified by a pair of integer coordinates, (x,y). The upper left corner has coordinates (0,0). The x coordinate increases from left to right, and the y coordinate increases from top to bottom. The illustration on the right shows a 12-by-8 pixel component (with very large pixels). A small line, rectangle, and oval are shown as they would be drawn by coloring individual pixels. (Note that, properly speaking, the coordinates don't belong to the pixels but to the grid lines between them.) For any component, you can find out the size of the rectangle that it occupies by calling the instance method http://math.hws.edu/eck/cs124/javanotes4/c6/s3.html (2 of 10) [6/11/2004 11:10:28 AM] Java Programming: Section 6.3 getSize(). This method returns an object that belongs to the class, java.awt.Dimension. A Dimension object has two integer instance variables, width and height. The width of the component is getSize().width pixels, and its height is getSize().height pixels. When you are writing an applet, you don't necessarily know the applet's size. The size of an applet is usually specified in an <APPLET> tag in the source code of a Web page, and it's easy for the Web-page author to change the specified size. In some cases, when the applet is displayed in some other kind of window instead of on a Web page, the applet can even be resized while it is running. So, it's not good form to depend on the size of the applet being set to some particular value. For other components, you have even less chance of knowing the component's size in advance. This means that it's good form to check the size of a component before doing any drawing on that component. For example, you can use a paintComponent() method that looks like: public void paintComponent(Graphics g) { int width = getSize().width; // Find out the width of component. int height = getSize().height; // Find out its height. . . . // Draw the contents of the component. } Of course, your drawing commands will have to take the size into account. That is, they will have to use (x,y) coordinates that are calculated based on the actual height and width of the applet. Colors Java is designed to work with the RGB color system. An RGB color is specified by three numbers that give the level of red, green, and blue, respectively, in the color. A color in Java is an object of the class, java.awt.Color. You can construct a new color by specifying its red, blue, and green components. For example, myColor = new Color(r,g,b); There are two constructors that you can call in this way. In the one that I almost always use, r, g, and b are integers in the range 0 to 255. In the other, they are numbers of type float in the range 0.0F to 1.0F. (You might recall that a literal of type float is written with an "F" to distinguish it from a double number.) Often, you can avoid constructing new colors altogether, since the Color class defines several named constants representing common colors: Color.white, Color.black, Color.red, Color.green, Color.blue, Color.cyan, Color.magenta, Color.yellow, Color.pink, Color.orange, Color.lightGray, Color.gray, and Color.darkGray. An alternative to RGB is the HSB color system. In the HSB system, a color is specified by three numbers called the hue, the saturation, and the brightness. The hue is the basic color, ranging from red through orange through all the other colors of the rainbow. The brightness is pretty much what it sounds like. A fully saturated color is a pure color tone. Decreasing the saturation is like mixing white or gray paint into the pure color. In Java, the hue, saturation and brightness are always specified by values of type float in the range from 0.0F to 1.0F. The Color class has a static member function named getHSBColor for creating HSB colors. To create the color with HSB values given by h, s, and b, you can say: myColor = Color.getHSBColor(h,s,b); For example, you could make a random color that is as bright and as saturated as possible with myColor = Color.getHSBColor( (float)Math.random(), 1.0F, 1.0F ); The type cast is necessary because the value returned by Math.random() is of type double, and Color.getHSBColor() requires values of type float. (By the way, you might ask why RGB colors are created using a constructor while HSB colors are created using a static member function. The problem is that we would need two different constructors, both of them with three parameters of type float. Unfortunately, this is impossible. You can only have two constructors if the number of parameters or the parameter types http://math.hws.edu/eck/cs124/javanotes4/c6/s3.html (3 of 10) [6/11/2004 11:10:28 AM] Java Programming: Section 6.3 differ.) The RGB system and the HSB system are just different ways of describing the same set of colors. It is possible to translate between one system and the other. The best way to understand the color systems is to experiment with them. In the following applet, you can use the scroll bars to control the RGB and HSB values of a color. A sample of the color is shown on the right side of the applet. Computer monitors differ as to the number of different colors they can display, so you might not get to see the full range of colors in this applet. (Applet "ColorChooserApplet" would be displayed here if Java were available.) One of the instance variables in a Graphics object is the current drawing color, which is used for all drawing of shapes and text. If g is a graphics context, you can change the current drawing color for g using the method g.setColor(c), where c is a Color. For example, if you want to draw in green, you would just say g.setColor(Color.green) before doing the drawing. The graphics context continues to use the color until you explicitly change it with another setColor() command. If you want to know what the current drawing color is, you can call the function g.getColor(), which returns an object of type Color. This can be useful if you want to change to another drawing color temporarily and then restore the previous drawing color. Every component has an associated foreground color and background color. Generally, the component is filled with the background color before anything else is drawn (although some components are "transparent," meaning that the background color is ignored). When a new graphics context is created for a component, the current drawing color is set to the foreground color. Note that the foreground color and background color are properties of the component, not of a graphics context. The foreground and background colors can be set by instance methods setForeground(c) and setBackground(c), which are defined in the Component class and therefore are available for use with any component. Fonts A font represents a particular size and style of text. The same character will appear different in different fonts. In Java, a font is characterized by a font name, a style, and a size. The available font names are system dependent, but you can always use the following four strings as font names: "Serif", "SansSerif", "Monospaced", and "Dialog". In the original Java 1.0, the font names were "TimesRoman", "Helvetica", and "Courier". You can still use the older names if you want. (A "serif" is a little decoration on a character, such as a short horizontal line at the bottom of the letter i. "SansSerif" means "without serifs." "Monospaced" means that all the characters in the font have the same width. The "Dialog" font is the one that is typically used in dialog boxes.) The style of a font is specified using named constants that are defined in the Font class. You can specify the style as one of the four values: ● Font.PLAIN, ● Font.ITALIC, ● Font.BOLD, or ● Font.BOLD + Font.ITALIC. The size of a font is an integer. Size typically ranges from about 10 to 36, although larger sizes can also be used. The size of a font is usually about equal to the height of the largest characters in the font, in pixels, but this is not a definite rule. The size of the default font is 12. Java uses the class named java.awt.Font for representing fonts. You can construct a new font by specifying its font name, style, and size in a constructor: http://math.hws.edu/eck/cs124/javanotes4/c6/s3.html (4 of 10) [6/11/2004 11:10:28 AM] Java Programming: Section 6.3 Font plainFont = new Font("Serif", Font.PLAIN, 12); Font bigBoldFont = new Font("SansSerif", Font.BOLD, 24); Every graphics context has a current font, which is used for drawing text. You can change the current font with the setFont() method. For example, if g is a graphics context and bigBoldFont is a font, then the command g.setFont(bigBoldFont) will set the current font of g to bigBoldFont. You can find out the current font of g by calling the method g.getFont(), which returns an object of type Font. Every component has an associated font. It can be set with the instance method setFont(font), which is defined in the Component class. When a graphics context is created for drawing on a component, the graphic context's current font is set equal to the font of the component. Shapes The Graphics class includes a large number of instance methods for drawing various shapes, such as lines, rectangles, and ovals. The shapes are specified using the (x,y) coordinate system described above. They are drawn in the current drawing color of the graphics context. The current drawing color is set to the foreground color of the component when the graphics context is created, but it can be changed at any time using the setColor() method. Here is a list of some of the most important drawing methods. With all these commands, any drawing that is done outside the boundaries of the component is ignored. Note that all these methods are in the Graphics class, so they all must be called through an object of type Graphics. drawString(String str, int x, int y) -- Draws the text given by the string str. The string is drawn using the current color and font of the graphics context. x specifies the position of the left end of the string. y is the y-coordinate of the baseline of the string. The baseline is a horizontal line on which the characters rest. Some parts of the characters, such as the tail on a y or g, extend below the baseline. drawLine(int x1, int y1, int x2, int y2) -- Draws a line from the point (x1,y1) to the point (x2,y2). The line is drawn as if with a pen that hangs one pixel to the right and one pixel down from the (x,y) point where the pen is located. For example, if g refers to an object of type Graphics, then the command g.drawLine(x,y,x,y), which corresponds to putting the pen down at a point, draws the single pixel located at the point (x,y). drawRect(int x, int y, int width, int height) -- Draws the outline of a rectangle. The upper left corner is at (x,y), and the width and height of the rectangle are as specified. If width equals height, then the rectangle is a square. If the width or the height is negative, then nothing is drawn. The rectangle is drawn with the same pen that is used for drawLine(). This means that the actual width of the rectangle as drawn is width+1, and similarly for the height. There is an extra pixel along the right edge and the bottom edge. For example, if you want to draw a rectangle around the edges of the component, you can say "g.drawRect(0, 0, getSize().width-1, getSize().height-1);", where g is a graphics context for the component. drawOval(int x, int y, int width, int height) -- Draws the outline of an oval. The oval is one that just fits inside the rectangle specified by x, y, width, and height. If width equals height, the oval is a circle. drawRoundRect(int x, int y, int width, int height, int xdiam, int ydiam) -- Draws the outline of a rectangle with rounded corners. The basic rectangle is specified by x, y, width, and height, but the corners are rounded. The degree of rounding is given by xdiam and ydiam. The corners are arcs of an ellipse with horizontal diameter http://math.hws.edu/eck/cs124/javanotes4/c6/s3.html (5 of 10) [6/11/2004 11:10:28 AM] Java Programming: Section 6.3 xdiam and vertical diameter ydiam. A typical value for xdiam and ydiam is 16. But the value used should really depend on how big the rectangle is. draw3DRect(int x, int y, int width, int height, boolean raised) -- Draws the outline of a rectangle that is supposed to have a three-dimensional effect, as if it is raised from the screen or pushed into the screen. The basic rectangle is specified by x, y, width, and height. The raised parameter tells whether the rectangle seems to be raised from the screen or pushed into it. The 3D effect is achieved by using brighter and darker versions of the drawing color for different edges of the rectangle. The documentation recommends setting the drawing color equal to the background color before using this method. The effect won't work well for some colors. drawArc(int x, int y, int width, int height, int startAngle, int arcAngle) -- Draws part of the oval that just fits inside the rectangle specified by x, y, width, and height. The part drawn is an arc that extends arcAngle degrees from a starting angle at startAngle degrees. Angles are measured with 0 degrees at the 3 o'clock position (the positive direction of the horizontal axis). Positive angles are measured counterclockwise from zero, and negative angles are measured clockwise. To get an arc of a circle, make sure that width is equal to height. fillRect(int x, int y, int width, int height) -- Draws a filled-in rectangle. This fills in the interior of the rectangle that would be drawn by drawRect(x,y,width,height). The extra pixel along the bottom and right edges is not included. The width and height parameters give the exact width and height of the rectangle. For example, if you wanted to fill in the entire component, you could say "g.fillRect(0, 0, getSize().width, getSize().height);" fillOval(int x, int y, int width, int height) -- Draws a filled-in oval. fillRoundRect(int x, int y, int width, int height, int xdiam, int ydiam) -- Draws a filled-in rounded rectangle. fill3DRect(int x, int y, int width, int height, boolean raised) -- Draws a filled-in three-dimensional rectangle. fillArc(int x, int y, int width, int height, int startAngle, int arcAngle) -- Draw a filled-in arc. This looks like a wedge of pie, whose crust is the arc that would be drawn by the drawArc method. Let's use some of the material covered in this section to write a JApplet. Since we will be drawing on the applet, we will need to create a drawing surface. The drawing surface will be a JComponent belonging to a subclass of the JPanel class. We will define this class as a nested class inside the main applet class. Nested classes were introduced in Section 5.6. All the drawing is done in the paintComponent() method of the drawing surface class. I will use nested classes consistently to define drawing surfaces, although it is perfectly legal to use an independent class instead of a nested class to define the drawing surface. A nested class can be either static or non-static. In general, a non-static class must be used if it needs access to instance variables or instance methods that are defined in the main class. This will be the case in most of my examples. The applet will draw multiple copies of a message on a black background. Each copy of the message is in a random color. Five different fonts are used, with different sizes and styles. The displayed message is the string "Java!", but a different message can be specified in an applet param. (Applet params were discussed at the end of the previous section.) The applet works OK no matter what size is specified for the applet in the <applet> tag. Here's the applet: (Applet "RandomStrings" would be displayed here if Java were available.) http://math.hws.edu/eck/cs124/javanotes4/c6/s3.html (6 of 10) [6/11/2004 11:10:28 AM] Java Programming: Section 6.3 The applet does have a problem. When the drawing surface's paintComponent() method is called, it chooses random colors, fonts, and locations for the messages. The information about which colors, fonts, and locations are used is not stored anywhere. The next time paintComponent() is called, it will make different random choices and will draw a different picture. For this particular applet, the problem only really appears when the applet is partially covered and then uncovered. Only the part that was covered will be redrawn, and in the part that's not redrawn, the old picture will remain. Try it. You'll see partial messages, cut off by the dividing line between the new picture and the old. (Actually, in some browsers, the entire applet might be repainted, even if only part of it was covered.) A better approach would be to compute the contents of the picture elsewhere, outside the paintComponent() method. Information about the picture should be stored in instance variables, and the paintComponent() method should use that information to draw the picture. If paintComponent() is called twice, it should draw the same picture twice, unless the data has changed in the meantime. Unfortunately, to store the data for the picture in this applet, we would need to use either arrays, which will not be covered until Chapter 8, or off-screen images, which will not be covered until Section 7.1. Other applets in this chapter will suffer from the same problem. The source for the applet is shown below. I use an instance variable called message to hold the message that the applet will display. There are five instance variables of type Font that represent different sizes and styles of text. These variables are initialized in the applet's init() method and are used in the drawing surface's paintComponent() method. I also use the init() method to create the drawing surface, add it to the applet, and set its background color to black. The paintComponent() method for the drawing surface simply draws 25 copies of the message. For each copy, it chooses one of the five fonts at random, and it calls g.setFont() to select that font for drawing the text. It creates a random HSB color and uses g.setColor() to select that color for drawing. It then chooses random (x,y) coordinates for the location of the message. The x coordinate gives the horizontal position of the left end of the string. The formula used for the x coordinate, "-50 + (int)(Math.random()*(width+40)" gives a random integer in the range from -50 to width-10. This makes it possible for the string to extend beyond the left edge or the right edge of the applet. Similarly, the formula for y allows the string to extend beyond the top and bottom of the applet. The drawing surface class, which is named Display, defines the paintComponent() method that draws all the strings that appear in the applet. The drawing surface is created in the applet's init() method as an object of type Display. This object is set to be the "content pane" of the applet. A JApplet's content pane fills the entire applet, except for an optional menu bar. An applet comes with a default content pane, and you can add components to that content pane. However, any JComponent can be a content pane, and in a case like this where a single component fills the applet, it makes sense to replace the content pane with the setContentPane() method. The paintComponent() method in the Display class begins with a call to super.paintComponent(g). The special variable super was discussed in Section 5.5. The command super.paintComponent(g) simply calls the paintComponent() method that is defined in the superclass, JPanel. The effect of this is to fill the component with its background color. Most paintComponent() methods begin with a call to super.paintComponent(g), but this is not necessary if the drawing commands in the method cover the background of the component completely. Here is the complete source code for the RandomStrings applet: /* This applet displays 25 copies of a message. The color and position of each message is selected at random. The font of each message is randomly chosen from among five possible fonts. The messages are displayed on a black background. Note: This applet uses bad style, because every time the paintComponent() method is called, new random values are used. This means that a different picture will be drawn each http://math.hws.edu/eck/cs124/javanotes4/c6/s3.html (7 of 10) [6/11/2004 11:10:28 AM] Java Programming: Section 6.3 time. This is particularly bad if only part of the applet needs to be redrawn, since then the applet will contain cut-off pieces of messages. When this file is compiled, it produces two classes, RandomStrings.class and RandomStrings$Display.class. Both classes are required to use the applet. */ import java.awt.*; import javax.swing.*; public class RandomStrings extends JApplet { String message; // The message to be displayed. This can be set in // an applet param with name "message". If no // value is provided in the applet tag, then // the string "Java!" is used as the default. Font font1, font2, font3, font4, font5; // The five fonts. Display drawingSurface; // This is the component on which the // drawing will actually be done. It // is defined by a nested class that // can be found below. public void init() { // Called by the system to initialize the applet. message = getParameter("message"); if (message == null) message = "Java!"; font1 = new Font("Serif", Font.BOLD, 14); font2 = new Font("SansSerif", Font.BOLD + Font.ITALIC, 24); font3 = new Font("Monospaced", Font.PLAIN, 20); font4 = new Font("Dialog", Font.PLAIN, 30); font5 = new Font("Serif", Font.ITALIC, 36); drawingSurface = new Display(); // Create the drawing surface. drawingSurface.setBackground(Color.black); setContentPane(drawingSurface); // Since drawingSurface will fill // the entire applet, we simply // replace the applet's content // pane with drawingSurface. } // end init() class Display extends JPanel { // This nested class defines a JPanel that is used // for displaying the content of the applet. An // object of this class is used as the content pane // of the applet. Note that since this is a nested // non-static class, it has access to the instance // variables of the main class such as message and font1. http://math.hws.edu/eck/cs124/javanotes4/c6/s3.html (8 of 10) [6/11/2004 11:10:28 AM] Java Programming: Section 6.3 public void paintComponent(Graphics g) { super.paintComponent(g); // Call the paintComponent method from // the superclass, JPanel. This simply // fills the entire component with the // component's background color. int width = getSize().width; // Get this component's width. int height = getSize().height; // Get this component's height. for (int i = 0; i < 25; i++) { // Draw one string. First, set the font to be one of the five // available fonts, at random. int fontNum = (int)(5*Math.random()) + 1; switch (fontNum) { case 1: g.setFont(font1); break; case 2: g.setFont(font2); break; case 3: g.setFont(font3); break; case 4: g.setFont(font4); break; case 5: g.setFont(font5); break; } // end switch // Set the color to a bright, saturated color, with random hue. float hue = (float)Math.random(); g.setColor( Color.getHSBColor(hue, 1.0F, 1.0F) ); // Select the position of the string, at random. int x,y; x = -50 + (int)(Math.random()*(width+40)); y = (int)(Math.random()*(height+20)); // Draw the message. g.drawString(message,x,y); } // end for } // end paintComponent() } // end nested class Display http://math.hws.edu/eck/cs124/javanotes4/c6/s3.html (9 of 10) [6/11/2004 11:10:28 AM] Java Programming: Section 6.3 } // end class RandomStrings [ Next Section | Previous Section | Chapter Index | Main Index ] http://math.hws.edu/eck/cs124/javanotes4/c6/s3.html (10 of 10) [6/11/2004 11:10:28 AM] Java Programming: Section 6.4 Section 6.4 Mouse Events EVENTS ARE CENTRAL to programming for a graphical user interface. A GUI program doesn't have a main() routine that outlines what will happen when the program is run, in a step-by-step process from beginning to end. Instead, the program must be prepared to respond to various kinds of events that can happen at unpredictable times and in an order that the program doesn't control. The most basic kinds of events are generated by the mouse and keyboard. The user can press any key on the keyboard, move the mouse, or press a button on the mouse. The user can do any of these things at any time, and the computer has to respond appropriately. In Java, events are represented by objects. When an event occurs, the system collects all the information relevant to the event and constructs an object to contain that information. Different types of events are represented by objects belonging to different classes. For example, when the user presses one of the buttons on a mouse, an object belonging to a class called MouseEvent is constructed. The object contains information such as the GUI component on which the user clicked, the (x,y) coordinates of the point in the component where the click occurred, and which button on the mouse was pressed. When the user presses a key on the keyboard, a KeyEvent is created. After the event object is constructed, it is passed as a parameter to a designated subroutine. By writing that subroutine, the programmer says what should happen when the event occurs. As a Java programmer, you get a fairly high-level view of events. There is lot of processing that goes on between the time that the user presses a key or moves the mouse and the time that a subroutine in your program is called to respond to the event. Fortunately, you don't need to know much about that processing. But you should understand this much: Even though your GUI program doesn't have a main() routine, there is a sort of main routine running somewhere that executes a loop of the form while the program is still running: Wait for the next event to occur Call a subroutine to handle the event This loop is called an event loop. Every GUI program has an event loop. In Java, you don't have to write the loop. It's part of "the system." If you write a GUI program in some other language, you might have to provide a main routine that runs an event loop. In this section, we'll look at handling mouse events in Java, and we'll cover the framework for handling events in general. The next section will cover keyboard events. Java also has other types of events, which are produced by GUI components. These will be introduced in Section 6 and covered in detail in Section 7.3. For an event to have any effect, a program must detect the event and react to it. In order to detect an event, the program must "listen" for it. Listening for events is something that is done by an object called an event listener. An event listener object must contain instance methods for handling the events for which it listens. For example, if an object is to serve as a listener for events of type MouseEvent, then it must contain the following method (among several others): public void mousePressed(MouseEvent evt) { . . . } The body of the method defines how the object responds when it is notified that a mouse button has been pressed. The parameter, evt, contains information about the event. This information can be used by the listener object to determine its response. The methods that are required in a mouse event listener are specified in an interface named http://math.hws.edu/eck/cs124/javanotes4/c6/s4.html (1 of 11) [6/11/2004 11:10:30 AM] Java Programming: Section 6.4 MouseListener. To be used as a listener for mouse events, an object must implement this MouseListener interface. Java interfaces were covered in Section 5.6. (To review briefly: An interface in Java is just a list of instance methods. A class can "implement" an interface by doing two things. First, the class must be declared to implement the interface, as in "class MyListener implements MouseListener" or "class RandomStrings extends JApplet implements MouseListener". Second, the class must include a definition for each instance method specified in the interface. An interface can be used as the type for a variable or formal parameter. We say that an object implements the MouseListener interface if it belongs to a class that implements the MouseListener interface. Note that it is not enough for the object to include the specified methods. It must also belong to a class that is specifically declared to implement the interface.) Every event in Java is associated with a GUI component. For example, when the user presses a button on the mouse, the associated component is the one that the user clicked on. Before a listener object can "hear" events associated with a given component, the listener object must be registered with the component. If a MouseListener object, mListener, needs to hear mouse events associated with a component object, comp, the listener must be registered with the component by calling "comp.addMouseListener(mListener);". The addMouseListener() method is an instance method in the class, Component, and so can be used with any GUI component object. In our first few examples, we will listen for events on a JPanel that is being used as the drawing surface of a JApplet. The event classes, such as MouseEvent, and the listener interfaces, such as MouseListener, are defined in the package java.awt.event. This means that if you want to work with events, you should include the line "import java.awt.event.*;" at the beginning of your source code file. Admittedly, there is a large number of details to tend to when you want to use events. To summarize, you must 1. Put the import specification "import java.awt.event.*;" at the beginning of your source code; 2. Declare that some class implements the appropriate listener interface, such as MouseListener; 3. Provide definitions in that class for the subroutines from the interface; 4. Register the listener object with the component that will generate the events by calling a method such as addMouseListener() in the component. Any object can act as an event listener, provided that it implements the appropriate interface. A component can listen for the events that it itself generates. An applet can listen for events from components that are contained in the applet. A special class can be created just for the purpose of defining a listening object. Many people consider it to be good form to use anonymous nested classes to define listening objects. (See Section 5.6 for information on anonymous nested classes.) You will see all of these patterns in examples in this textbook. MouseEvent and MouseListener The MouseListener interface specifies five different instance methods: public void mousePressed(MouseEvent evt); public void mouseReleased(MouseEvent evt); public void mouseClicked(MouseEvent evt); public void mouseEntered(MouseEvent evt); public void mouseExited(MouseEvent evt); The mousePressed method is called as soon as the user presses down on one of the mouse buttons, and mouseReleased is called when the user releases a button. These are the two methods that are most commonly used, but any mouse listener object must define all five methods. You can leave the body of a method empty if you don't want to define a response. The mouseClicked method is called if the user http://math.hws.edu/eck/cs124/javanotes4/c6/s4.html (2 of 11) [6/11/2004 11:10:30 AM] Java Programming: Section 6.4 presses a mouse button and then releases it quickly, without moving the mouse. (When the user does this, all three routines -- mousePressed, mouseReleased, and mouseClicked -- will be called in that order.) In most cases, you should define mousePressed instead of mouseClicked. The mouseEntered and mouseExited methods are called when the mouse cursor enters or leaves the component. For example, if you want the component to change appearance whenever the user moves the mouse over the component, you could define these two methods. As an example, let's look at an applet that does something when the user clicks on it. Here's an improved version of the RandomStrings applet from the end of the previous section. In this version, the applet will redraw itself when you click on it: (Applet "ClickableRandomStrings" would be displayed here if Java were available.) For this version of the applet, we need to make four changes in the source code. First, add the line "import java.awt.event.*;" before the class definition. Second, declare that some class implements the MouseListener interface. If we want to use the applet itself as the listener, we would do this by saying: class RandomStrings extends JApplet implements MouseListener { ... Third, define the five methods of the MouseListener interface. Only mousePressed will do anything. We want to repaint the drawing surface of the applet when the user clicks the mouse. The drawing surface is represented in this applet by an instance variable named drawingSurface, so the mousePressed() just needs to call drawingSurface.repaint() to force the drawing surface to be redrawn. The other mouse listener methods are empty. The following methods are added to the applet class definition: public void mousePressed(MouseEvent evt) { // When user presses the mouse, tell the system to // call the drawing surface's paintComponent() method. drawingSurface.repaint(); } // The following empty routines are required by the // MouseListener interface: public void mouseEntered(MouseEvent evt) { } public void mouseExited(MouseEvent evt) { } public void mouseClicked(MouseEvent evt) { } public void mouseReleased(MouseEvent evt) { } Fourth and finally, the applet must be registered to listen for mouse events. Since the drawing surface fills the entire applet, it is actually the drawing surface on which the user clicks. We want the applet to listen for mouse events on the drawing surface. This can be arranged by adding this line to the applet's init() method: drawingSurface.addMouseListener(this); This calls the addMouseListener() method in the drawing surface object. It tells that object where to send the mouse events that it generates. The parameter to this method is the object that will be listening for the events. In this case, the listening object is the applet itself. The special variable "this" is used here to refer to the applet. (See Section 5.5. When used in the definition of an instance method, "this" refers to the object that contains the method.) We could make all these changes in the source code of the original RandomStrings applet. However, since we are supposed to be doing object-oriented programming, it might be instructive to write a subclass that contains the changes. This will let us build on previous work and concentrate just on the modifications. http://math.hws.edu/eck/cs124/javanotes4/c6/s4.html (3 of 11) [6/11/2004 11:10:31 AM] Java Programming: Section 6.4 Here's the actual source code for the above applet. It uses "super", another special variable from Section 5.5. import java.awt.event.*; public class ClickableRandomStrings extends RandomStrings implements MouseListener { public void init() { // When the applet is created, do the initialization // of the superclass, RandomStrings. Then set this // applet to listen for mouse events on the // "drawingSurface". (The drawingSurface variable // is defined in the RandomStrings class and // represents a component that fills the entire applet.) super.init(); drawingSurface.addMouseListener(this); } public void mousePressed(MouseEvent evt) { // When user presses the mouse, tell the system to // call the drawingSurface's paintComponent() method. drawingSurface.repaint(); } // The next four empty routines are required by the // MouseListener interface. public void mouseEntered(MouseEvent evt) { } public void mouseExited(MouseEvent evt) { } public void mouseClicked(MouseEvent evt) { } public void mouseReleased(MouseEvent evt) { } } // end class ClickableRandomStrings Often, when a mouse event occurs, you want to know the location of the mouse cursor. This information is available from the parameter to the event-handling method, evt. This parameter is an object of type MouseEvent, and it contains instance methods that return information about the event. To find out the coordinates of the mouse cursor, call evt.getX() and evt.getY(). These methods return integers which give the x and y coordinates where the mouse cursor was positioned. The coordinates are expressed in the coordinate system of the component that generated the event, where the top left corner of the component is (0,0). The user can hold down certain modifier keys while using the mouse. The possible modifier keys include: the Shift key, the Control key, the ALT key (called the Option key on the Macintosh), and the Meta key (called the Command or Apple key on the Macintosh and with no equivalent in Windows). You might want to respond to a mouse event differently when the user is holding down a modifier key. The boolean-valued instance methods evt.isShiftDown(), evt.isControlDown(), evt.isAltDown(), and evt.isMetaDown() can be called to test whether the modifier keys are pressed. You might also want to have different responses depending on whether the user presses the left mouse button, the middle mouse button, or the right mouse button. Now, not every mouse has a middle button and a right button, so Java handles the information in a peculiar way. It treats pressing the right button as equivalent to holding down the Meta key while pressing the left mouse button. That is, if the right button is pressed, then the instance method evt.isMetaDown() will return true (even if the Meta key is not http://math.hws.edu/eck/cs124/javanotes4/c6/s4.html (4 of 11) [6/11/2004 11:10:31 AM] Java Programming: Section 6.4 pressed). Similarly, pressing the middle mouse button is equivalent to holding down the ALT key. In practice, what this really means is that pressing the right mouse button under Windows is equivalent to holding down the Command key while pressing the mouse button on Macintosh. A program tests for either of these by calling evt.isMetaDown(). As an example, consider the following applet. Click on the applet with the left mouse button to place a red rectangle on the applet. Click with the right mouse button (or hold down the Command key and click on a Macintosh) to place a blue oval on the applet. Hold down the Shift key and click to clear the applet. (Applet "SimpleStamper" would be displayed here if Java were available.) This applet is a JApplet which uses a nested class named Display to define its drawing surface. There are many ways to write this applet, but I decided in this case to let the drawing surface object listen for mouse events on itself. The main applet class does nothing but set up the drawing surface. In order to respond to mouse clicks, the Display class implements the MouseListener interface, and the constructor for the display class includes the command "addMouseListener(this)". Since this command is in a method in the Display class, the addMouseListener() method in the display object is being called, and "this" also refers to the display object. That is, the display object will send any mouse events that it generates to itself. The source code for this applet is shown below. You can see how the instance methods in the MouseEvent object are used. You can also check for the Four Steps of Event Handling ("import java.awt.event.*", "implements MouseListener", "addMouseListener", and the event-handling methods). The Display class in this example violates the rule that all drawing should be done in a paintComponent() method. The rectangles and ovals are drawn directly in the mousePressed() routine. To make this possible, I need to obtain a graphics context by saying "g = getGraphics()". (After using g for drawing, I call g.dispose() to inform the operating system that I will no longer be using g for drawing. It is a good idea to do this to free the system resources that are used by the graphics context.) I do not advise doing this type of direct drawing if it can be avoided, but you can see that it does work in this case. By the way, this applet still has the problem that it does not save information about what has been drawn on the applet. So if the applet is covered up and uncovered, the contents of the applet are erased. Here is the source code: import java.awt.*; import java.awt.event.*; import javax.swing.*; public class SimpleStamper extends JApplet { public void init() { // This method is called by the system to initialize // the applet. An object belonging to the nested class // Display is created and installed as the content // pane of the applet. This Display object does // all the real work. Display display = new Display(); setContentPane(display); } http://math.hws.edu/eck/cs124/javanotes4/c6/s4.html (5 of 11) [6/11/2004 11:10:31 AM] Java Programming: Section 6.4 class Display extends JPanel implements MouseListener { // A nested class to represent the drawing surface that // fills the applet. Display() { // This constructor simply sets the background color // of the panel to be black and sets the panel to // listen for mouse events on itself. setBackground(Color.black); addMouseListener(this); } public void mousePressed(MouseEvent evt) { // Since this panel has been set to listen for mouse // events on itself, this method will be called when the // user clicks the mouse on the panel. (Since the panel // fills the whole applet, that means clicking anywhere // on the applet.) if ( evt.isShiftDown() ) { // The user was holding down the Shift key. Just // repaint the panel. Since this class does not // define a paintComponent() method, the method // from the superclass, JPanel, is called. That // method simply fills the panel with its background // color, which is black. This has the effect of // erasing the contents of the applet. repaint(); return; } int x = evt.getX(); // x-coordinate where user clicked. int y = evt.getY(); // y-coordinate where user clicked. Graphics g = getGraphics(); // Graphics context for drawing // directly on this JPanel. if ( evt.isMetaDown() ) { // User right-clicked at the point (x,y). // Draw a blue oval centered at the point (x,y). // (A black outline around the oval will make it // more distinct when ovals and rects overlap.) g.setColor(Color.blue); g.fillOval( x - 30, y - 15, 60, 30 ); g.setColor(Color.black); g.drawOval( x - 30, y - 15, 60, 30 ); } else { // User left-clicked (or middle-clicked) at (x,y). // Draw a red rectangle centered at (x,y). g.setColor(Color.red); g.fillRect( x - 30, y - 15, 60, 30 ); g.setColor(Color.black); g.drawRect( x - 30, y - 15, 60, 30 ); } http://math.hws.edu/eck/cs124/javanotes4/c6/s4.html (6 of 11) [6/11/2004 11:10:31 AM] Java Programming: Section 6.4 g.dispose(); // We are finished with the graphics context, // so dispose of it. } // end mousePressed(); // The next four empty routines are required by the // MouseListener interface. public void mouseEntered(MouseEvent evt) { } public void mouseExited(MouseEvent evt) { } public void mouseClicked(MouseEvent evt) { } public void mouseReleased(MouseEvent evt) { } } // end nested class Display } // end class SimpleStamper MouseMotionListeners and Dragging Whenever the mouse is moved, it generates events. The operating system of the computer detects these events and uses them to move the mouse cursor on the screen. It is also possible for a program to listen for these "mouse motion" events and respond to them. The most common reason to do so is to implement dragging. Dragging occurs when the user moves the mouse while holding down a mouse button. The methods for responding to mouse motion events are defined in an interface named MouseMotionListener. This interface specifies two event-handling methods: public void mouseDragged(MouseEvent evt); public void mouseMoved(MouseEvent evt); The mouseDragged method is called if the mouse is moved while a button on the mouse is pressed. If the mouse is moved while no mouse button is down, then mouseMoved is called instead. The parameter, evt, is an object of type MouseEvent. It contains the x and y coordinates of the mouse's location. As long as the user continues to move the mouse, one of these methods will be called over and over. (So many events are generated that it would be inefficient for a program to hear them all, if it doesn't want to do anything in response. This is why the mouse motion event-handlers are defined in a separate interface from the other mouse events: You can listen for the mouse events defined in MouseListener without automatically hearing all mouse motion events as well.) If you want your program to respond to mouse motion events, you must create an object that implements the MouseMotionListener interface, and you must register that object to listen for events. The registration is done by calling a component's addMouseMotionListener method. The object will then listen for mouseDragged and mouseMoved events associated with that component. In most cases, the listener object will also implement the MouseListener interface so that it can respond to the other mouse events as well. For example, if we want an applet to listen for all mouse events associated with a drawingSurface object, then the definition of the applet class might have the form: import java.awt.*; import java.awt.event.*; import javax.swing.*; public class Mouser extends JApplet http://math.hws.edu/eck/cs124/javanotes4/c6/s4.html (7 of 11) [6/11/2004 11:10:31 AM] Java Programming: Section 6.4 implements MouseListener, MouseMotionListener { public void init() { // set up the applet drawingSurface.addMouseListener(this); drawingSurface.addMouseMotionListener(this); . . . // other initializations } . . // Define the seven MouseListener and . // MouseMotionListener methods. Also, there . // can be other variables and methods. Here is a small sample applet that displays information about mouse events. It is programmed to respond to any of the seven different kinds of mouse events by displaying the coordinates of the mouse, the type of event, and a list of the modifier keys that are down (Shift, Control, Meta, and Alt). Experiment to see what happens when you use the mouse on the applet. The source code for this applet can be found in SimpleTrackMouse.java. I encourage you to read the source code. You should now be familiar with all the techniques that it uses. (Applet "SimpleTrackMouse" would be displayed here if Java were available.) It is interesting to look at what a program needs to do in order to respond to dragging operations. In general, the response involves three methods: mousePressed(), mouseDragged(), and mouseReleased(). The dragging gesture starts when the user presses a mouse button, it continues while the mouse is dragged, and it ends when the user releases the button. This means that the programming for the response to one dragging gesture must be spread out over the three methods! Furthermore, the mouseDragged() method can be called many times as the mouse moves. To keep track of what is going on between one method call and the next, you need to set up some instance variables. In many applications, for example, in order to process a mouseDragged event, you need to remember the previous coordinates of the mouse. You can store this information in two instance variables prevX and prevY of type int. I also suggest having a boolean variable, dragging, which is set to true while a dragging gesture is being processed. This is necessary because not every mousePressed event is the beginning of a dragging gesture. The mouseDragged and mouseReleased methods can use the value of dragging to check whether a drag operation is actually in progress. You might need other instance variables as well, but in general outline, the code for handling dragging looks like this: private int prevX, prevY; // Most recently processed mouse coords. private boolean dragging; // Set to true when dragging is in process. . . . // other instance variables for use in dragging public void mousePressed(MouseEvent evt) { if ( we-want-to-start-dragging ) { dragging = true; prevX = evt.getX(); // Remember starting position. prevY = evt.getY(); } . . // Other processing. . } public void mouseDragged(MouseEvent evt) { if ( dragging == false ) // First, check if we are return; // processing a dragging gesture. http://math.hws.edu/eck/cs124/javanotes4/c6/s4.html (8 of 11) [6/11/2004 11:10:31 AM] Java Programming: Section 6.4 int x = evt.getX(); // Current position of Mouse. int y = evt.getY(); . . // Process a mouse movement from (prevX, prevY) to (x,y). . prevX = x; // Remember the current position for the next call. prevY = y; } public void mouseReleased(MouseEvent evt) { if ( dragging == false ) // First, check if we are return; // processing a dragging gesture. dragging = false; // We are done dragging. . . // Other processing and clean-up. . } As an example, let's look at a typical use of dragging: allowing the user to sketch a curve by dragging the mouse. This example also shows many other features of graphics and mouse processing. In the following applet, you can draw a curve by dragging the mouse on the large white area. Select a color for drawing by clicking on one of the colored rectangles on the right. Note that the selected color is framed with a white border. Clear your drawing by clicking in the square labeled "CLEAR". (This applet still has the old problem that the drawing will disappear if you cover the applet and uncover it.) (Applet "SimplePaint" would be displayed here if Java were available.) You'll find the complete source code for this applet in the file SimplePaint.java. I will discuss a few aspects of it here, but I encourage you to read it carefully in its entirety. There are lots of informative comments in the source code. (This is actually an old-style non-Swing Applet which uses a paint() method to draw on the applet instead of a paintComponet() method to draw on a drawing surface.) The applet class for this example is designed to work for any reasonable applet size, that is, unless the applet is too small. This means that coordinates are computed in terms of the actual width and height of the applet. (The width and height are obtained by calling getSize().width and getSize().height.) This makes things quite a bit harder than they would be if we assumed some particular fixed size for the applet. Let's look at some of these computations in detail. For example, the command used to fill in the large white drawing area is g.fillRect(3, 3, width - 59, height - 6); There is a 3-pixel border along each edge, so the height of the drawing area is 6 less than the height of the applet. As for the width: The colored rectangles are 50 pixels wide. There is a 3-pixel border on each edge of the applet. And there is a 3-pixel divider between the drawing area and the colored rectangles. All that adds up to make 59 pixels that are not included in the width of the drawing area, so the width of the drawing area is 59 less than the width of the applet. The white square labeled "CLEAR" occupies a 50-by-50 pixel region beneath the colored rectangles. Allowing for this square, we can figure out how much vertical space is available for the seven colored rectangles, and then divide that space by 7 to get the vertical space available for each rectangle. This quantity is represented by a variable, colorSpace. Out of this space, 3 pixels are used as spacing between the rectangles, so the height of each rectangle is colorSpace - 3. The top of the N-th rectangle is located (N*colorSpace + 3) pixels down from the top of the applet, assuming that we start counting at zero. This is because there are N rectangles above the N-th rectangle, each of which uses colorSpace pixels. The extra 3 is for the border at the top of the applet. After all that, we can write down the command for drawing the N-th rectangle: http://math.hws.edu/eck/cs124/javanotes4/c6/s4.html (9 of 11) [6/11/2004 11:10:31 AM] Java Programming: Section 6.4 g.fillRect(width - 53, N*colorSpace + 3, 50, colorSpace - 3); That was not easy! But it shows the kind of careful thinking and precision graphics that are sometimes necessary to get good results. The mouse in this applet is used to do three different things: Select a color, clear the drawing, and draw a curve. Only the third of these involves dragging, so not every mouse click will start a dragging operation. The mousePressed routine has to look at the (x,y) coordinates where the mouse was clicked and decide how to respond. If the user clicked on the CLEAR rectangle, the drawing area is cleared by calling repaint(). If the user clicked somewhere in the strip of colored rectangles, the selected color is changed. This involves computing which color the user clicked on, which is done by dividing the y coordinate by colorSpace. Finally, if the user clicked on the drawing area, a drag operation is initiated. A boolean variable, dragging, is set to true so that the mouseDragged and mouseReleased methods will know that a curve is being drawn. The code for this follows the general form given above. The actual drawing of the curve is done in the mouseDragged method, which draws a line from the previous location of the mouse to its current location. Some effort is required to make sure that the line does not extend beyond the white drawing area of the applet. This is not automatic, since as far as the computer is concerned, the border and the color bar are part of the drawing surface. If the user drags the mouse outside the drawing area while drawing a line, the mouseDragged routine changes the x and y coordinates to make them lie within the drawing area. Anonymous Event Handlers and Adapter Classes As I mentioned above, it is a fairly common practice to use anonymous nested classes to define listener objects. As discussed in Section 5.6, a special form of the new operator is used to create an object that belongs to an anonymous class. For example, a mouse listener object can be created with an expression of the form: new MouseListener() { public void mousePressed(MouseEvent evt) { . . . } public void mouseReleased(MouseEvent evt) { . . . } public void mouseClicked(MouseEvent evt) { . . . } public void mouseEntered(MouseEvent evt) { . . . } public void mouseExited(MouseEvent evt) { . . . } } This is all just one long expression that both defines an un-named class and creates an object that belongs to that class. To use the object as a mouse listener, it should be passed as the parameter to some component's addMouseListener() method in a command of the form: component.addMouseListener( new MouseListener() { public void mousePressed(MouseEvent evt) { . . . } public void mouseReleased(MouseEvent evt) { . . . } public void mouseClicked(MouseEvent evt) { . . . } public void mouseEntered(MouseEvent evt) { . . . } public void mouseExited(MouseEvent evt) { . . . } } ); Now, in a typical application, most of the method definitions in this class will be empty. A class that implements an interface must provide definitions for all the methods in that interface, even if the definitions are empty. To avoid the tedium of writing empty method definitions in cases like this, Java provides adapter classes. An adapter class implements a listener interface by providing empty definitions for all the methods in the interface. An adapter class is only useful as a basis for making subclasses. In the subclass, you can define just those methods that you actually want to use. For the remaining methods, the empty definitions that are provided by the adapter class will be used. The adapter class for the MouseListener interface is named MouseAdapter. For example, if you want a mouse listener that http://math.hws.edu/eck/cs124/javanotes4/c6/s4.html (10 of 11) [6/11/2004 11:10:31 AM] Java Programming: Section 6.4 only responds to mouse-pressed events, you can use a command of the form: component.addMouseListener( new MouseAdapter() { public void mousePressed(MouseEvent evt) { . . . } } ); To see how this works in a real example, let's write another version of the ClickableRandomStrings applet that uses an anonymous class based on MouseAdapter to handle mouse events: import java.awt.event.*; public class ClickableRandomStrings2 extends RandomStrings { public void init() { // When the applet is created, do the initialization // of the superclass, RandomStrings. Then add a // mouse listener to listen for mouse events on the // "drawingSurface". (drawingSurface is defined // in the superclass, RandomStrings.) super.init(); drawingSurface.addMouseListener( new MouseAdapter() { public void mousePressed(MouseEvent evt) { // When user presses the mouse, tell the system to // call the drawingSurface's paintComponent() method. drawingSurface.repaint(); } } ); } // end init() } // end class ClickableRandomStrings2 [ Next Section | Previous Section | Chapter Index | Main Index ] http://math.hws.edu/eck/cs124/javanotes4/c6/s4.html (11 of 11) [6/11/2004 11:10:31 AM] Java Programming: Section 6.5 Section 6.5 Keyboard Events IN JAVA, EVENTS are associated with GUI components. When the user presses a button on the mouse, the event that is generated is associated with the component that contains the mouse cursor. What about keyboard events? When the user presses a key, what component is associated with the key event that is generated? A GUI uses the idea of input focus to determine the component associated with keyboard events. At any given time, exactly one interface element on the screen has the input focus, and that is where all keyboard events are directed. If the interface element happens to be a Java component, then the information about the keyboard event becomes a Java object of type KeyEvent, and it is delivered to any listener objects that are listening for KeyEvents associated with that component. The necessity of managing input focus adds an extra twist to working with keyboard events. It's a good idea to give the user some visual feedback about which component has the input focus. For example, if the component is the typing area of a word-processor, the feedback is usually in the form of a blinking text cursor. Another common visual clue is to draw a brightly colored border around the edge of a component when it has the input focus, as I do in the sample applet later on this page. A component that wants to have the input focus can call the method requestFocus(), which is defined in the Component class. Calling this method does not absolutely guarantee that the component will actually get the input focus. Several components might request the focus; only one will get it. This method should only be used in certain circumstances in any case, since it can be a rude surprise to the user to have the focus suddenly pulled away from a component that the user is working with. In a typical user interface, the user can choose to give the focus to a component by clicking on that component with the mouse. And pressing the tab key will often move the focus from one component to another. Some components do not automatically receive the input focus when the user clicks on them. To solve this problem, a program has to register a mouse listener with the component to detect user clicks. In response to a user click, the mousePressed() method should call requestFocus() for the component. This is true, in particular, for the components that are used as drawing surfaces in the examples in this chapter. These components are defined as subclasses of JPanel, and JPanel objects do not receive the input focus automatically. If you want to be able to use the keyboard to interact with a JPanel named drawingSurface, you have to register a listener to listen for mouse events on the drawingSurface and call drawingSurface.requestFocus() in the mousePressed() method of the listener object. Here is a sample applet that processes keyboard events. If the applet has the input focus, the arrow keys can be used to move the colored square. Furthermore, pressing the 'R', 'G', 'B', or 'K' key will set the color of the square to red, green, blue, or black. When the applet has the input focus, the border of the applet is a bright cyan (blue-green) color. When the applet does not have the focus, the border is gray, and a message, "Click to activate," is displayed. When the user clicks on an unfocused applet, it requests the input focus. (In some browsers, you also have to leave the mouse positioned inside the applet, in order for it to have the input focus.) The complete source code for this applet is in the file KeyboardAndFocusDemo.java. I will discuss some aspects of it below. After reading this section, you should be able to understand the source code in its entirety. (Applet "KeyboardAndFocusDemo" would be displayed here if Java were available.) In Java, keyboard event objects belong to a class called KeyEvent. An object that needs to listen for KeyEvents must implement the interface named KeyListener. Furthermore, the object must be http://math.hws.edu/eck/cs124/javanotes4/c6/s5.html (1 of 7) [6/11/2004 11:10:32 AM] Java Programming: Section 6.5 registered with a component by calling the component's addKeyListener() method. The registration is done with the command "component.addKeyListener(listener);" where listener is the object that is to listen for key events, and component is the object that will generate the key events (when it has the input focus). It is possible for component and listener to be the same object. All this is, of course, directly analogous to what you learned about mouse events in the previous section. The KeyListener interface defines the following methods, which must be included in any class that implements KeyListener: public void keyPressed(KeyEvent evt); public void keyReleased(KeyEvent evt); public void keyTyped(KeyEvent evt); Java makes a careful distinction between the keys that you press and the characters that you type. There are lots of keys on a keyboard: letter keys, number keys, modifier keys such as Control and Shift, arrow keys, page up and page down keys, keypad keys, function keys. In many cases, pressing a key does not type a character. On the other hand, typing a character sometimes involves pressing several keys. For example, to type an uppercase 'A', you have to press the Shift key and then press the A key before releasing the Shift key. On my Macintosh computer, I can type an accented e, é, by holding down the Option key, pressing the E key, releasing the Option key, and pressing E again. Only one character was typed, but I had to perform three key-presses and I had to release a key at the right time. In Java, there are three types of KeyEvent. The types correspond to pressing a key, releasing a key, and typing a character. The keyPressed method is called when the user presses a key, the keyReleased method is called when the user releases a key, and the keyTyped method is called when the user types a character. Note that one user action, such as pressing the E key, can be responsible for two events, a keyPressed event and a keyTyped event. Typing an upper case 'A' could generate two keyPressed, two keyReleased, and one keyTyped event. Usually, it is better to think in terms of two separate streams of events, one consisting of keyPressed and keyReleased events and the other consisting of keyTyped events. For some applications, you want to monitor the first stream; for other applications, you want to monitor the second one. Of course, the information in the keyTyped stream could be extracted from the keyPressed/keyReleased stream, but it would be difficult (and also system-dependent to some extent). Some user actions, such as pressing the Shift key, can only be detected as keyPressed events. I have a solitaire game on my computer that hilites every card that can be moved, when I hold down the Shift key. You could do something like that in Java by hiliting the cards when the Shift key is pressed and removing the hilite when the Shift key is released. There is one more complication. Usually, when you hold down a key on the keyboard, that key will auto-repeat. This means that it will generate multiple keyPressed events, as long as it is held down. It can also generate multiple keyTyped events. For the most part, this will not affect your programming, but you should not expect every keyPressed event to have a corresponding keyReleased event. Every key on the keyboard has an integer code number. (Actually, this is only true for keys that Java knows about. Many keyboards have extra keys that can't be used with Java.) When the keyPressed or keyReleased method is called, the parameter, evt, contains the code of the key that was pressed or released. The code can be obtained by calling the function evt.getKeyCode(). Rather than asking you to memorize a table of code numbers (which can be different for different platforms in any case), Java provides a named constant for each key. These constants are defined in the KeyEvent class. For example the constant for the shift key is KeyEvent.VK_SHIFT. If you want to test whether the key that the user pressed is the Shift key, you could say "if (evt.getKeyCode() == KeyEvent.VK_SHIFT)". The key codes for the four arrow keys are KeyEvent.VK_LEFT, KeyEvent.VK_RIGHT, KeyEvent.VK_UP, and KeyEvent.VK_DOWN. Other keys have similar codes. (The "VK" stands for "Virtual Keyboard". In reality, different keyboards use different key codes, but Java translates the actual codes from the keyboard into its own "virtual" codes. Your program only sees these virtual key codes, so it will work with various keyboards on various platforms without modification.) http://math.hws.edu/eck/cs124/javanotes4/c6/s5.html (2 of 7) [6/11/2004 11:10:32 AM] Java Programming: Section 6.5 In the case of a keyTyped event, you want to know which character was typed. This information can be obtained from the parameter, evt, in the keyTyped method by calling the function evt.getKeyChar(). This function returns a value of type char representing the character that was typed. In the KeyboardAndFocusDemo applet, shown above, I use the keyPressed routine to respond when the user presses one of the arrow keys. The applet includes instance variables, squareLeft and squareTop that give the position of the upper left corner of the square. When the user presses one of the arrow keys, the keyPressed routine modifies the appropriate instance variable and calls canvas.repaint() to redraw the whole applet. ("canvas" is the name I use for the drawing surface component in this applet.) Note that the values of squareLeft and squareRight are restricted so that the square never moves outside the white area of the applet: public void keyPressed(KeyEvent evt) { // Called when the user has pressed a key, which can be // a special key such as an arrow key. If the key pressed // was one of the arrow keys, move the square (but make sure // that it doesn't move off the edge, allowing for a // 3-pixel border all around the applet). SQUARE_SIZE is // a named constant that specifies the size of the square. // squareLeft and squareRight give the position of the // top-left corner of the square. int key = evt.getKeyCode(); // Keyboard code for the pressed key. if (key == KeyEvent.VK_LEFT) { // left-arrow key; move square up squareLeft -= 8; if (squareLeft < 3) squareLeft = 3; canvas.repaint(); } else if (key == KeyEvent.VK_RIGHT) { // right-arrow key; move right squareLeft += 8; if (squareLeft > getSize().width - 3 - SQUARE_SIZE) squareLeft = getSize().width - 3 - SQUARE_SIZE; canvas.repaint(); } else if (key == KeyEvent.VK_UP) { // up-arrow key; move up squareTop -= 8; if (squareTop < 3) squareTop = 3; canvas.repaint(); } else if (key == KeyEvent.VK_DOWN) { // down-arrow key; move down squareTop += 8; if (squareTop > getSize().height - 3 - SQUARE_SIZE) squareTop = getSize().height - 3 - SQUARE_SIZE; canvas.repaint(); } } // end keyPressed() Color changes -- which happen when the user types the characters 'R', 'G', 'B', and 'K', or the lower case equivalents -- are handled in the keyTyped method. I won't include it here, since it is so similar to the http://math.hws.edu/eck/cs124/javanotes4/c6/s5.html (3 of 7) [6/11/2004 11:10:32 AM] Java Programming: Section 6.5 keyPressed method. Finally, to complete the KeyListener interface, the keyReleased method must be defined. In the sample applet, the body of this method is empty since the applet does nothing to respond to keyReleased events. Focus Events If a component is to change its appearance when it has the input focus, it needs some way to know when it has the focus. In Java, objects are notified about changes of input focus by events of type FocusEvent. An object that wants to be notified of changes in focus can implement the FocusListener interface. This interface declares two methods: public void focusGained(FocusEvent evt); public void focusLost(FocusEvent evt); Furthermore, the addFocusListener() method must be used to set up a listener for the focus events. When a component gets the input focus, it calls the focusGained() method of any object that has been registered with that component as a FocusListener. When it loses the focus, it calls the listener's focusLost() method. Often, it is the component itself that listens for focus events. In my sample applet, there is a boolean-valued instance variable named focussed. This variable is true when the applet has the input focus and is false when the applet does not have focus. The applet implements the FocusListener interface and listens for focus events from the canvas. The paintComponent() method of the canvas looks at the value of focussed to decide what color the border should be. The value of focussed is set in the focusGained() and focusLost() methods. These methods call canvas.repaint() so that the drawing surface will be redrawn with the correct border color. The method definitions are very simple: public void focusGained(FocusEvent evt) { // The canvas now has the input focus. focussed = true; canvas.repaint(); // redraw with cyan border } public void focusLost(FocusEvent evt) { // The canvas has now lost the input focus. focussed = false; canvas.repaint(); // redraw with gray border } The other aspect of handling focus is to make sure that the canvas gets the focus when the user clicks on it. To do this, the applet implements the MouseListener interface and listens for mouse events on the canvas. It defines a mousePressed routine that asks that the input focus be given to the canvas: public void mousePressed(MouseEvent evt) { canvas.requestFocus(); } The other four methods of the mouseListener interface are defined to be empty. Note that the applet implements three listener interfaces, so the class definition begins: public class KeyboardAndFocusDemo extends JApplet implements KeyListener, FocusListener, MouseListener The applet's init() method registers the applet to listen for all three types of events. To do this, the init() method includes the lines http://math.hws.edu/eck/cs124/javanotes4/c6/s5.html (4 of 7) [6/11/2004 11:10:32 AM] Java Programming: Section 6.5 canvas.addFocusListener(this); canvas.addKeyListener(this); canvas.addMouseListener(this); There are, of course, other ways to organize this applet. It would be possible, for example, to use the canvas object instead of the applet object to listen for events. Or anonymous classes could be used to define separate listening objects. State Machines The information stored in an object's instance variables is said to represent the state of that object. When one of the object's methods is called, the action taken by the object can depend on its state. (Or, in the terminology we have been using, the definition of the method can look at the instance variables to decide what to do.) Furthermore, the state can change. (That is, the definition of the method can assign new values to the instance variables.) In computer science, there is the idea of a state machine, which is just something that has a state and can change state in response to events or inputs. The response of a state machine to an event or input depends on what state it's in. An object is a kind of state machine. Sometimes, this point of view can be very useful in designing classes. The state machine point of view can be especially useful in the type of event-oriented programming that is required by graphical user interfaces. When designing an applet, you can ask yourself: What information about state do I need to keep track of? What events can change the state of the applet? How will my response to a given event depend on the current state? Should the appearance of the applet be changed to reflect a change in state? How should the paintComponent() method take the state into account? All this is an alternative to the top-down, step-wise-refinement style of program design, which does not apply to the overall design of an event-oriented program. In the KeyboardAndFocusDemo applet, shown above, the state of the applet is recorded in the instance variables focussed, squareLeft, and squareTop. These state variables are used in the paintComponent() method to decide how to draw the applet. They are set in the various event-handling methods. In the rest of this section, we'll look at another example, where the state of the applet plays an even bigger role. In this example, the user plays a simple arcade-style game by pressing the arrow keys. The example is based on one of my frameworks, called KeyboardAnimationApplet2. (See Section 3.7 for a discussion of frameworks and a sample framework that supports animation.) The game is written as an extension of the KeyboardAnimationApplet2 class. It includes a method, drawFrame(), that draws one frame in the animation. It also defines keyPressed to respond when the user presses the arrow keys. The source code for the game is in the file SubKillerGame.java. You can also look at the source code in KeyboardAnimationApplet2.java, but it uses some advanced techniques that I haven't covered yet. You have to click on the game to activate it. The applet shows a black "submarine" moving back and forth erratically near the bottom. Near the top, there is a blue "boat". You can move this boat back and forth by pressing the left and right arrow keys. Attached to the boat is a red "depth charge." You can drop the depth charge by hitting the down arrow key. The objective is to blow up the submarine by hitting it with the depth charge. If the depth charge falls off the bottom of the screen, you get a new one. If the sub explodes, a new sub is created and you get a new depth charge. Try it! Make sure to hit the sub at least once, so you can see the explosion. (Applet "SubKillerGame" would be displayed here if Java were available.) Let's think about how this applet can be programmed. What constitutes the "state" of the applet? That is, what things change from time to time and affect the appearance or behavior of the applet? Of course, the state includes the positions of the boat, submarine, and depth charge, so I need instance variables to store http://math.hws.edu/eck/cs124/javanotes4/c6/s5.html (5 of 7) [6/11/2004 11:10:32 AM] Java Programming: Section 6.5 the positions. Anything else, possibly less obvious? Well, sometimes the depth charge is falling, and sometimes it's not. That is a difference in state. Since there are two possibilities, I represent this aspect of the state with a boolean variable, bombIsFalling. Sometimes the submarine is moving left and sometimes it is moving right. The difference is represented by another boolean variable, subIsMovingLeft. Sometimes, the sub is exploding. This is also part of the state, but representing it requires a little more thought. While an explosion is in progress, the sub looks different in each frame, since the size of the explosion increases. Also, I need to know when the explosion is over so that I can go back to drawing the sub as usual. So, I use a variable, explosionFrameNumber, of type int, which tells how many frames have been drawn since the explosion started. I represent the fact that no explosion is happening by setting the value of explosionFrameNumber to zero. Alternatively, I could have used another boolean variable to keep track of whether or not an explosion is in progress. How and when do the values of these instance variables change? Some of them can change when the user presses certain keys. In the program, this is checked in the keyPressed() method. If the user presses the left or right arrow key, the position of the boat is changed. If the user presses the down arrow key, the depth charge changes from not-falling to falling. This is coded as follows: public void keyPressed(KeyEvent evt) { int code = evt.getKeyCode(); // which key was pressed if (code == KeyEvent.VK_LEFT) { // Move the boat left. boatCenterX -= 15; } else if (code == KeyEvent.VK_RIGHT) { // Move the boat right. boatCenterX += 15; } else if (code == KeyEvent.VK_DOWN) { // Start the bomb falling, if it is not already falling. if ( bombIsFalling == false ) bombIsFalling = true; } } // end keyPressed() Note that it's not necessary to call repaint() when the state changes, since this applet is an animation that is constantly being redrawn anyway. Any changes in the state will become visible to the user as soon as the next frame is drawn. At some point in the program, I have to make sure that the user does not move the boat off the screen. I could have done this in keyPressed(), but I choose to check for this in another routine, just before drawing the boat. Other aspects of the state are changed in the drawFrame() routine. From the point of view of programming, this method is handling an event ("Hey, it's time to draw the next frame!"). It just happens to be an event that is generated by the KeyboardAnimationApplet2 framework rather than by the user. In my applet, the drawFrame() routine calls three other methods that I wrote to organize the process of computing and drawing a new frame: doBombFrame(), doBoatFrame(), and doSubFrame(). Consider doBombFrame(). This routine draws the depth charge. What happens in this routine depends on the current state, and the routine can make changes to the state when it is executed. The state of the bomb can be falling or not-falling, as recorded in the variable, bombIsFalling. If bombIsFalling is false, then the bomb is simply drawn at the bottom of the boat. If bombIsFalling is true, the vertical coordinate of the bomb has to be increased by some amount to make the bomb move down a bit from one frame to the next. Several other things can also happen. If the bomb has fallen off the bottom of the applet http://math.hws.edu/eck/cs124/javanotes4/c6/s5.html (6 of 7) [6/11/2004 11:10:32 AM] Java Programming: Section 6.5 -- something that we can test by looking at its vertical coordinate -- then bombIsFalling becomes false. This puts the bomb back at the boat in the next frame. Also, the bomb might hit the sub. This can be tested by comparing the locations of the bomb and the sub. If the bomb hits the sub, then the state changes in two ways: the bomb is no longer falling and the sub is exploding. These state changes are implemented by setting bombIsFalling to false and explosionFrameNumber to 1. Most interesting is the submarine. What happens with the submarine depends on whether it is exploding or not. If it is (that is, if explosionFrameNumber > 0), then yellow and red ovals are drawn at the sub's position. The sizes of these ovals depend on the value of explosionFrameNumber, so they grow with each frame of the explosion. After the ovals are drawn, the value of explosionFrameNumber is incremented. If its value has reached 14, it is reset to 0. This reflects a change of state: The sub is no longer exploding. It's important for you to understand what is happening here. There is no loop in the program to draw the stages of the explosion. Each frame is a new event and is drawn separately, based on values stored in instance variables. The state can change, which will make the next frame look different from the current one. In a frame where the sub is not exploding, it moves left or right. This is accomplished by adding or subtracting a small amount to the horizontal coordinate of the sub. Whether it moves left or right is determined by the value of the variable, subIsMovingLeft. It's interesting to consider how and when this variable changes value. If the sub reaches the left edge of the applet, subIsMovingLeft is set to false to make the sub start moving right. Similarly, if the sub reaches the right edge. But the sub can also reverse direction at random times. The way this is implemented is that in each frame, there is a small chance that the sub will reverse direction. This is done with the statement if ( Math.random() < 0.04 ) sumIsMovingLeft = !subIsMovingLeft; Since Math.random() is between 0 and 1, the condition "Math.random() < 0.04" has a 4 in 100, or 1 in 25, chance of being true. In those frames where this conditions happens to evaluate to true, the sub reverses direction. (The value of the expression "!subIsMovingLeft" is false when subIsMovingLeft is true, and it is true when subIsMovingLeft is false, so it effectively reverses the value of subIsMovingLeft.) While it's not very sophisticated as arcade games go, the SubKillerGame applet does use some interesting programming. And it nicely illustrates how to apply state-machine thinking in event-oriented programming. [ Next Section | Previous Section | Chapter Index | Main Index ] http://math.hws.edu/eck/cs124/javanotes4/c6/s5.html (7 of 7) [6/11/2004 11:10:32 AM] Java Programming: Section 6.6 Section 6.6 Introduction to Layouts and Components IN PRECEDING SECTIONS, YOU'VE SEEN how to use a graphics context to draw on the screen and how to handle mouse events and keyboard events. In one sense, that's all there is to GUI programming. If you're willing to program all the drawing and handle all the mouse and keyboard events, you have nothing more to learn. However, you would either be doing a lot more work than you need to do, or you would be limiting yourself to very simple user interfaces. A typical user interface uses standard GUI components such as buttons, scroll bars, text-input boxes, and menus. These components have already been written for you, so you don't have to duplicate the work involved in developing them. They know how to draw themselves, and they can handle the details of processing the mouse and keyboard events that concern them. Consider one of the simplest user interface components, a push button. The button has a border, and it displays some text. This text can be changed. Sometimes the button is disabled, so that clicking on it doesn't have any effect. When it is disabled, its appearance changes. When the user clicks on the push button, the button changes appearance while the mouse button is pressed and changes back when the mouse button is released. In fact, it's more complicated than that. If the user moves the mouse outside the push button before releasing the mouse button, the button changes to its regular appearance. To implement this, it is necessary to respond to mouse exit or mouse drag events. Furthermore, on many platforms, a button can receive the input focus. The button changes appearance when it has the focus. If the button has the focus and the user presses the space bar, the button is triggered. This means that the button must respond to keyboard and focus events as well. Fortunately, you don't have to program any of this, provided you use an object belonging to the standard class javax.swing.JButton. A JButton object draws itself and processes mouse, keyboard, and focus events on its own. You only hear from the Button when the user triggers it by clicking on it or pressing the space bar while the button has the input focus. When this happens, the JButton object creates an event object belonging to the class java.awt.event.ActionEvent. The event object is sent to any registered listeners to tell them that the button has been pushed. Your program gets only the information it needs -- the fact that a button was pushed. Another aspect of GUI programming is laying out components on the screen, that is, deciding where they are drawn and how big they are. You have probably noticed that computing coordinates can be a difficult problem, especially if you don't assume a fixed size for the applet. Java has a solution for this, as well. Components are the visible objects that make up a GUI. Some components are containers, which can hold other components. An applet's content pane is an example of a container. The standard class JPanel, which we have only used as a drawing surface up till now, is another example of a container. Because a JPanel object is a container, it can hold other components. So JPanels are dual purpose: You can draw on them, and you can add other components to them. Because a JPanel is itself a component, you can add a JPanel to an applet's content pane or even to another JPanel. This makes complex nesting of components possible. JPanels can be used to organize complicated user interfaces. The components in a container must be "laid out," which means setting their sizes and positions. It's possible to program the layout yourself, but ordinarily layout is done by a layout manager. A layout manager is an object associated with a container that implements some policy for laying out the components in that container. Different types of layout manager implement different policies. In this section, we'll look at a few examples of using components and layout managers, leaving the details until Section 7.2 and Section 7.3. The applets that we look at in this section have a large drawing area with a row of controls below it. http://math.hws.edu/eck/cs124/javanotes4/c6/s6.html (1 of 9) [6/11/2004 11:10:34 AM] Java Programming: Section 6.6 Our first example is rather simple. It's another "Hello World" applet, in which the color of the message can be changed by clicking one of the buttons at the bottom of the applet: (Applet "HelloWorldJApplet" would be displayed here if Java were available.) In the previous JApplets that we've looked at, the entire applet was filled with a JPanel that served as a drawing surface. In this example, there are two JPanels: the large black area at the top that displays the message and the smaller area at the bottom that holds the three buttons. Let's first consider the panel that contains the buttons. This panel is created in the applet's init() method as a variable named buttonBar, of type JPanel: JPanel buttonBar = new JPanel(); When a panel is to be used as a drawing surface, it is necessary to create a subclass of the JPanel class and include a paintComponent() method to do the drawing. However, when a JPanel is just being used as a container, there is no need to create a subclass. A standard JPanel is already capable of holding components of any type. Once the panel has been created, the three buttons are created and are added to the panel. A button is just an object belonging to the class javax.swing.JButton. When a button is created, the text that will be shown on the button is provided as a parameter to the constructor. The first button in the panel is created with the command: JButton redButton = new JButton("Red"); This button is added to the buttonBar panel with the command: buttonBar.add(redButton); Every JPanel comes automatically with a layout manager. This default layout manager will simply line up the components that are added to it in a row. That's exactly the behavior we want here, so there is nothing more to do. If we wanted a different kind of layout, it's possible to change the panel's layout manager. One more step is required to make the button useful: an object must be registered with the button to listen for ActionEvents. The button will generate an ActionEvent when the user clicks on it. ActionEvents are similar to MouseEvents or KeyEvents. To use them, a class should import java.awt.event.*. The object that is to do the listening must implement an interface named ActionListener. This interface requires a definition for the method "public void actionPerformed(ActionEvent evt);". Finally, the listener must be registered with the button by calling the button's addActionListener() method. In this case, the applet itself will act as listener, and the registration is done with the command: redButton.addActionListener(this); After doing the same three commands for each of the other two buttons -- and setting the background color for the sake of aesthetics -- the buttonBar panel is ready to use. It just has to be added to the applet. As we have seen, components are not added directly to an applet. Instead, they are added to the applet's content pane, which is itself a container. The content pane comes with a default layout manager that is capable of displaying up to five components. Four of these components are placed along the edges of the applet, in the so-called "North", "South", "East", and "West" positions. A component in the "Center" position fills in all the remaining space. This type of layout is called a BorderLayout. In our example, the button bar occupies the "South" position and the drawing area fills the "Center" position. When you add a component to a BorderLayout, you have to specify its position using a constant such as BorderLayout.SOUTH or BorderLayout.CENTER. In this example, buttonBar is added to the applet with the command: getContentPane().add(buttonBar, BorderLayout.SOUTH); http://math.hws.edu/eck/cs124/javanotes4/c6/s6.html (2 of 9) [6/11/2004 11:10:34 AM] Java Programming: Section 6.6 The display area of the applet is a drawing surface like those we have seen in other examples. A nested class named Display is created as a subclass of JPanel, and the display area is created as an object belonging to that class. The applet class has an instance variable named display of type Display to represent the drawing surface. The display object is simply created and added to the applet with the commands: display = new Display(); getContentPane().add(display, BorderLayout.CENTER); Putting this all together, the complete init() method for the applet becomes: public void init() { display = new Display(); // The component that displays "Hello World". getContentPane().add(display, BorderLayout.CENTER); // Adds the display panel to the CENTER position of the // JApplet's content pane. JPanel buttonBar = new JPanel(); // This panel will hold three buttons and will appear // at the bottom of the applet. buttonBar.setBackground(Color.gray); // Change the background color of the button panel // so that the buttons will stand out better. JButton redButton = new JButton("Red"); // Create a new button. "Red" is the text // displayed on the button. redButton.addActionListener(this); // Set up the button to send an "action event" to this applet // when the user clicks the button. The parameter, this, // is a name for the applet object that we are creating, // so action events from the button will be handled by // calling the actionPerformed() method in this class. buttonBar.add(redButton); // Add the button to the buttonBar panel. JButton greenButton = new JButton("Green"); // the second button greenButton.addActionListener(this); buttonBar.add(greenButton); JButton blueButton = new JButton("Blue"); // the third button blueButton.addActionListener(this); buttonBar.add(blueButton); getContentPane().add(buttonBar, BorderLayout.SOUTH); // Add button panel to the bottom of the content pane. } // end init() Notice that the variables buttonBar, redButton, greenButton, and blueButton are local to the http://math.hws.edu/eck/cs124/javanotes4/c6/s6.html (3 of 9) [6/11/2004 11:10:34 AM] Java Programming: Section 6.6 init() method. This is because once the buttons and panel have been added to the applet, the variables are no longer needed. The objects continue to exist, since they have been added to the applet. But they will take care of themselves, and there is no need to manipulate them elsewhere in the applet. The display variable, on the other hand, is an instance variable that can be used throughout the applet. This is because we are not finished with the display object after adding it to the applet. When the user clicks a button, we have to change the color of the display. We need a way to keep the variable around so that we can refer to it in the actionPerformed() method. In general, you don't need an instance variable for every component in an applet -- just for the components that will be referred to outside the init() method. The drawing surface in our example is defined by a nested class named Display which is a subclass of JPanel. The class contains a paintComponent() method that is responsible for drawing the message "Hello World" on a black background. The Display class also contains a variable that it uses to remember the current color of the message and a method that can be called to change the color. This class is more self-contained than than most of the drawing surface classes that we have looked at, and in fact it could have been defined as an independent class instead of as a nested class. Here is the definition of the nested class, Display: class Display extends JPanel { // This nested class defines a component that displays // the string "Hello World". The color and font for // the string are recorded in the variables colorNum // and textFont. int colorNum; // Keeps track of which color is displayed; // 1 for red, 2 for green, 3 for blue. Font textFont; // The font in which the message is displayed. // A font object represents a certain size and // style of text drawn on the screen. Display() { // Constructor for the Display class. Set the background // color and assign initial values to the instance // variables, colorNum and textFont. setBackground(Color.black); colorNum = 1; // The color of the message is set to red. textFont = new Font("Serif",Font.BOLD,36); // Create a font object representing a big, bold font. } void setColor(int code) { // This method is provided to be called by the // main class when it wants to set the color of the // message. The parameter value should be 1, 2, or 3 // to indicate the desired color. colorNum = code; repaint(); // Tell the system to repaint this component. } public void paintComponent(Graphics g){ // This routine is called by the system whenever this // panel needs to be drawn or redrawn. It first calls // super.paintComponent() to fill the panel with the // background color. It then displays the message http://math.hws.edu/eck/cs124/javanotes4/c6/s6.html (4 of 9) [6/11/2004 11:10:34 AM] Java Programming: Section 6.6 // "Hello World" in the proper color and font. super.paintComponent(g); switch (colorNum) { // Set the color. case 1: g.setColor(Color.red); break; case 2: g.setColor(Color.green); break; case 3: g.setColor(Color.blue); break; } g.setFont(textFont); // Set the font. g.drawString("Hello World!", 25,50); // Draw the message. } // end paintComponent } // end nested class Display The main class has an instance variable named display of type Display. When the user clicks one of the buttons in the applet, this variable is used to call the setColor() method in the drawing surface object. This is done in the applet's actionPerformed() method. This method is called when the user clicks any one of the three buttons, so it needs some way to tell which button was pressed. This information is provided in the parameter to the actionPerformed() method. This parameter contains an "action command," which in the case of a button is just the string that is displayed on the button: public void actionPerformed(ActionEvent evt) { // This routine is called by the system when the user clicks // on one of the buttons. The response is to set the display's // color accordingly. String command = evt.getActionCommand(); // The "action command" associated with the event // is the text on the button that was clicked. if (command.equals("Red")) // Set the color. display.setColor(1); else if (command.equals("Green")) display.setColor(2); else if (command.equals("Blue")) display.setColor(3); } // end actionPerformed() We have now looked at all the pieces of the sample applet. You can find the entire source code in the file HelloWorldJApplet.java. For a second example, let's look at something a little more interesting. Here's a simple card game in which you look at a playing card and try to predict whether the next card will be higher or lower in value. (Aces have the lowest value in this game.) You've seen a text-oriented version of the same game in Section 5.3. That section also defined Deck, Hand, and Card classes that are used in this applet. In this GUI version of the game, you click on a button to make your prediction. If you predict wrong, you lose. If you make three http://math.hws.edu/eck/cs124/javanotes4/c6/s6.html (5 of 9) [6/11/2004 11:10:34 AM] Java Programming: Section 6.6 correct predictions, you win. After completing one game, you can click the "New Game" button to start a new game. Try it! See what happens if you click on one of the buttons at a time when it doesn't make sense to do so. (Applet "HighLowGUI" would be displayed here if Java were available.) The overall form of this applet is the same as that of the previous example: It has three buttons in a panel at the bottom of the applet and a large drawing surface that displays the cards and a message. However, I've organized the code a little differently in this example. In this case, it's the drawing surface object, rather than the applet, that listens for events from the buttons, and I've put almost all the programming into the display surface class. The applet object is only responsible for creating the components and adding them to the applet. This is done in the following init() method, which has almost the same form as the init() method in the previous example: public void init() { // The init() method lays out the applet. A HighLowCanvas // occupies the CENTER position of the layout. On the // bottom is a panel that holds three buttons. The // HighLowCanvas object listens for ActionEvents from the // buttons and does all the real work of the program. setBackground( new Color(130,50,40) ); HighLowCanvas board = new HighLowCanvas(); getContentPane().add(board, BorderLayout.CENTER); JPanel buttonPanel = new JPanel(); buttonPanel.setBackground( new Color(220,200,180) ); getContentPane().add(buttonPanel, BorderLayout.SOUTH); JButton higher = new JButton( "Higher" ); higher.addActionListener(board); buttonPanel.add(higher); JButton lower = new JButton( "Lower" ); lower.addActionListener(board); buttonPanel.add(lower); JButton newGame = new JButton( "New Game" ); newGame.addActionListener(board); buttonPanel.add(newGame); } // end init() In programming the drawing surface class, HighLowCanvas, it is important to think in terms of the states that the game can be in, how the state can change, and how the response to events can depend on the state. The approach that produced the original, text-oriented game in Section 5.3 is not appropriate here. Trying to think about the game in terms of a process that goes step-by-step from beginning to end is more likely to confuse you than to help you. The state of the game includes the cards and the message. The cards are stored in an object of type Hand. The message is a String. These values are stored in instance variables. There is also another, less obvious aspect of the state: Sometimes a game is in progress, and the user is supposed to make a prediction about http://math.hws.edu/eck/cs124/javanotes4/c6/s6.html (6 of 9) [6/11/2004 11:10:34 AM] Java Programming: Section 6.6 the next card. Sometimes we are between games, and the user is supposed to click the "New Game" button. It's a good idea to keep track of this basic difference in state. The canvas class uses a boolean variable named gameInProgress for this purpose. The state of the applet can change whenever the user clicks on a button. The HighLowCanvas class implements the ActionListener interface and defines an actionPerformed() method to respond to the user's clicks. This method simply calls one of three other methods, doHigher(), doLower(), or newGame(), depending on which button was pressed. It's in these three event-handling methods that the action of the game takes place. We don't want to let the user start a new game if a game is currently in progress. That would be cheating. So, the response in the newGame() method is different depending on whether the state variable gameInProgress is true or false. If a game is in progress, the message instance variable should be set to show an error message. If a game is not in progress, then all the state variables should be set to appropriate values for the beginning of a new game. In any case, the board must be repainted so that the user can see that the state has changed. The complete newGame() method is as follows: void doNewGame() { // Called by the constructor, and called by actionPerformed() // when the user clicks the "New Game" button. Start a new game. if (gameInProgress) { // If the current game is not over, it is an error to try // to start a new game. message = "You still have to finish this game!"; repaint(); return; } deck = new Deck(); // Create a deck and hand to use for this game. hand = new Hand(); deck.shuffle(); hand.addCard( deck.dealCard() ); // Deal the first card. message = "Is the next card higher or lower?"; gameInProgress = true; // State changes! A game has started. repaint(); } The doHigher() and doLower() methods are almost identical to each other (and could probably have been combined into one method with a parameter, if I were more clever). Let's look at the doHigher() routine. This is called when the user clicks the "Higher" button. This only makes sense if a game is in progress, so the first thing doHigher() should do is check the value of the state variable gameInProgress. If the value is false, then doHigher() should just set up an error message. If a game is in progress, a new card should be added to the hand and the user's prediction should be tested. The user might win or lose at this time. If so, the value of the state variable gameInProgress must be set to false because the game is over. In any case, the board is repainted to show the new state. Here is the doHigher() method: void doHigher() { // Called by actionPerformed() when user clicks "Higher". // Check the user's prediction. Game ends if user guessed // wrong or if the user has made three correct predictions. if (gameInProgress == false) { // If the game has ended, it was an error to click "Higher", // so set up an error message and abort processing. message = "Click \"New Game\" to start a new game!"; repaint(); return; } http://math.hws.edu/eck/cs124/javanotes4/c6/s6.html (7 of 9) [6/11/2004 11:10:34 AM] Java Programming: Section 6.6 hand.addCard( deck.dealCard() ); // Deal a card to the hand. int cardCt = hand.getCardCount(); // How many cards in the hand? Card thisCard = hand.getCard( cardCt - 1 ); // Card just dealt. Card prevCard = hand.getCard( cardCt - 2 ); // The previous card. if ( thisCard.getValue() < prevCard.getValue() ) { gameInProgress = false; message = "Too bad! You lose."; } else if ( thisCard.getValue() == prevCard.getValue() ) { gameInProgress = false; message = "Too bad! You lose on ties."; } else if ( cardCt == 4) { gameInProgress = false; message = "You win! You made three correct guesses."; } else { message = "Got it right! Try for " + cardCt + "."; } repaint(); } The paintComponent() method of the HighLowCanvas class uses the values in the state variables to decide what to show. It displays the string stored in the message variable. It draws each of the cards in the hand. There is one little tricky bit: If a game is in progress, it draws an extra face-down card, which is not in the hand, to represent the next card in the deck. Drawing the cards requires some care and computation. I wrote a method, "void drawCard(Graphics g, Card card, int x, int y)", which draws a card with its upper left corner at the point (x,y). The paintComponent() routine decides where to draw each card and calls this routine to do the drawing. You can check out all the details in the source code, HighLowGUI.java. As a final example, let's look quickly at an improved paint program, similar to the one from Section 4. The user can draw on the large white area. In this version, the user selects the drawing color from the pop-up menu at the bottom-left of the applet. If the user hits the "Clear" button, the drawing area is filled with the background color. I've added one feature: If the user hits the "Set Background" button, the background color of the drawing area is set to the color currently selected in the pop-up menu, and the drawing area is cleared. This lets you draw in cyan on a magenta background if you have a mind to. (Applet "SimplePaint2" would be displayed here if Java were available.) The drawing area in this applet is a component, belonging to the nested class SimplePaintCanvas. I wrote this class, as usual, as a sub-class of JPanel and programmed it to listen for mouse events and to respond by drawing a curve. As in the HighLowGUI applet, all the action takes place in the nested class. The main applet class just does the set up. One new feature of interest is the pop-up menu. This component is an object belonging to the standard class, JComboBox. We'll cover this component class in Chapter 7. What you should note about this version of the paint applet is that in many ways, it was easier to write than the original. There are no computations about where to draw things and how to decode user mouse clicks. We don't have to worry about the user drawing outside the drawing area. The graphics context that is used for drawing on the canvas can only draw on the canvas. If the user tries to extend a curve outside the canvas, the part that lies outside the canvas is automatically ignored. We don't have to worry about giving the user visual feedback about which color is selected. That is handled by the text displayed on the pop-up menu. http://math.hws.edu/eck/cs124/javanotes4/c6/s6.html (8 of 9) [6/11/2004 11:10:34 AM] Java Programming: Section 6.6 You'll find the source code for this example in the file SimplePaint2.java. After struggling through this chapter, you should be equipped to understand it almost in its entirety! End of Chapter 6 [ Next Chapter | Previous Section | Chapter Index | Main Index http://math.hws.edu/eck/cs124/javanotes4/c6/s6.html (9 of 9) [6/11/2004 11:10:34 AM] Java Programming: Chapter 6 Exercises Programming Exercises For Chapter 6 THIS PAGE CONTAINS programming exercises based on material from Chapter 6 of this on-line Java textbook. Each exercise has a link to a discussion of one possible solution of that exercise. Exercise 6.1: Write an applet that shows a pair of dice. When the user clicks on the applet, the dice should be rolled (that is, the dice should be assigned newly computed random values). Each die should be drawn as a square showing from 1 to 6 dots. Since you have to draw two dice, its a good idea to write a subroutine, "void drawDie(Graphics g, int val, int x, int y)", to draw a die at the specified (x,y) coordinates. The second parameter, val, specifes the value that is showing on the die. Assume that the size of the applet is 100 by 100 pixels. Here is a working version of the applet. (My applet plays a clicking sound when the dice are rolled. See the solution to see how this is done.) See the solution! Exercise 6.2: Improve your dice applet from the previous exercise so that it also responds to keyboard input. When the applet has the input focus, it should be hilited with a colored border, and the dice should be rolled whenever the user presses a key on the keyboard. This is in addition to rolling them when the user clicks the mouse on the applet. Here is an applet that solves this exercise: See the solution! Exercise 6.3: In Exercise 6.1, above, you wrote a pair-of-dice applet where the dice are rolled when the clicks on the applet. Now make a pair-of-dice applet that uses the methods discussed in Section 6.6. Draw the dice on a JPanel, and place a "Roll" button at the bottom of the applet. The dice should be rolled when the user clicks the Roll button. Your applet should look and work like this one: (Note: Since there was only one button in this applet, I added it directly to the applet's content pane, rather than putting it in a "buttonBar" panel and adding the panel to the content pane.) See the solution! Exercise 6.4: In Exercise 3.5, you drew a checkerboard. For this exercise, write a checkerboard applet where the user can select a square by clicking on it. Hilite the selected square by drawing a colored border around it. When the applet is first created, no square is selected. When the user clicks on a square that is not currently selected, it becomes selected. If the user clicks the square that is selected, it becomes unselected. Assume that the size of the applet is 160 by 160 pixels, so that each square on the checkerboard is 20 by 20 pixels. Here is a working version of the applet: See the solution! Exercise 6.5: Write an applet that shows two squares. The user should be able to drag either square with the mouse. (You'll need an instance variable to remember which square the user is dragging.) The user can drag the square off the applet if she wants; if she does this, it's gone. You can try it here: See the solution! http://math.hws.edu/eck/cs124/javanotes4/c6/exercises.html (1 of 2) [6/11/2004 11:10:34 AM] Java Programming: Chapter 6 Exercises Exercise 6.6: For this exercise, you should modify the SubKiller game from Section 6.5. You can start with the existing source code, from the file SubKillerGame.java. Modify the game so it keeps track of the number of hits and misses and displays these quantities. That is, every time the depth charge blows up the sub, the number of hits goes up by one. Every time the depth charge falls off the bottom of the screen without hitting the sub, the number of misses goes up by one. There is room at the top of the applet to display these numbers. To do this exercise, you only have to add a half-dozen lines to the source code. But you have to figure out what they are and where to add them. To do this, you'll have to read the source code closely enough to understand how it works. See the solution! (A working version of the applet can be found here.) Exercise 6.7: Section 3.7 discussed SimpleAnimationApplet2, a framework for writing simple animations. You can define an animation by writing a subclass and defining a drawFrame() method. It is possible to have the subclass implement the MouseListener interface. Then, you can have an animation that responds to mouse clicks. Write a game in which the user tries to click on a little square that jumps erratically around the applet. To implement this, use instance variables to keep track of the position of the square. In the drawFrame() method, there should be a certain probability that the square will jump to a new location. (You can experiment to find a probability that makes the game play well.) In your mousePressed method, check whether the user clicked on the square. Keep track of and display the number of times that the user hits the square and the number of times that the user misses it. Don't assume that you know the size of the applet in advance. See the solution! (A working version of the applet can be found here.) Exercise 6.8:Write a Blackjack applet that lets the user play a game of Blackjack, with the computer as the dealer. The applet should draw the user's cards and the dealer's cards, just as was done for the graphical HighLow card game in Section 6.6. You can use the source code for that game, HighLowGUI.java, for some ideas about how to write your Blackjack game. The structures of the HighLow applet and the Blackjack applet are very similar. You will certainly want to use the drawCard() method from that applet. You can find a description of the game of Blackjack in Exercise 5.5. Add the following rule to that description: If a player takes five cards without going over 21, that player wins immediately. This rule is used in some casinos. For your applet, it means that you only have to allow room for five cards. You should assume that your applet is just wide enough to show five cards, and that it is tall enough to show the user's hand and the dealer's hand. Note that the design of a GUI Blackjack game is very different from the design of the text-oriented program that you wrote for Exercise 5.5. The user should play the game by clicking on "Hit" and "Stand" buttons. There should be a "New Game" button that can be used to start another game after one game ends. You have to decide what happens when each of these buttons is pressed. You don't have much chance of getting this right unless you think in terms of the states that the game can be in and how the state can change. Your program will need the classes defined in Card.java, Hand.java, BlackjackHand.java, and Deck.java. Here is a working version of the applet: See the solution! [ Chapter Index | Main Index ] http://math.hws.edu/eck/cs124/javanotes4/c6/exercises.html (2 of 2) [6/11/2004 11:10:34 AM] Java Programming: Chapter 6 Quiz Quiz Questions For Chapter 6 THIS PAGE CONTAINS A SAMPLE quiz on material from Chapter 6 of this on-line Java textbook. You should be able to answer these questions after studying that chapter. Sample answers to all the quiz questions can be found here. Question 1: Programs written for a graphical user interface have to deal with "events." Explain what is meant by the term event. Give at least two different examples of events, and discuss how a program might respond to those events. Question 2: What is an event loop? Question 3: Explain carefully what the repaint() method does. Question 4: What is HTML? Question 5: Draw the picture that will be produced by the following paint() method: public static void paint(Graphics g) { for (int i=10; i <= 210; i = i + 50) for (int j = 10; j <= 210; j = j + 50) g.drawLine(i,10,j,60); } Question 6: Suppose you would like an applet that displays a green square inside a red circle, as illustrated. Write a paint() method that will draw the image. Question 7: Suppose that you are writing an applet, and you want the applet to respond in some way when the user clicks the mouse on the applet. What are the four things you need to remember to put into the source code of your applet? Question 8: Java has a standard class called MouseEvent. What is the purpose of this class? What does an object of type MouseEvent do? Question 9: Explain what is meant by input focus. How is the input focus managed in a Java GUI program? Question 10: Java has a standard class called JPanel. Discuss two ways in which JPanels can be used. [ Answers | Chapter Index | Main Index ] http://math.hws.edu/eck/cs124/javanotes4/c6/quiz.html [6/11/2004 11:10:35 AM] Java Programming: Chapter 7 Index Chapter 7 Advanced GUI Programming IT'S POSSIBLE TO PROGRAM A WIDE VARIETY of GUI applications using only the techniques covered in the previous chapter. In many cases, the basic events, components, layouts, and graphics routines covered in that chapter suffice. But the Swing graphical user interface library is far richer than what we have seen so far, and it can be used to build highly sophisticated applications. This chapter is a further introduction to Swing. Although the title of the chapter is "Advanced GUI Programming," it is still just an introduction. Full coverage of Swing would require at least another complete book. In this chapter, we'll take a more detailed look at Swing, starting with a few more features of the Graphics class. We'll cover a number of new layout managers, component classes, and event types, and we'll see how to open independent windows and dialog boxes on the screen. We'll also look at two other programming techniques, timers and threads. The material in this chapter will be used in a number of examples and programming exercises in future chapters. Aside from that, this chapter is not a prerequisite the rest of this textbook. If you skip it, you will not miss out on any fundamental programming concepts -- just a lot of the fun of GUI programming. Contents Chapter 7: ● Section 1: More about Graphics ● Section 2: More about Layouts and Components ● Section 3: Basic Components and Their Events ● Section 4: Programming with Components ● Section 5: Menus and Menubars ● Section 6: Timers, Animation, and Threads ● Section 7: Frames and Applications ● Programming Exercises ● Quiz on this Chapter [ First Section | Next Chapter | Previous Chapter | Main Index ] http://math.hws.edu/eck/cs124/javanotes4/c7/ [6/11/2004 11:10:53 AM] Java Programming: Section 7.1 Section 7.1 More About Graphics IN THIS SECTION, we'll look at some additional aspects of graphics in Java. Most of the section deals with Images, which are pictures stored in files or in the computer's memory. But we'll also consider a few other techniques that can be used to draw better or more efficiently. Images To a computer, an image is just a set of numbers. The numbers specify the color of each pixel in the image. The numbers that represent the image on the computer's screen are stored in a part of memory called a frame buffer. Many times each second, the computer's video card reads the data in the frame buffer and colors each pixel on the screen according to that data. Whenever the computer needs to make some change to the screen, it writes some new numbers to the frame buffer, and the change appears on the screen a fraction of a second later, the next time the screen is redrawn by the video card. Since it's just a set of numbers, the data for an image doesn't have to be stored in a frame buffer. It can be stored elsewhere in the computer's memory. It can be stored in a file on the computer's hard disk. Just like any other data file, an image file can be downloaded over the Internet. Java includes standard classes and subroutines that can be used to copy image data from one part of memory to another and to get data from an image file and use it to display the image on the screen. The standard class java.awt.Image is used to represent images. A particular object of type Image contains information about some particular image. There are actually two kinds of Image objects. One kind represents an image in an image data file. The second kind represents an image in the computer's memory. Either type of image can be displayed on the screen. The second kind of Image can also be modified while it is in memory. We'll look at this second kind of Image below. Every image is coded as a set of numbers, but there are various ways in which the coding can be done. For images in files, there are two main coding schemes which are used in Java and on the Internet. One is used for GIF images, which are usually stored in files that have names ending in ".gif". The other is used for JPEG images, which are stored in files that have names ending in ".jpg" or ".jpeg". Both GIF and JPEG images are compressed. That is, redundancies in the data are exploited to reduce the number of numbers needed to represent the data. In general, the compression method used for GIF images works well for line drawings and other images with large patches of uniform color. JPEG compression generally works well for photographs. The Applet class defines a method, getImage, that can be used for loading images stored in GIF and JPEG files. (As we will see later, stand-alone applications use a different technique for loading image files.) For example, suppose that the image of an ace of clubs, shown at the right, is contained in a file named "ace.gif". And suppose that img is a variable of type Image. Then the following command could be used in the source code of your applet: img = getImage( getCodeBase(), "ace.gif" ); This would create an Image object to represent the ace. The second parameter is the name of the file that contains the image. The first parameter specifies the directory that contains the image file. The value "getCodeBase()" specifies that the image file is in the code base directory for the applet. Assuming that the applet is in the default package, as usual, that just means that the image file is in the same directory as the compiled class file of the applet. http://math.hws.edu/eck/cs124/javanotes4/c7/s1.html (1 of 9) [6/11/2004 11:10:54 AM] Java Programming: Section 7.1 Once you have an object of type Image, however you obtain it, you can draw the image in any graphics context. Most commonly, this will be done in the paintComponent() method of a JPanel (or some other JComponent.) If g is the Graphics object that is provided as a parameter to the paintComponent() method, then the command: g.drawImage(img, x, y, this); will draw the image img in a rectangular area in the component. The parameters x and y give the position of the upper-left corner of the rectangle in which the image is displayed, and the rectangle is just large enough to hold the image. The fourth parameter, this, is the special variable from Section 5.5 that refers to the component itself. This parameter is there for technical reasons having to do with the funny way Java treats image files. (Although you don't really need to know this, here is how it works: When you use getImage() to create an Image object from an image file, the file is not downloaded immediately. The Image object simply remembers where the file is. The file will be downloaded the first time you draw the image. However, when the image needs to be downloaded, the drawImage() method only initiates the downloading. It doesn't wait for the data to arrive. So, after drawImage() has finished executing, it's quite possible that the image has not actually been drawn! But then, when does it get drawn? That's where the fourth parameter to the drawImage() command comes in. The fourth parameter is something called an ImageObserver. After the image has been downloaded, the system will inform the ImageObserver that the image is available, and the ImageObserver will actually draw the image at that time. For large images, it's even possible that the image will be drawn in several parts as it is downloaded. Any JComponent object can act as an ImageObserver. If you are sure that the image that you are drawing has already been downloaded, you can set the fourth parameter of drawImage() to null.) There are a few useful variations of the drawImage() command. For example, it is possible to scale the image as it is drawn to a specified width and height. This is done with the command g.drawImage(img, x, y, width, height, this); The parameters width and height give the size of the rectangle in which the image is displayed. Another version makes it possible to draw just part of the image. In the command: g.drawImage(img, dest_x1, dest_y1, dest_x2, dest_y2, source_x1, source_y1, source_x2, source_y2, this); the integers source_x1, source_y1, source_x2, and source_y2 specify the top-left and bottom-right corners of a rectangular region in the source image. The integers dest_x1, dest_y1, dest_x2, and dest_y2 specify the corners of a region in the destination graphics context. The specified rectangle in the image is drawn, with scaling if necessary, to the specified rectangle in the graphics context. For an example in which this is useful, consider a card game that needs to display 52 different cards. Dealing with 52 image files can be cumbersome and inefficient, especially for downloading over the Internet. So, all the cards might be put into a single image: http://math.hws.edu/eck/cs124/javanotes4/c7/s1.html (2 of 9) [6/11/2004 11:10:54 AM] Java Programming: Section 7.1 Now, only one Image object is needed. Drawing one card means drawing a rectangular region from the image. This technique is used in the following version of the HighLow card game from Section 6.6: (Applet "HighLowGUI2" would be displayed here if Java were available.) In this applet, the cards are drawn by the following method. The variable, cardImages, is a variable of type Image that represents the image of 52 cards that is shown above. Each card is 40 by 60 pixels. These numbers are used, together with the suit and value of the card, to compute the corners of the source and destination rectangles for the drawImage() command: void drawCard(Graphics g, Card card, int x, int y) { // Draws a card as a 40 by 60 rectangle with // upper left corner at (x,y). The card is drawn // in the graphics context g. If card is null, then // a face-down card is drawn. The cards are taken // from an Image object that loads the image from // the file smallcards.gif. if (card == null) { // Draw a face-down card g.setColor(Color.blue); g.fillRect(x,y,40,60); g.setColor(Color.white); g.drawRect(x+3,y+3,33,53); g.drawRect(x+4,y+4,31,51); } else { int row = 0; // Which of the four rows contains this card? switch (card.getSuit()) { case Card.CLUBS: row = 0; break; case Card.HEARTS: row = 1; break; case Card.SPADES: row = 2; break; case Card.DIAMONDS: row = 3; break; } int sx, sy; // Coords of upper left corner in the source image. http://math.hws.edu/eck/cs124/javanotes4/c7/s1.html (3 of 9) [6/11/2004 11:10:54 AM] Java Programming: Section 7.1 sx = 40*(card.getValue() - 1); sy = 60*row; g.drawImage(cardImages, x, y, x+40, y+60, sx, sy, sx+40, sy+60, this); } } // end drawCard() The variable cardImages is defined as an instance variable in the applet, and the image object is created in the init() method of the applet with the command: cardImages = getImage( getCodeBase(), "smallcards.gif" ); The complete source code for this applet can be found in HighLowGUI2.java. Off-screen Images and Double Buffering In addition to images in image files, objects of type Image can be used to represent images stored in the computer's memory. What makes such images particularly useful is that it is possible to draw to an Image in the computer's memory. This drawing is not visible to the user. Later, however, the image can be copied very quickly to the screen. In fact, this technique is used automatically in Swing to draw the components that you see on the screen. When the on-screen picture needs to be redrawn, the new picture is drawn step-by-step to an off-screen image. This can take some time. If all this drawing were done on screen, the user would see the image flicker as it is drawn. Instead, a complete new image replaces the old one on the screen almost instantaneously. The user doesn't see all the steps involved in redrawing. This technique makes smooth, flicker-free animation and dragging easy in Swing. (It is not at all easy or automatic when using the older AWT GUI components. This is one big advantage of Swing.) The technique of drawing an off-screen image and then quickly copying the image to the screen is called double buffering. The name comes from the term "frame buffer," which refers to the region in memory that holds the image on the screen. (In fact, true double buffering uses two frame buffers. The video card can display either frame buffer on the screen and can switch instantaneously from one frame buffer to the other. One frame buffer is used to draw a new image for the screen. Then the video card is told to switch from one frame buffer to the other. No copying of memory is involved. Double-buffering as it is implemented in Java does require copying, which takes some time and is not perfectly flicker-free.) It's possible to turn off double buffering in Swing (although there is little reason to do so). To help you understand the effect of double buffering, here are two applets that are identical, except that one uses double buffering and one does not. You can drag the red squares around the applets. I've added a lot of lines in the background to increase the time it takes to redraw the applet. You should notice an annoying flicker in the non-double-buffered applet on the left: (Applets "NonDoubleBufferedDrag" and "DoubleBufferedDrag" would be displayed here if Java were available.) Swing's double buffering uses an off-screen image. Sometimes, it's useful to create your own off-screen images for other purposes. An off-screen Image object can be created by calling the instance method createImage(). This method is defined in the Component class, and so can be used just about anywhere in an applet's source code. The createImage() method takes two parameters to specify the width and height of the image to be created. For example, Image offScreenImage = createImage(width, height); Drawing to an off-screen image is done in the same way as any other drawing in Java, by using a graphics context. The Image class defines an instance method getGraphics() that returns a Graphics object that can be used for drawing on the off-screen image. (This works only for off-screen images. If you try to do this with an Image from a file, an error will occur.) That is, if offScreenImage is a variable of type http://math.hws.edu/eck/cs124/javanotes4/c7/s1.html (4 of 9) [6/11/2004 11:10:54 AM] Java Programming: Section 7.1 Image that refers to an off-screen image, you can say Graphics offscreenGraphics = offScreenImage.getGraphics(); Then, any drawing operations performed with the graphics context offscreenGraphics are applied to the off-screen image. For example, "offscreenGraphics.drawRect(10,10,50,100);" will draw a 50-by-100-pixel rectangle on the off-screen image. Once a picture has been drawn on the off-screen image, the picture can be copied into another graphics context, using the graphics context's drawImage() method. For example: g.drawImage(offScreenImage,0,0,null). For an off-screen image, the file parameter to drawImage() can be null. (Since the image is already in memory, there is no need for an "ImageObserver" to wait for the image to be loaded from a file.) Off-screen images can be used to solve one problem that we have seen in many of our sample applets. In many cases, we have had no convenient way of remembering what was drawn on an applet, so that we were unable restore the drawing when necessary. For example, in the paint applet in Section 6.6, the user's sketch will disappear if the applet is covered up and then uncovered. An off-screen image can be used to solve this problem. The idea is simple: Keep a copy of the drawing in an off-screen image. When the component needs to be redrawn, copy the off-screen image onto the screen. This method is used in the improved paint program at the end of this section. When used in this way, the off-screen image should always contain a copy of the picture on the screen. The paintComponent() method copies this off-screen image to the screen. This will refresh the picture when it is covered and uncovered. The actual drawing of the picture should take place elsewhere. (Occasionally, it makes sense to draw some extra stuff on the screen, on top of the image from the off-screen image. For example, a hilite or a shape that is being dragged might be treated in this way. These things are not permanently part of the image. The permanent image is safe in the off-screen image, and it can be used to restore the on-screen image when the hilite is removed or the shape is dragged to a different location. We will use this technique in the next example.) There are two approaches to keeping the image on the screen synchronized with the image in the off-screen image. In the first approach, in order to change the image, you make the change to the off-screen image and then call repaint() to copy the modified image to the screen. This is safe and easy, but not always efficient. The second approach is to make every change twice, once to the off-screen image and once to the screen. This keeps the two images the same, but it requires some care to make sure that exactly the same drawing is done in both (and it violates the rule about doing drawing operations only inside paintComponent() methods). When using an off-screen image as a backup for the picture displayed on a component, the size of the off-screen image should be the same as the size of the component. This raises the problem of where in the program the image should be created. If the off-screen image is to fill an entire applet, then the image can be created in the applet's init() method with the command: offScreenImage = createImage(getSize().width,getSize().height); However, components other than applets do not have convenient init() methods for initialization. They have constructors, but the size of a component is not known when its constructor is executed, so the above command will not work in a constructor. An alternative is to create the off-screen image on demand, when it is needed. We can even allow for changes in size of a component if we make a new off-screen image whenever the size changes. Here is some sample code that implements this idea. A method named checkOffScreenImage() will create the off-screen image when necessary. This method should always be called before using the off-screen image. For example, it is called in the paintComponent() method before copying the image to the screen. /* Some variables used for double-buffering. */ Image OSI; // The off-screen image (created in paintComponent()). http://math.hws.edu/eck/cs124/javanotes4/c7/s1.html (5 of 9) [6/11/2004 11:10:54 AM] Java Programming: Section 7.1 int widthOfOSI, heightOfOSI; // Current width and height of OSI. // These are checked against the size // of the component, to detect any change // in the component's size. If the size // has changed, a new OSI is created. // The picture in the off-screen image // is lost when that happens. void checkOffScreenImage() { // This method will create the off-screen image if it has not // already been created or if the component's size has changed. // It should always be called before using the off-screen // image in any way. if (OSI == null || widthOfOSI != getSize().width || heightOfOSI != getSize().height) { // OSI doesn't yet exist, or else it exists but has a // different size from the component's current size. // Create a new OSI, and fill it with the component's // background color. OSI = null; // If OSI already exists, this frees up the memory. widthOfOSI = getSize().width; heightOfOSI = getSize().height; OSI = createImage(widthOfOSI, heightOfOSI); Graphics OSGr = OSC.getGraphics(); OSGr.setColor(getBackground()); OSGr.fillRect(0, 0, widthOfOSC, heightOfOSC); OSGr.dispose(); // Free operating system resources. } } public void paintComponent(Graphics g) { // Paint the component by copying the off-screen image onto // the screen. First, call checkOffScreenImage() to make // sure that the off-screen image is ready. // (Note that since the image fills the entire component, // it is not necessary to call super.paintComponent(g).) checkOffScreenImage(); g.drawImage(OSI, 0, 0, null); // Copy OSI onto the screen. // Note: At this point, we could draw hiliting or other extra // stuff on top of the picture in the off-screen image. } Note that the contents of the off-screen image are lost if the size changes. If this is a problem, you can consider copying the contents of the old off-screen image to the new one before discarding the old image. You can do this with drawImage(), and you can even scale the image to fit the new size if you want. However, the results of scaling are not always attractive. Here is an applet that demonstrates some of these ideas. Draw red lines by clicking and dragging on the applet. Draw blue rectangles by right-clicking and dragging. Hold down the shift key and click to clear the applet. Notice that as you drag the mouse, the figure that you are drawing stretches between the current mouse position and the point where you started dragging. This effect is sometimes called a rubber band cursor: http://math.hws.edu/eck/cs124/javanotes4/c7/s1.html (6 of 9) [6/11/2004 11:10:54 AM] Java Programming: Section 7.1 (Applet "RubberBand" would be displayed here if Java were available.) In this applet, a copy of the picture that you've drawn is kept in an off-screen image. If you cover the applet and uncover it, the picture is restored by copying this backup image onto the screen. When you drag the mouse, the figure that you are drawing is not added to the off-screen image. The paintComponent() method simply draws the new figure on top of the backup image. The backup image is not changed, and as you move the mouse around, you can see that it is still there, "underneath" the figure you are sketching. The new figure is only added to the off-screen image when you release the mouse button. To see how all this works in detail, check the source code, RubberBand.java. There is one other point of interest in the above applet. To draw a rectangle in Java, you need to know the coordinates of the upper left corner, the width, and the height. However, when a rectangle is drawn in this applet, the available data consists of two corners of the rectangle: the starting position of the mouse and its current position. From these two corners, the left edge, the top edge, the width, and the height of the rectangle have to be computed. This can be done as follows: void drawRectUsingCorners(Graphics g, int x1, int y1, int x2, int y2) { // Draw a rectangle with corners at (x1,y1) and (x2,y2). int x,y; // Coordinates of the top-left corner. int w,h; // Width and height of rectangle. if (x1 < x2) { // x1 is the left edge x = x1; w = x2 - x1; } else { // x2 is the left edge x = x2; w = x1 - x2; } if (y1 < y2) { // y1 is the top edge y = y1; h = y2 - y1; } else { // y2 is the top edge y = y2; h = y1 - y2; } g.drawRect(x, y, w, h); // Draw the rect. } Rectangles, Clipping, and Repainting The example we've just looked at has one glaring inefficiency: Every time the user drags the mouse, the entire applet is repainted, even though only a small part of the picture might need to be changed. It's possible to improve on this by repainting only a part of the applet. There is a second version of the repaint() command that makes this possible. If comp is a variable that refers to some component, then comp.repaint( x, y, width, height ); tells the system that a rectangular area in the component needs to be repainted. The first two parameters, x and y, specify the upper left corner of the rectangle and the next two parameters give the width and height of the rectangle. In response to this, the system will call paintComponent() as usual, but the graphics context will be set up for drawing only in the specified region. This is done by setting the clip region of the graphics context. The clip region of a graphics context specifies the area where drawing can occur. Any http://math.hws.edu/eck/cs124/javanotes4/c7/s1.html (7 of 9) [6/11/2004 11:10:54 AM] Java Programming: Section 7.1 attempt to use the graphics context to draw outside the clip region is ignored. (If part of a shape lies outside the clip region, that part is "clipped off" before the shape is drawn on the screen.) Only the pixels inside the clip region need to have their color set, and this can be much more efficient than setting the color of every pixel in the component. When an off-screen image is copied onto the component, only the part that lies within the clip region is actually copied. The techniques covered in this section can be used to improve the simple painting program from Section 6.6. The new version uses an off-screen image to save a copy of the user's work, and it uses the version of repaint() discussed above. As before, the user can draw a free-hand sketch. However, in this version, the user can also choose to draw several shapes by selecting from the pop-up menu in the upper right. Try it out! Check that when you cover up the applet with another window, your drawing is still there when you uncover it. (Applet "SimplePaint3" would be displayed here if Java were available.) The source code for this improved paint applet is in the file SimplePaint3.java. It uses an off-screen image pretty much in the way described above. The paintComponent() method copies the off-screen image to the screen, and as the user drags the mouse, clipping is used to restrict the drawing to the region that actually needs to be changed. In this applet, curves are handled differently from the other shapes. Suppose that the user is sketching a curve and that the user moves the mouse from the point (prevX,prevY) to the point (mouseX,mouseY). The applet responds to this by drawing a line segment in the off-screen image from (prevX,prevY) to (mouseX,mouseY). To make this change appear on the screen, a rectangle that contains these two points must be copied from the off-screen image onto the screen. This is accomplished in the applet by calling repaint(x,y,w,h) with appropriate values for the parameters. When the user is sketching one of the other shapes in the applet, the rubber band cursor technique is used. That is, while the user is dragging the mouse, the shape is drawn by the paintComponent() method on top of the picture from the off-screen image. Let's say, for example, that the user is drawing a rectangle. Suppose that the user starts by pressing the mouse at the point (startX,startY). Consider what happens later, when the user drags the mouse from the point (prevX,prevY) to the point (mouseX,mouseY). At the beginning of this motion, a rectangle is shown on the screen with corners at (startX,startY) and (prevX,prevY). In response to the motion, this rectangle must be removed and a new one with corners at (startX,startY) and (mouseX,mouseY) should appear. This can be accomplished by changing the values of the variables that tell paintComponent() where to draw the rectangle and by calling repaint(x,y,w,h) twice: once to repaint the area occupied by the old rectangle and once to repaint the area that will be occupied by the new rectangle. (The system will actually combine the two operations into a single call to paintComponent().) This version of "SimplePaint" is not really all that simple. There are a lot of details to take care of. I urge you to look at the source code to see how it's done. FontMetrics In the rest of this section, we turn from Images to look briefly at another aspect of Java graphics. Often, when drawing a string, it's important to know how big the image of the string will be. You need this information if you want to center a string on an applet. Or if you want to know how much space to leave between two lines of text, when you draw them one above the other. Or if the user is typing the string and you want to position a cursor at the end of the string. In Java, questions about the size of a string are answered by an object belonging to the standard class java.awt.FontMetrics. http://math.hws.edu/eck/cs124/javanotes4/c7/s1.html (8 of 9) [6/11/2004 11:10:54 AM] Java Programming: Section 7.1 There are several lengths associated with any given font. Some of them are shown in this illustration: The red lines in the illustration are the baselines of the two lines of text. The suggested distance between two baselines, for single-spaced text, is known as the lineheight of the font. The ascent is the distance that tall characters can rise above the baselines, and the descent is the distance that tails like the one on the letter g can descend below the baseline. The ascent and descent do not add up to the lineheight, because there should be some extra space between the tops of characters in one line and the tails of characters on the line above. The extra space is called leading. All these quantities can be determined by calling instance methods in a FontMetrics object. There are also methods for determining the width of a character and the width of a string. If F is a font and g is a graphics context, you can get a FontMetrics object for the font F by calling g.getFontMetrics(F). If fm is a variable that refers to the FontMetrics object, then the ascent, descent, leading, and lineheight of the font can be obtained by calling fm.getAscent(), fm.getDescent(), fm.getLeading(), and fm.getHeight(). If ch is a character, then fm.charWidth(ch) is the width of the character when it is drawn in that font. If str is a string, then fm.stringWidth(str) is the width of the string. For example, here is a paintComponent() method that shows the message "Hello World" in the exact center of the component: public void paintComponent(Graphics g) { int width, height; // Width and height of the string. int x, y; // Starting point of baseline of string. Font F = g.getFont(); // What font will g draw in? FontMetrics fm = g.getFontMetrics(F); width = fm.stringWidth("Hello World"); height = fm.getAscent(); // Note: There are no tails on // any of the chars in the string! x = getSize().width / 2 - width / 2; // Go to center and back up // half the width of the // string. y = getSize().height / 2 + height / 2; // Go to center, then move // down half the height of // the string. g.drawString("Hello World", x, y); } [ Next Section | Previous Chapter | Chapter Index | Main Index ] http://math.hws.edu/eck/cs124/javanotes4/c7/s1.html (9 of 9) [6/11/2004 11:10:54 AM] Java Programming: Section 7.2 Section 7.2 More about Layouts and Components SWING INCLUDES A VARIETY of GUI components. We have already encountered a few of these, such as JApplet, JButton, and JPanel. In then next few sections, we will be studying Swing components in more detail. Most Swing components are defined by subclasses of the class javax.swing.JComponent. A JComponent cannot stand on its own. It must be contained in some other component. We have seen, for example, that JPanels can act as containers for other JComponents. At the top level of this containment hierarchy are classes such as JApplet. A JApplet is not a JComponent, but it can serve as a container for JComponents. A JApplet is a top-level container that is meant to appear on a Web page. In Section 7, we'll see two more top-level container classes, JFrame and JDialog, which can be used to create independent windows on the computer screen. The basic properties of components and containers are actually defined by the AWT classes java.awt.Component and java.awt.Container. Occasionally, you will see these classes used in Swing. For example, the getContentPane() method in a JApplet has a return type of Container rather than JPanel or JComponent as you might expect. A JPanel is a container that is itself a JComponent. A JPanel can contain other components, and it can in turn be contained in another component. The fact that panels can contain other panels means that you can have many levels of components containing other components, as shown in the illustration on the right. Several other classes, such as Box and TabbedPane, also define components that can be used as containers. This leads to two questions: How are components added to a container? How are their sizes and positions controlled? The sizes and positions of the components in a container are usually controlled by a layout manager. Different layout managers implement different ways of arranging components. There are several predefined layout manager classes, including FlowLayout, GridLayout, BorderLayout, BoxLayout, CardLayout and GridBagLayout. All these classes are defined in the package java.awt. It is also possible to define new layout managers, if none of these suit your purpose. Every container is assigned a default layout manager when it is first created. For JPanels, the default layout manager belongs to the class FlowLayout. The content pane of a JApplet uses a BorderLayout by default. You can change the layout manager of a container using its setLayout() method. It is even possible to set the LayoutManager of a container to be null. This allows you to take complete charge of laying out the components in the container. I will discuss this possibility and give an example in the last part of Section 4. As for adding components to a container, that's easy. You just use one of the container's add() methods. There are several add() methods. Which one you should use depends on what type of LayoutManager is being used by the container, so I will discuss the appropriate add() methods as I go along. I have often found it to be fairly difficult to get the exact layout that I want in my applets and windows. I will briefly discuss several layout manager classes here, but using them well will require practice and experimentation. FlowLayout A FlowLayout simply lines up its components without trying to be particularly neat about it. After laying out as many items as will fit in a row across the container, it will move on to the next row. The components in a given row can be either left-aligned, right-aligned, or centered, and there can be horizontal and vertical gaps between components. If the default constructor, "new FlowLayout()" is used, then the components on each row will be http://math.hws.edu/eck/cs124/javanotes4/c7/s2.html (1 of 6) [6/11/2004 11:10:56 AM] Java Programming: Section 7.2 centered and the horizontal and vertical gaps will be five pixels. The default layout for a JPanel uses gaps of this size. The constructor FlowLayout(int align, int hgap, int vgap) can be used to specify alternative alignment and gaps. The possible values of align are FlowLayout.LEFT, FlowLayout.RIGHT, and FlowLayout.CENTER. A nifty trick is to use a very large value of hgap. This forces the FlowLayout to put exactly one component in each row, since there won't be room on a single row for two components and the horizontal gap between them. The appropriate add() method for FlowLayouts has a single parameter of type Component, specifying the component to be added. For example, suppose that we wanted an applet to contain one button, located in the upper right corner of the applet. The default layout manager for an applet's content pane is a BorderLayout. We need to give the content pane a FlowLayout with right alignment. This will shove the button to the right edge of the applet. The following init() method will do this: public void init() { getContentPane().setLayout( new FlowLayout(FlowLayout.RIGHT, 5, 5) ); getConetntPane().add( new JButton("Press me!") ); } Note again that it is the applet's content pane that actually holds components, and it is the content pane that needs a layout manager. It is an error to try to set a layout manager for a JApplet itself. BoxLayout and the Box Class A BoxLayout simply lines up components in a single horizontal row or in a single vertical column. BoxLayouts are generally used with objects belonging to the class javax.swing.Box. A Box is just a container that uses a BoxLayout. The Box class contains two static methods for creating boxes: Box.createHorizontalBox(); and Box.createVerticalBox(); These methods are used instead of a constructor to create box objects. For example, if you want a Box to contain a horizontal row of components, you can create it with the command: Box hbox = Box.createHorizontalBox(); Components are added to a box using an add() method with one parameter, which specifies the component that is to be added. The Box class has several static methods that can be used to create specialized components for adding space to a box layout. For example, if width is an integer, then Box.createHorizontalStrut(width) creates a component that is invisible except that it has the specified width and so takes up that amount of horizontal space. You can add a horizontal strut between two components in a horizontal box layout to leave space between the components. Similarly, Box.createVerticalStrut(height) creates an invisible component that has the specified height. For example, the following commands create a Box that contains four (useless) buttons in a horizontal row, with ten pixels of space between the second and third button: Box hbox = Box.createHorizontalBox(); hbox.add( new JButton("First") ); hbox.add( new JButton("Second") ); hbox.add( Box.createHorizontalStrut(10) ); hbox.add( new JButton("Third") ); hbox.add( new JButton("Fourth") ); Horizontal Boxes can be used for the "toolbars" that you see in many graphical user interfaces. http://math.hws.edu/eck/cs124/javanotes4/c7/s2.html (2 of 6) [6/11/2004 11:10:56 AM] Java Programming: Section 7.2 BorderLayout A BorderLayout places one component in the center of a container. This central component is surrounded by up to four other components that border it to the "North", "South", "East", and "West", as shown in the diagram at the right. Each of the four bordering components is optional. The layout manager first allocates space to the bordering components. Any space that is left over goes to the center component. If a container uses a BorderLayout, then components should be added to the container using a version of the add() method that has two parameters. The first parameter is the component that is being added to the container. The second parameter specifies where the component is to be placed. It must be one of the constants BorderLayout.CENTER, BorderLayout.NORTH, BorderLayout.SOUTH, BorderLayout.EAST, or BorderLayout.WEST. If the second parameter is omitted, then BorderLayout.CENTER is used by default. For example, the following code creates a panel with drawArea as its center component and with scroll bars to the right and below: JPanel panel = new JPanel(); panel.setLayout(new BorderLayout()); // To use BorderLayout with a JPanel, you have // to change the panel's layout manager; otherwise, // a FlowLayout is used. Alternatively, you // can provide the layout manager as a // parameter to the constructor: // panel = new JPanel( new BorderLayout() ); panel.add(drawArea, BorderLayout.CENTER); // Assume drawArea already exists. panel.add(hScroll, BorderLayout.SOUTH); // Assume hScroll is a horizontal scroll bar // component that already exists. panel.add(vScroll, BorderLayout.EAST); // Assume vScroll is a vertical scroll bar // component that already exists. Sometimes, you want to leave space between the components in a container. You can specify horizontal and vertical gaps in the constructor of a BorderLayout object. For example, if you say panel.setLayout(new BorderLayout(5,7)); then the layout manager will insert horizontal gaps of 5 pixels between components and vertical gaps of 7 pixels between components. The horizontal gap is inserted between the center and west components and between the center and east components; the vertical gap is inserted between the center and north components and between the center and south components. (The default layout for a JApplet's content pane is a BorderLayout with no horizontal or vertical gap.) GridLayout A GridLayout lays out components in a grid of equal sized rectangles. The illustration shows how the components would be arranged in a grid layout with 3 rows and 2 columns. If a container uses a GridLayout, the appropriate add method takes a single parameter of type Component (for example: add(myButton)). Components are added to the grid in the order shown; that is, each row is filled from left to right before going on the next row. The constructor for a GridLayout with R rows and C columns takes the form "new GridLayout(R,C)". If you want to leave horizontal gaps of H pixels between columns and vertical gaps of V pixels between rows, use "new GridLayout(R,C,H,V)" instead. When you use a GridLayout, it's probably good form to add just enough components to fill the grid. However, this http://math.hws.edu/eck/cs124/javanotes4/c7/s2.html (3 of 6) [6/11/2004 11:10:56 AM] Java Programming: Section 7.2 is not required. In fact, as long as you specify a non-zero value for the number of rows, then the number of columns is essentially ignored. The system will use just as many columns as are necessary to hold all the components that you add to the container. If you want to depend on this behavior, you should probably specify zero as the number of columns. You can also specify the number of rows as zero. In that case, you must give a non-zero number of columns. The system will use the specified number of columns, with just as many rows as necessary to hold the components that are added to the container. Horizontal grids, with a single row, and vertical grids, with a single column, are very common. For example, suppose that button1, button2, and button3 are buttons and that you'd like to display them in a horizontal row in a panel. If you use a horizontal grid for the panel, then the buttons will completely fill that panel and will all be the same size. The panel can be created as follows: JPanel buttonBar = new JPanel(); buttonBar.setLayout(new GridLayout(1,3)); // (Note: The "3" here is pretty much ignored, and // you could also say "new GridLayout(1,0)". // To leave gaps between the buttons, you could use // "new GridLayout(1,0,5,5)".) buttonBar.add(button1); buttonBar.add(button2); buttonBar.add(button3); You might find this button bar to be more attractive than the ones in the examples in the Section 6.6, which used the default FlowLayout layout manager. GridBagLayout A GridBagLayout is similar to a GridLayout in that the container is broken down into rows and columns of rectangles. However, a GridBagLayout is much more sophisticated because the rows do not all have to be of the same height, the columns do not all have to be of the same width, and a component in the container can spread over several rows and several columns. There is a separate class, GridBagConstraints, that is used to specify the position of a component, the number of rows and columns that it occupies, and several additional properties of the component. Using a GridBagLayout is rather complicated, and I have used it on exactly two occasions in my own Java programming career. I will not explain it here; if you are interested, you should consult a Java reference. CardLayout CardLayouts differ from other layout managers in that in a container that uses a CardLayout, only one of its components is visible at any given time. Think of the components as a set of "cards". Only one card is visible at a time, but you can flip from one card to another. Methods are provided in the CardLayout class for flipping to the first card, to the last card, and to the next card in the deck. A name can be specified for each card as it is added to the container, and there is a method in the CardLayout class for flipping directly to the card with a specified name. Suppose, for example, that you want to create a JPanel that can show any one of three JPanels: panel1, panel2, and panel3. Assume that panel1, panel2, and panel3 have already been created: cardPanel = new JPanel(); // assume cardPanel is declared as an instance variable // so that it can be used in other methods cards = new CardLayout(); // assume cards is declared as an instance variable // so that it can be used in other methods cardPanel.setLayout(cards); cardPanel.add(panel1, "First"); // add panel1 with name "First" cardPanel.add(panel2, "Second"); http://math.hws.edu/eck/cs124/javanotes4/c7/s2.html (4 of 6) [6/11/2004 11:10:56 AM] Java Programming: Section 7.2 // add panel2 with name "Second" cardPanel.add(panel3, "Third"); // add panel3 with name "Third" Elsewhere in your program, you could show panel1 by saying cards.show(cardPanel, "First"); or cards.first(cardPanel); Other methods that are available are cards.last(cardPanel), cards.next(cardPanel), and cards.previous(cardPanel). Note that each of these methods takes the container as a parameter. To use a CardLayout effectively, you'll need to have instance variables to record both the layout manager (cards in the example) and the container (cardPanel in the example). You need both of these objects in order to flip from one card to another. An Example To finish this survey of layout managers, here is an applet that demonstrates layout managers of various types: (Applet "LayoutDemo" would be displayed here if Java were available.) The applet itself uses a BorderLayout with vertical gaps of 3 pixels. These gaps show up in blue. The Center component of the applet is a JPanel, which uses a CardLayout as its layout manager. The layout contains eight cards. Each card is itself another panel that contains several buttons. Each card uses a different type of layout manager (several of which are extremely stupid choices for laying out buttons). The North component of the applet is a JComboBox, which contains the names of the eight panels in the card layout. The user can switch among the cards by selecting items from this menu. The South component of the applet is a JLabel that displays an appropriate message whenever the user clicks on a button or chooses an item from the JComboBox. The source code for this applet is in the file LayoutDemo.java. It consists mainly of a long init() method that creates all the buttons, panels, and other components and lays out the applet. Borders and Insets Swing makes it very easy to add decorative borders around the edges of a JComponent. The class javax.swing.BorderFactory contains a large number of static methods for creating borders. For example, the function BorderFactory.createLineBorder(Color.black) returns an object that represents a one-pixel wide black line around the outside of a component. If comp is a JComponent, a border can be added to comp using its setBorder() method. For example: comp.setBorder( BorderFactory.createLineBorder(Color.black) ); When a border has been set for a JComponent, the border is drawn automatically, without any further effort on the part of the programmer. The border is drawn along the edges of the component, just inside its boundary. The layout manager of a JPanel or other container will take the space occupied by the border into account. The components that are added to the container will be displayed in the area inside the border. I don't recommend using a border on a JPanel that is being used as a drawing surface. However, if you do this, you should take the border into account. If you draw in the area occupied by the border, that part of your drawing will be covered by the border. Here are some of the static methods that can be used to create borders: ● BorderFactory.createEmptyBorder(top,left,bottom,right) -- leaves an empty border http://math.hws.edu/eck/cs124/javanotes4/c7/s2.html (5 of 6) [6/11/2004 11:10:56 AM] Java Programming: Section 7.2 around the edges of a component. Nothing is drawn in this space, so the background color will appear in the area occupied by the border. The parameters are integers that give the width of the border along the top, left, bottom, and right edges of the component. This is actually very useful when used on a JPanel that contains other components. It puts some space between the components and the edge of the panel. ● BorderFactory.createLineBorder(color,thickness) -- draws a line around all four edges of a component. The first parameter is of type Color and specifies the color of the line. The second parameter is an integer that specifies the thickness of the border. If the second parameter is omitted, a line of thickness 1 is drawn. ● BorderFactory.createMatteBorder(top,left,bottom,right,color) -- is similar to createLineBorder, except that you can specify individual thicknesses for the top, left, bottom, and right edges of the component. ● BorderFactory.createEtchedBorder() -- creates a border that looks like a groove etched around the boundary of the component. The effect is achieved using lighter and darker shades of the component's background color, and it does not work well with every background color. ● BorderFactory.createLoweredBevelBorder() -- gives a component a three-dimensional effect that makes it look like it is lowered into the computer screen. As with an EtchedBorder, this only works well for certain background colors. ● BorderFactory.createRaisedBevelBorder() -- similar to a LoweredBevelBorder, but the component looks like it is raised above the computer screen. ● BorderFactory.createTitledBorder(title) -- creates a border with a title. The title is a String, which is displayed in the upper left corner of the border. There are many other methods in the BorderFactory class, most of them providing variations of the basic border styles given here. The following applet shows six components with six different border styles. The text in each component is the command that created the border for that component: (Applet "BorderDemo" would be displayed here if Java were available.) Since a JApplet is not a JComponent, it's not possible to set a Border object for a JApplet. There is, however, another way to add a border of color around the edges. An applet can use "insets" to leave space around the edges of the applet where the background color of the applet will show through. To do this, define the method public Insets getInsets() in your subclass of JApplet. This method should return an object of type Insets, which specifies the width of the border along the top, left, bottom, and right edges of the applet. The system will call your method to determine how much space to leave. For example, if your subclass of JApplet includes the method definition: public Insets getInsets() { return new Insets(5,5,5,5); } then there will be a 5-pixel-wide border around the edges of the applet where the background color of the applet will show. To specify the color, you can set the applet's background color in its init() method. Note that Insets should not be used with JComponents. For a JComponent, you can use BorderFactory.createEmptyBorder() to accomplish the same thing. The LayoutDemo applet uses Insets to leave a 3-pixel border around the outside of the applet, where the blue background color of the applet shows through. This is different from the 3-pixel blue gap between the components in the applet's content pane, where the blue gap is a feature of the content pane's BorderLayout. It's the background color of the content pane, not of the applet, that shows though the spaces in the BorderLayout. To set up the colors, the init() method of the applet sets the background color for both the applet and for its content pane to blue. Since the default layout used for a content pane has no vertical gap, the init() method also installs a different layout manager for the content pane. All this is done with the following commands: setBackground(Color.blue); getContentPane().setBackground(Color.blue); getContentPane().setLayout(new BorderLayout(3,3)); [ Next Section | Previous Section | Chapter Index | Main Index ] http://math.hws.edu/eck/cs124/javanotes4/c7/s2.html (6 of 6) [6/11/2004 11:10:56 AM] Java Programming: Section 7.3 Section 7.3 Basic Components and Their Events THIS SECTION DISCUSSES some of the GUI interface elements that are represented by subclasses of JComponent. The treatment here is brief and covers only the basic uses of each component type. After you become familiar with the basics, you might want to consult a Java reference for more details. I will give some examples of programming with components in the next section. The exact appearance of a Swing component and the way that the user interacts with the component are not fixed. They depend on the look-and-feel of the user interface. While Swing supports a default look-and-feel, which is probably the one that you will see most often, it is possible to change the look-and-feel. For example, a Windows look-and-feel could be used to make a Java program that is running on a Windows computer look more like a standard Windows program. While this can improve the user's experience, it means that some of the details that I discuss will have to be qualified with the phrase "depending on the look-and-feel." The JComponent class itself defines many useful methods that can be used with components of any type. We've already used some of these in examples. Let comp be a variable that refers to any JComponent. Then the following methods are available (among many others): ● comp.getSize() is a function that returns an object belonging to the class Dimension. This object contains two instance variables, comp.getSize().width and comp.getSize().height, that give the current size of the component. You can also get the height and width more directly by calling comp.getHeight() and comp.getWidth(). One warning: When a component is first created, its size is zero. The size will be set later, probably by a layout manager. A common mistake is to check the size of a component before that size has been set, such as in a constructor. ● comp.setEnabled(true) and comp.setEnabled(false) can be used to enable and disable the component. When a component is disabled, its appearance changes, and the user cannot do anything with it. There is a boolean-valued function, comp.getEnabled() that you can call to discover whether the component is enabled. ● comp.setVisible(true) and comp.setVisible(false) can be called to hide or show the component. ● comp.setBackground(color) and comp.setForeground(color) set the background and foreground colors for the component. If no colors are set for a component, the colors are determined by the look-and-feel. ● comp.setOpaque(true) tells the component that the area occupied by the component should be filled with the component's background color before the content of the component is painted. In the default look-and-feel, only JLabels are non-opaque. A non-opaque, or "transparent", component ignores its background color and simply paints its content over the content of its container. This usually means that it inherits the background color from its container. ● comp.setFont(font) sets the font that is used for text displayed on the component. The parameter is an object of type java.awt.Font. ● comp.setToolTipText(string) sets the specified string as a "tool tip" for the component. The tool tip is displayed if mouse cursor is in the component and the mouse is not moved for a few seconds. The tool tip should give some information about the meaning of the component or how to use it. ● comp.setCursor(cursor) sets the cursor image that represents the mouse position when the mouse cursor is inside this component. The parameter is an object belonging to the class java.awt.Cursor. Generally, this parameter takes the form http://math.hws.edu/eck/cs124/javanotes4/c7/s3.html (1 of 11) [6/11/2004 11:10:58 AM] Java Programming: Section 7.3 Cursor.getPredefinedCursor(id) where id is one of several constants defined in the Cursor class. The most useful values are probably Cursor.HAND_CURSOR, Cursor.CROSSHAIR_CURSOR, Cursor.WAIT_CURSOR, and Cursor.DEFAULT_CURSOR. For example, if you would like the cursor to appear as a little pointing hand when the mouse is in the component comp, you can use: comp.setCursor(Cursor.getPredefinedCursor(Cursor.HAND_CURSOR)); ● comp.setPreferredSize(size) sets the size at which the component should be displayed, if possible. The parameter is of type Dimension, and a call to this method usually looks something like "setPreferredSize(new Dimension(100,50))". The preferred size is used as a hint by layout managers, but will not be respected in all cases. In a BorderLayout, for example, the preferred size of the Center component is irrelevant, but the preferred sizes of the North, South, East, and West components are used by the layout manager to decide how much space to allow for those components. Standard components generally compute a correct preferred size automatically, but it can be useful to set it in some cases. For example, if you use a JPanel as a drawing surface, it might be a good idea to set a preferred size for it. ● comp.getParent() is a function that returns a value of type java.awt.Container. The return value is the container that directly contains the component, if any. For a top-level component such as a JApplet, the value will be null. ● comp.getLocation() is a function that returns the location of the top-left corner of the component. The location is specified in the coordinate system of the component's parent. The returned value is an object of type Point. An object of type Point contains two instance variables, x and y. For the rest of this section, we'll look at subclasses of JComponent that represent common GUI components. Remember that using any component is a multi-step process. The component object must be created with a constructor. It must be added to a container. In many cases, a listener must be registered to respond to events from the component. And in some cases, a reference to the component must be saved in an instance variable so that the component can be manipulated by the program after it has been created. The JButton Class An object of class JButton is a push button. You've already seen buttons used in the previous chapter, but we can use a review of JButtons as a reminder of what's involved in using components, events, and listeners. (Some of the methods described here are new.) ● Constructors: The JButton class has a constructor that takes a string as a parameter. This string becomes the text displayed on the button. For example: stopGoButton = new JButton("Go") ● Events: When the user clicks on a button, the button generates an event of type ActionEvent. This event is sent to any listener that has been registered with the button. ● Listeners: An object that wants to handle events generated by buttons must implement the ActionListener interface. This interface defines just one method, "pubic void actionPerformed(ActionEvent evt)", which is called to notify the object of an action event. ● Registration of Listeners: In order to actually receive notification of an event from a button, an ActionListener must be registered with the button. This is done with the button's addActionListener() method. For example: stopGoButton.addActionListener(buttonHandler); ● Event methods: When actionPerformed(evt) is called by the button, the parameter, evt, contains information about the event. This information can be retrieved by calling methods in the ActionEvent class. In particular, evt.getActionCommand() returns a String giving the http://math.hws.edu/eck/cs124/javanotes4/c7/s3.html (2 of 11) [6/11/2004 11:10:58 AM] Java Programming: Section 7.3 command associated with the button. By default, this command is the text that is displayed on the button. The method evt.getSource() returns a reference to the Object that produced the event, that is, to the JButton that was pressed. The return value is of type Object, not JButton, because other types of components can also produce ActionEvents. ● Component methods: There are several useful methods in the JButton class. For example, stopGoButton.setText("Stop") changes the text displayed on the button to "Stop". And stopGoButton.setActionCommand("sgb") changes the action command associated to this button for action events. Of course, JButtons also have all the general Component methods, such as setEnabled() and setFont(). The setEnabled() and setText() methods of a button are particularly useful for giving the user information about what is going on in the program. A disabled button is better than a button that gives an obnoxious error message such as "Sorry, you can't click on me now!" By the way, it's possible for a JButton to display an icon instead of or in addition to the text that it displays. An icon is simply a small image. Several other components can also display icons. However, I will not cover this aspect of Swing in this book. Consult a Java reference if you are interested. The JLabel Class JLabels are certainly the simplest type of component. An object of type JLabel is just a single line of text. The text cannot be edited by the user, although it can be changed by your program. The constructor for a JLabel specifies the text to be displayed: JLabel message = new JLabel("Hello World!"); There is another constructor that specifies where in the label the text is located, if there is extra space. The possible alignments are given by the constants JLabel.LEFT, JLabel.CENTER, and JLabel.RIGHT. For example, JLabel message = new JLabel("Hello World!", JLabel.CENTER); creates a label whose text is centered in the available space. You can change the text displayed in a label by calling the label's setText() method: message.setText("Goodby World!"); Since the JLabel class is a subclass of Component, you can use methods such as setForeground() with labels. If you want the background color to have any effect, you should call setOpaque(true) on the label, since otherwise the JLabel might not fill in its background (depending on the look-and-feel). For example: JLabel message = new JLabel("Hello World!"); message.setForeground(Color.red); // Display red text... message.setBackground(Color.black); // on a black background... message.setFont(new Font("Serif",Font.BOLD,18)); // in a bold font. message.setOpaque(true); // Make sure background is filled in. The JCheckBox Class A JCheckBox is a component that has two states: selected or unselected. The user can change the state of a check box by clicking on it. The state of a checkbox is represented by a boolean value that is true if the box is selected and false if the box is unselected. A checkbox has a label, which is specified when the http://math.hws.edu/eck/cs124/javanotes4/c7/s3.html (3 of 11) [6/11/2004 11:10:58 AM] Java Programming: Section 7.3 box is constructed: JCheckBox showTime = new JCheckBox("Show Current Time"); Usually, it's the user who sets the state of a JCheckBox, but you can also set the state in your program. The current state of a checkbox is set using its setSelected(boolean) method. For example, if you want the checkbox showTime to be checked, you would say "showTime.setSelected(true);". To uncheck the box, say "showTime.setSelected(false);". You can determine the current state of a checkbox by calling its isSelected() method, which returns a boolean value. In many cases, you don't need to worry about events from checkboxes. Your program can just check the state whenever it needs to know it by calling the isSelected() method. However, a checkbox does generate an event when its state changes, and you can detect this event and respond to it if you want something to happen at the moment the state changes. When the state of a checkbox is changed by the user, it generates an event of type ActionEvent. If you want something to happen when the user changes the state of a checkbox, you must register an ActionListener with the checkbox. (Note that if you change the state by calling the setSelected() method, no ActionEvent is generated. However, there is another method in the JCheckBox class, doClick(), which simulates a user click on the checkbox and does generate an ActionEvent.) When handling an ActionEvent, you can call evt.getSource() in the actionPerformed() method to find out which object generated the event. (Of course, if you are only listening for events from one component, you don't even have to do this.) The returned value is of type Object, but you can type-cast it to another type if you want. Once you know the object that generated the event, you can ask the object to tell you its current state. For example, if you know that the event had to come from one of two checkboxes, cb1 or cb2, then your actionPerformed() method might look like this: public void actionPerformed(ActionEvent evt) { Object source = evt.getSource(); if (source == cb1) { boolean newState = ((JCheckBox)cb1).getSelected(); ... // respond to the change of state } else if (source == cb2) { boolean newState = ((JCheckBox)cb2).getSelected(); ... // respond to the change of state } } Alternatively, you can use evt.getActionCommand() to retrieve the action command associated with the source. For a JCheckBox, the action command is, by default, the label of the checkbox. The JRadioButton and ButtonGroup Classes Closely related to checkboxes are radio buttons. Radio buttons occur in groups. At most one radio button in a group can be selected at any given time. In Java, a radio button is represented by an object of type JRadioButton. When used in isolation, a JRadioButton acts just like a JCheckBox, and it has the same methods and events. Ordinarily, however, a JRadioButton is used in a group. A group of radio buttons is represented by an object belonging to the class ButtonGroup. A ButtonGroup is not a component and does not itself have a visible representation on the screen. A ButtonGroup works behind the scenes to organize a group of radio buttons, so that at most one button in the group can be selected at any given time. To use a group of radio buttons, you must create a JRadioButton object for each button in the group, and you must create one object of type ButtonGroup to organize the individual buttons into a group. http://math.hws.edu/eck/cs124/javanotes4/c7/s3.html (4 of 11) [6/11/2004 11:10:58 AM] Java Programming: Section 7.3 Each JRadioButton must be added individually to some container, so that it will appear on the screen. (A ButtonGroup plays no role in the placement of the buttons on the screen.) Each JRadioButton must also be added to the ButtonGroup, which has an add() method for this purpose. If you want one of the buttons to be selected at start-up, you can call setSelected(true) for that button. If you don't do this, then none of the buttons will be selected until the user clicks on one of them. As an example, here is how you could set up a set of radio buttons that can be used to select a color: JRadioButton redRadio, blueRadio, greenRadio, blackRadio; // Variables to represent the radio buttons. // These should probably be instance variables, so // that they can be used throughout the program. ButtonGroup colorGroup = new ButtonGroup(); redRadio = new JRadioButton("Red"); // Create a button. colorGroup.add(redRadio); // Add it to the group. blueRadio = new JRadioButton("Blue"); colorGroup.add(blueRadio); greenRadio = new JRadioButton("Green"); colorGroup.add(greenRadio); blackRadio = new JRadioButton("Black"); colorGroup.add(blackRadio); redRadio.setSelected(true); // Make an initial selection. The individual buttons must still be added to a container if they are to appear on the screen. If you want to respond immediately when the user clicks on one of the radio buttons, you should register an ActionListener for each button. Here is an applet that demonstrates this. When you click one of the radio buttons, the background color of the label is changed: (Applet "RadioButtonDemo" would be displayed here if Java were available.) The source code for the applet is in the file RadioButtonDemo.java. Just as for checkboxes, it is not always necessary to register listeners for radio buttons. In many cases, you can simply check the state of each button when you need to know it, using the isSelected() method. The JComboBox Class The JComboBox class represents another way of letting the user select one option from a list of options. But in this case, the options are presented as a kind of pop-up menu, and only the currently selected option is visible on the screen. The painting applet at the end of Section 6.6 used a JComboBox for selecting a color. When a JComboBox object is first constructed, it initially contains no items. An item is added to the bottom of the menu by calling its instance method, addItem(str), where str is a string that will be displayed. (In fact, you can add any type of object to a JComboBox. The toString() method of the object is called to determine what string to display.) For example, the following code will create an object of type JComboBox that contains the options Red, Blue, Green, and Black: http://math.hws.edu/eck/cs124/javanotes4/c7/s3.html (5 of 11) [6/11/2004 11:10:58 AM] Java Programming: Section 7.3 JComboBox colorChoice = new JComboBox(); colorChoice.addItem("Red"); colorChoice.addItem("Blue"); colorChoice.addItem("Green"); colorChoice.addItem("Black"); You can call the getSelectedIndex() method of a JComboBox to find out which item is currently selected. This method returns an integer that gives the position of the selected item in the list, where the items are numbered starting from zero. Alternatively, you can call getSelectedItem() to get the selected item itself. (This method returns a value of type Object.) You can change the selection by calling the method setSelectedIndex(n), where n is an integer giving the position of the item that you want to select. The most common way to use a JComboBox menu is to call its getSelectedIndex() method when you have a need to know which item is currently selected. However, like other components that we have seen, JComboBox components generate ActionEvents. You can register an ActionListener with the JComboBox if you want to respond to such events as they occur. JComboBoxes have a nifty feature, which is probably not all that useful in practice. You can make a JComboBox "editable" by calling its method setEditable(true). If you do this, the user can edit the selection by clicking on the JComboBox and typing. This allows the user to make a selection that is not in the pre-configured list that you provide. (The "Combo" in the name "JComboBox" refers to the fact that it's a kind of combination of menu and text-input box.) If the user has edited the selection in this way, then the getSelectedIndex() method will return the value -1, and getSelectedItem() will return the string that the user typed. An ActionEvent is triggered if the user presses return in the JComboBox. The JSlider Class A JSlider provides a way for the user to select an integer value from a range of possible values. The user does this by dragging a "knob" along a bar. A slider can, optionally, be decorated with tick marks and with labels. This demonstration applet shows three sliders with different decorations and with different ranges of values: (Applet "SliderDemo" would be displayed here if Java were available.) In this applet, the second slider is decorated with ticks, and the third one is decorated with labels. It's possible for a single slider to have both types of decorations. The most commonly used constructor for JSliders specifies the start and end of the range of values for the slider and its initial value when it first appears on the screen: JSlider(int minimum, int maximum, int value) If the parameters are omitted, the values 0, 100, and 50 are used. By default, a slider is horizontal, but you can make it vertical by calling its method setOrientation(JSlider.VERTICAL). The current value of a JSlider can be read at any time with its getValue() method. This method returns a value of type int. If you want to change the value, you can do so with the method setValue(n), which takes a parameter of type int. If you want to respond immediately when the user changes the value of a slider, you can register a listener with the slider. JSliders, unlike other components we have seen, do not generate ActionEvents. Instead, they generate events of type ChangeEvent. ChangeEvent and related classes are defined in the package javax.swing.event rather than java.awt.event, so if you want to use ChangeEvents, you should import javax.swing.event.* at the beginning of your program. http://math.hws.edu/eck/cs124/javanotes4/c7/s3.html (6 of 11) [6/11/2004 11:10:58 AM] Java Programming: Section 7.3 You must also define some object to implement the ChangeListener interface, and you must register the change listener with the slider by calling its addChangeListener() method. A ChangeListener must provide a definition for the method: void stateChanged(ChangeEvent evt) This method will be called whenever the value of the slider changes. (Note that it will be called when you change the value with the setValue() method, as well as when the user changes the value.) In the stateChanged() method, you can call evt.getSource() to find out which object generated the event. Using tick marks on a slider is a two-step process: Specify the interval between the tick marks, and tell the slider that the tick marks should be displayed. There are actually two types of tick marks, "major" tick marks and "minor" tick marks. You can have one or the other or both. Major tick marks are a bit longer than minor tick marks. The method setMinorTickSpacing(i) indicates that there should be a minor tick mark every i units along the slider. The parameter is an integer. (The spacing is in terms of values on the slider, not pixels.) For the major tick marks, there is a similar command, setMajorTickSpacing(i). Calling these methods is not enough to make the tick marks appear. You also have to call setPaintTicks(true). For example, the second slider in the above applet was created and configured using the commands: slider2 = new JSlider(); slider2.addChangeListener(this); slider2.setMajorTickSpacing(25); slider2.setMinorTickSpacing(5); slider2.setPaintTicks(true); getContentPane().add(slider2); Labels on a slider are handled similarly. You have to specify the labels and tell the slider to paint them. Specifying labels is a tricky business, but the JSlider class has a method to simplify it. Create a set of labels and add them to a slider named sldr with the command: sldr.setLabelTable( sldr.createStandardLabels(i) ); where i is an integer giving the spacing between the labels. To arrange for the labels to be displayed, call setPaintLabels(true). For example, the third slider in the above applet was created and configured with the commands: slider3 = new JSlider(2000,2100,2002); slider3.addChangeListener(this); slider3.setLabelTable(slider3.createStandardLabels(50)); slider3.setPaintLabels(true); getContentPane().add(slider3); JScrollBar and JScrollPane A JScrollBar, like a JSlider, allows the user to select an integer value from a range of values. A scroll bar, however, is generally used to control the scrolling of another component such as the text in a text editor. A scroll bar can be either horizontal or vertical. It has five parts: http://math.hws.edu/eck/cs124/javanotes4/c7/s3.html (7 of 11) [6/11/2004 11:10:58 AM] Java Programming: Section 7.3 The position of the tab specifies the currently selected value. The user can move the tab by dragging it or by clicking on any of the other parts of the scroll bar. The size of the tab tells what portion of a scrolling region is currently visible. It is actually the position of the bottom or left edge of the tab that represents the currently selected value. A scroll bar has four associated integer values: ● min, which specifies the starting point of the range of values represented by the scrollbar, corresponding to the left or bottom edge of the bar ● max, which specifies the end point of the range of values, corresponding to the right or top edge of the bar ● visible, which specifies the size of the tab ● value, which gives the currently selected value, somewhere in the range between min and (max - visible). Note that the largest possible value is (max - visible), not max, since the value represents the position of the left or bottom edge of the tab. The largest possible value allows space for the tab, whose size is given by visible. The four values can be specified when the scroll bar is created. The constructor takes the form JScrollbar(int orientation, int value, int visible, int min, int max); The orientation, which specifies whether the scroll bar is horizontal or vertical, must be one of the constants JScrollbar.HORIZONTAL or JScrollbar.VERTICAL. The value must be between min and (max - visible). You can leave out all the int parameters to get a scroll bar with default values. You can set the value of the scroll bar at any time with the method setValue(int). Similarly, the other values can be set with setMinimum(int), setMaximum(int), and setVisibleAmount(int). You can also set all four values at once by calling: void setValues(int value, int visible, int min, int max); Methods getValue(), getVisibleAmount(), getMinimum() and getMaximum() are provided for reading the current values of each of these parameters. The user can drag the tab or click elsewhere on the scroll bar. How far does the tab move when the user clicks on the up-arrow or down-arrow or in the page-up or page-down region of a scrollbar? The amount by which the value changes when the user clicks on the up-arrow or down-arrow is called the unit increment. The amount by which it changes when the user clicks in the page-up or page-down region is called the block increment. By default, both of these values are 1. They can be set using the methods: void setUnitIncrement(int unitIncrement); http://math.hws.edu/eck/cs124/javanotes4/c7/s3.html (8 of 11) [6/11/2004 11:10:59 AM] Java Programming: Section 7.3 void setBlockIncrement(int blockIncrement); Let's look at an example. Suppose that you want to use a very large drawing area, which is too large to fit on the screen. You might decide to display only part of the JPanel and to provide scroll bars to allow the user to scroll through the entire panel. Let's say that the actual panel is 1000 by 1000 pixels, and that you will show a 200-by-200 region of the panel at any one time. Let's look at how you would set up the vertical scroll bar. The horizontal bar would be essentially the same. The visible of the scroll bar would be 200, since that is how many pixels would actually be displayed. The value of the scroll bar would represent the vertical coordinate of the pixel that is at the top of the display. (Whenever the value changes, you have to redraw the display.) The min would be 0, and the max would be 1000. The range of values that can be set on the scroll bar is from 0 to 800. (Remember that the largest possible value is the maximum minus the visible amount.) The page increment for the scroll bar could be set to some value a little less than 200, say 190 or 175. Then, when the user clicks in the page-up or page-down region, the display will scroll by an amount almost equal to its size. The line increment could be left at 1, but it is likely that this would be too small since it represents a scrolling increment of just one pixel. A line increment of 15 would be better, since then the display would scroll by a more reasonable 15 pixels when the user clicks the up-arrow or down-arrow. (Of course, all these values would have to be reset if the display area is resized.) A scroll bar generates an event of type AdjustmentEvent whenever the user changes the value of the scroll bar. The associated AdjustmentListener interface defines one method, "adjustmentValueChanged(AdjustmentEvent evt)", which is called by the scroll bar to notify the listener that the value on the scroll bar has been changed. This method should repaint the display or make whatever other change is appropriate for the new value. The method evt.getValue() returns the current value on the scroll bar. If you are using more than one scroll bar and need to determine which scroll bar generated the event, use evt.getSource() to determine the source of the event. Scrolling is complicated. Fortunately, Swing provides a class that can take care of many of the details. A JScrollPane is is a component that provides scrolling for another component. That component is specified as a parameter to the constructor: JScrollPane(Component content) The content component appears in the center of the scroll pane. If it is too large to be displayed entirely, then horizontal and/or vertical scroll bars will appear that can be used for scrolling the content. You have to add the scroll pane to a container to make both the scroll pane and its content appear on the screen. This makes scrolling very easy, and makes it unusual to work with scroll bars directly. A JScrollPane can use any component as content, but several Swing components, including the JTextArea that will be discussed below, are designed specifically to work with JScrollPane. The JTextField and JTextArea Classes JTextFields and JTextAreas are boxes where the user can type in and edit text. The difference between them is that a JTextField contains a single line of editable text, while a JTextArea can display multiple lines. It is also possible to set a JTextField or JTextArea to be read-only so that the user can read the text that it contains but cannot edit the text. Both JTextField and JTextArea are subclasses of javax.swing.text.JTextComponent, which defines their common behavior. The JTextComponent class supports the idea of a selection. A selection is a subset of the characters in the JTextComponent, including all the characters from some starting position to some ending position. The selection is hilited on the screen. The user selects text by dragging the mouse over it. Some useful methods in class JTextComponent include the following. They http://math.hws.edu/eck/cs124/javanotes4/c7/s3.html (9 of 11) [6/11/2004 11:10:59 AM] Java Programming: Section 7.3 can, of course, be used for both JTextFields and JTextAreas. void setText(String newText); // substitute newText // for current contents String getText(); // return a copy of the current contents String getSelectedText(); // return the selected text void select(int start, int end); // change the selection; // characters in the range start <= pos < end are // selected; characters are numbered starting from zero void selectAll(); // select the entire text int getSelectionStart(); // get starting point of selection int getSelectionEnd(); // get end point of selection void setEditable(boolean canBeEdited); // specify whether or not the text in the component // can be edited by the user The requestFocus() method, inherited from JComponent, is also useful for text components. The constructor for a JTextField takes the form JTextField(int columns); where columns specifies the number of characters that should be visible in the text field. This is used to determine the preferred width of the text field. (Because characters can be of different sizes, the number of characters visible in the text field might not be exactly equal to columns.) You don't have to specify the number of columns; for example, you might use the text field in a context where it will expand to the maximum size available. In that case, you can use the constructor JTextField(), with no parameters. You can also use the following constructors, which specify the initial contents of the text field: JTextField(String contents); JTextField(String contents, int columns); JTextField has a subclass, JPasswordField, which is identical except that it does not reveal the text that it contains. The characters in a JPasswordField are all displayed as asterisks (or some other fixed character). A password field is, obviously, designed to let the user enter a password without showing that password on the screen. The constructors for a JTextArea are JTextArea(); JTextArea(int lines, int columns); JTextArea(String contents); JTextArea(String contents, int lines, int columns); The parameter lines specifies how many lines of text should be visible in the text area. This determines the preferred height of the text area. (The text area can actually contain any number of lines; the text area can be scrolled to reveal lines that are not currently visible.) It is common to use a JTextArea as the Center component of a BorderLayout. In that case, it isn't useful to specify the number of lines and columns, since the TextArea will expand to fill all the space available in the center area of the container. The JTextArea class adds a few useful procedures to those inherited from JTextComponent: void append(String text); // add the specified text at the end of the current // contents; line breaks can be inserted by using the // special character \n void insert(String text, int pos); // insert the text, starting at specified position void replaceRange(String text, int start, int end); // delete the text from position start to position end http://math.hws.edu/eck/cs124/javanotes4/c7/s3.html (10 of 11) [6/11/2004 11:10:59 AM] Java Programming: Section 7.3 // and then insert the specified text in its place void setLineWrap(boolean wrap); // If wrap is true, then a line that is too long to be // displayed in the text area will be "wrapped" onto // the next line. The default value is false. A JTextField generates an ActionEvent when the user presses return while typing in the JTextField. The JTextField class includes an addActionListener() method that can be used to register a listener with a JTextField. In the actionPerformed() method, the evt.getActionCommand() method will return a copy of the text from the JTextField. It is also common to use a JTextField by checking its contents, when needed, with the getText() method. JTextAreas do not generate action events. A JTextArea does not have scroll bars, but scroll bars can be added easily by putting the text area in a scroll pane: JTextArea inputArea = new JTextArea(); JScrollPane scroller = new JScrollPane( inputArea ); The scroll bars will appear only when needed. Remember to add the scroll pane, not the text area, to a container. Other Components This section has introduced many, but not all, Swing components. We will look at menus and menu bars in Section 5. Some Swing components will not be covered at all. These include: ● JList -- displays a list of items to the user, and allows the user to select one or several items from the list. ● JTable -- displays a two-dimensional table of items, and possibly allows the user to edit them. ● JTree -- displays hierarchical data in a tree-like structure. ● JToolBar -- holds a row of tools, such as icons and buttons. The user can drag the tool bar away from the window that contains it, and it becomes a separate, floating tool window. ● JSplitPane -- a container that displays two components. The user can drag the dividing line between the components to adjust their relative sizes. ● JTabbedPane -- a container that displays one of a set of panels. The user selects which panel is visible by clicking on a "tab" at the top of the pane. [ Next Section | Previous Section | Chapter Index | Main Index ] http://math.hws.edu/eck/cs124/javanotes4/c7/s3.html (11 of 11) [6/11/2004 11:10:59 AM] Java Programming: Section 7.4 Section 7.4 Programming with Components THE TWO PREVIOUS SECTIONS described some raw materials that are available in the form of layout managers and standard GUI components. This section presents some programming examples that make use of those raw materials. An Example with Text Input Boxes As a first example, let's look at a simple calculator applet. This example demonstrates typical uses of JTextFields, JButtons, and JLabels, and it uses several layout managers. In the applet, you can enter two real numbers in the text-input boxes and click on one of the buttons labeled "+", "-", "*", and "/". The corresponding operation is performed on the numbers, and the result is displayed in a JLabel at the bottom of the applet. If one of the input boxes contains an illegal entry -- a word instead of a number, for example -- an error message is displayed in the JLabel. (Applet "SimpleCalculator" would be displayed here if Java were available.) When designing an applet such as this one, you should start by asking yourself questions like: How will the user interact with the applet? What components will I need in order to support that interaction? What events can be generated by user actions, and what will the applet do in response? What data will I have to keep in instance variables to keep track of the state of the applet? What information do I want to display to the user? Once you have answered these questions, you can decide how to lay out the components. You might want to draw the layout on paper. At that point, you are ready to begin writing the program. In the simple calculator applet, the user types in two numbers and clicks a button. The computer responds by doing a computation with the user's numbers and displaying the result. The program uses two JTextField components to get the user's input. The JTextFields do a lot of work on their own. They respond to mouse, focus, and keyboard events. They show blinking cursors when they are active. They collect and display the characters that the user types. The program only has to do three things with each JTextField: Create it, add it to the applet, and get the text that the user has input by calling its getText() method. The first two things are done in the applet's init() method. The third -- getting the user's input from the input boxes -- is done in an actionPerformed() method, which responds when the user clicks on one of the buttons. When a component is created in one method and used in another, as the input boxes are in this case, we need an instance variable to refer to it. In this case, I use two instance variables, xInput and yInput, of type JTextField to refer to the input boxes. The JLabel that is used to display the result is treated similarly: A JLabel is created and added to the applet in the init() method. When an answer is computed in the actionPerformed() method, the JLabel's setText() method is used to display the answer in the label. I use an instance variable named answer, of type JLabel, to refer to the label. The applet also has four JButtons and two more JLabels. (The two extra labels display the strings "x =" and "y =".) I use local variables rather than instance variables for these components because I don't need to refer to them outside the init() method. The applet as a whole uses a GridLayout with four rows and one column. The bottom row is occupied by the JLabel, answer. The other three rows each contain several components. Each of the first three rows is filled by a JPanel, which has its own layout manager and contains several components. The row that contains the four buttons is a JPanel which uses a GridLayout with one row and four columns. The JPanels that contain the input boxes use BorderLayouts. The input box occupies the Center position of the BorderLayout, with a JLabel on the West. (This example shows that http://math.hws.edu/eck/cs124/javanotes4/c7/s4.html (1 of 11) [6/11/2004 11:11:01 AM] Java Programming: Section 7.4 BorderLayouts are more versatile than it might appear at first.) All the work of setting up the applet is done in its init() method: public void init() { /* Since I will be using the content pane several times, declare a variable to represent it. Note that the return type of getContentPane() is Container. */ Container content = getContentPane(); /* Assign a background color to the applet and its content panel. This color will show through between components and around the edges of the applet. */ setBackground(Color.gray); content.setBackground(Color.gray); /* Create the input boxes, and make sure that their background color is white. (They are likely to be white by default.) */ xInput = new JTextField("0"); xInput.setBackground(Color.white); yInput = new JTextField("0"); yInput.setBackground(Color.white); /* Create panels to hold the input boxes and labels "x =" and "y = ". By using a BorderLayout with the JTextField in the Center position, the JTextField will take up all the space left after the label is given its preferred size. */ JPanel xPanel = new JPanel(); xPanel.setLayout(new BorderLayout()); xPanel.add( new Label(" x = "), BorderLayout.WEST ); xPanel.add(xInput, BorderLayout.CENTER); JPanel yPanel = new JPanel(); yPanel.setLayout(new BorderLayout()); yPanel.add( new Label(" y = "), BorderLayout.WEST ); yPanel.add(yInput, BorderLayout.CENTER); /* Create a panel to hold the four buttons for the four operations. A GridLayout is used so that the buttons will all have the same size and will fill the panel. The applet serves as ActionListener for the buttons. */ JPanel buttonPanel = new JPanel(); buttonPanel.setLayout(new GridLayout(1,4)); JButton plus = new JButton("+"); plus.addActionListener(this); buttonPanel.add(plus); JButton minus = new JButton("-"); minus.addActionListener(this); buttonPanel.add(minus); http://math.hws.edu/eck/cs124/javanotes4/c7/s4.html (2 of 11) [6/11/2004 11:11:01 AM] Java Programming: Section 7.4 JButton times = new JButton("*"); times.addActionListener(this); buttonPanel.add(times); JButton divide = new JButton("/"); divide.addActionListener(this); buttonPanel.add(divide); /* Create the label for displaying the answer in red on a white background. The label is set to be "opaque" to make sure that the white background is painted. */ answer = new JLabel("x + y = 0", JLabel.CENTER); answer.setForeground(Color.red); answer.setBackground(Color.white); answer.setOpaque(true); /* Set up the layout for the applet, using a GridLayout, and add all the components that have been created. */ content.setLayout(new GridLayout(4,1,2,2)); content.add(xPanel); content.add(yPanel); content.add(buttonPanel); content.add(answer); /* Try to give the input focus to xInput, which is the natural place for the user to start. */ xInput.requestFocus(); } // end init() The action of the applet takes place in the actionPerformed() method. The algorithm for this method is simple: get the number from the input box xInput get the number from the input box yInput get the action command (the name of the button) if the command is "+" add the numbers and display the result in the answer label else if the command is "-" subtract the numbers and display the result in the label else if the command is "*" multiply the numbers and display the result in the label else if the command is "/" divide the numbers and display the result in the label There is only one problem with this. When we call xInput.getText() and yInput.getText() to get the contents of the input boxes, the results are Strings, not numbers. We need a method to convert a string such as "42.17" into the number that it represents. The standard class Double contains a static method, Double.parseDouble(String) for doing just that. So we can get the first number entered by the user with the commands: f http://math.hws.edu/eck/cs124/javanotes4/c7/s4.html (3 of 11) [6/11/2004 11:11:01 AM] Java Programming: Section 7.4 String xStr = xInput.getText(); x = Double.parseDouble(xStr); where x is a variable of type double. Similarly, if we wanted to get an integer value from the string, xStr, we could use a static method in the standard Integer class: x = Integer.parseInt(xStr). This makes it easy to get numerical values from a JTextField, but one problem remains: We can't be sure that the user has entered a string that represents a legal real number. We could ignore this problem and assume that a user who doesn't enter a valid input shouldn't expect to get an answer. However, a more friendly program would notice the error and display an error message to the user. This requires using a "try...catch" statement, which is not covered until Chapter 9 of this book. My program does in fact use a try...catch statement to handle errors, so you can get a preview of how it works. Here is the actionPerformed() method that responds when the user clicks on one of the buttons in the applet: public void actionPerformed(ActionEvent evt) { // When the user clicks a button, get the numbers // from the input boxes and perform the operation // indicated by the button. Put the result in // the answer label. If an error occurs, an // error message is put in the label. double x, y; // The numbers from the input boxes. /* Get a number from the xInput JTextField. Use xInput.getText() to get its contents as a String. Convert this String to a double. The try...catch statement will check for errors in the String. If the string is not a legal number, the error message "Illegal data for x." is put into the answer and the actionPerformed() method ends. */ try { String xStr = xInput.getText(); x = Double.parseDouble(xStr); } catch (NumberFormatException e) { // The string xStr is not a legal number. answer.setText("Illegal data for x."); return; } /* Get a number from yInput in the same way. */ try { String yStr = yInput.getText(); y = Double.parseDouble(yStr); } catch (NumberFormatException e) { answer.setText("Illegal data for y."); return; } /* Perform the operation based on the action command from the button. Note that division by zero produces an error message. */ http://math.hws.edu/eck/cs124/javanotes4/c7/s4.html (4 of 11) [6/11/2004 11:11:01 AM] Java Programming: Section 7.4 String op = evt.getActionCommand(); if (op.equals("+")) answer.setText( "x + y = " + (x+y) ); else if (op.equals("-")) answer.setText( "x - y = " + (x-y) ); else if (op.equals("*")) answer.setText( "x * y = " + (x*y) ); else if (op.equals("/")) { if (y == 0) answer.setText("Can't divide by zero!"); else answer.setText( "x / y = " + (x/y) ); } } // end actionPerformed() The complete source code for the applet can be found in the file SimpleCalculator.java. (It contains very little in addition to the two methods shown above.) An Example with Sliders As a second example, let's look more briefly at another applet. In this example, the user manipulates three JSliders to set the red, green, and blue levels of a color. The value of each color level is displayed in a JLabel, and the color itself is displayed in a large rectangle: (Applet "RGBColorChooser" would be displayed here if Java were available.) The layout manager for the applet is a GridLayout with one row and three columns. The first column contains a JPanel, which in turn contains the JSliders. This panel uses another GridLayout, with three rows and one column. The second column, which contains the JLabels, is similar. The third column contains the colored rectangle. The component in this column is a JPanel which contains no components. The displayed color is the background color of the JPanel. When the user changes the color, the background color of the panel is changed and the panel is repainted to show the new color. This is one of the few cases where an object of type JPanel is used without either making a subclass or adding components to it. When the user changes the value on a JSlider, an event of type ChangeEvent is generated. In order to respond to such events, the applet implements the ChangeListener interface, which specifies the method "public void stateChanged(ChangeEvent evt)". The applet registers itself to listen for change events from each slider. The applet has instance variables to refer to the sliders, the labels, and the color patch. Note that since the ChangeEvent and ChangeListener classes are defined in the package javax.swing.event, the command "import javax.swing.event.*;" is added to the beginning of the program. Let's look at the code from the init() method for setting up one of the JSliders, redSlider: redSlider = new JSlider(0, 255, 0); redSlider.addChangeListener(this); The first line constructs a horizontal slider whose value can range from 0 to 255. These are the possible values of the red level in a color. The initial value of the slider, which is specified by the third parameter to the constructor, is 0. The second line registers the applet ("this") to listen for change events from the slider. The other two sliders are initialized in a similar way. In the stateChanged() method, the applet must respond to the fact that the user has changed the value http://math.hws.edu/eck/cs124/javanotes4/c7/s4.html (5 of 11) [6/11/2004 11:11:01 AM] Java Programming: Section 7.4 of one of the sliders. The response is to read the values of all the sliders, set the labels to display those values, and change the color displayed on the color patch. (This is slightly lazy programming, since only one of the labels actually needs to be changed. However, there is no rule against setting the text of a label to the same text that it is already displaying.) public void stateChanged(ChangeEvent evt) { // This is called when the user has changed the value on // one of the sliders. All the sliders are checked, // the labels are set to display the correct values, and // the color patch is set to correspond to the new color. int r = redSlider.getValue(); int g = greenSlider.getValue(); int b = blueSlider.getValue(); redLabel.setText(" R = " + r); greenLabel.setText(" G = " + g); blueLabel.setText(" B = " + b); colorPatch.setBackground(new Color(r,g,b)); } // end stateChanged() The complete source code can be found in the file RGBColorChooser.java. Custom Component Examples Java's standard component classes are often all you need to construct a user interface. Sometimes, however, you need a component that Java doesn't provide. In that case, you can write your own component class, building on one of the components that Java does provide. We've already done this, actually, every time we've written a subclass of the JPanel class to use as a drawing surface. A JPanel is a blank slate. By defining a subclass, you can make it show any picture you like, and you can program it to respond in any way to mouse and keyboard events. Sometimes, if you are lucky, you don't need such freedom, and you can build on one of Java's more sophisticated component classes. For example, suppose I have a need for a "stopwatch" component. When the user clicks on the stopwatch, I want it to start timing. When the user clicks again, I want it to display the elapsed time since the first click. The textual display can be done with a JLabel, but we want a JLabel that can respond to mouse clicks. We can get this behavior by defining a StopWatch component as a subclass of the JLabel class. A StopWatch object will listen for mouse clicks on itself. The first time the user clicks, it will change its display to "Timing..." and remember the time when the click occurred. When the user clicks again, it will check the time again, and it will compute and display the elapsed time. (Of course, I don't necessarily have to define a subclass. I could use a regular label in my applet, set the applet to listen for mouse events on the label, and let the applet do the work of keeping track of the time and changing the text displayed on the label. However, by writing a new class, I have something that is reusable in other projects. I also have all the code involved in the stopwatch function collected together neatly in one place. For more complicated components, both of these considerations are very important.) The StopWatch class is not very hard to write. I need an instance variable to record the time when the user started the stopwatch. Times in Java are measured in milliseconds and are stored in variables of type long (to allow for very large values). In the mousePressed() method, I need to know whether the timer is being started or stopped, so I need another instance variable to keep track of this aspect of the component's state. There is one more item of interest: How do I know what time the mouse was clicked? The method System.currentTimeMillis() returns the current time. But there can be some delay between the time the user clicks the mouse and the time when the mousePressed() routine is called. I don't want to know the current time. I want to know the exact time when the mouse was pressed. When I wrote the StopWatch class, this need sent me on a search in the Java documentation. I found that if evt is an object of type MouseEvent(), then the function evt.getWhen() returns the time when the event http://math.hws.edu/eck/cs124/javanotes4/c7/s4.html (6 of 11) [6/11/2004 11:11:01 AM] Java Programming: Section 7.4 occurred. I call this function in the mousePressed() routine. The complete StopWatch class is rather short: import java.awt.event.*; import javax.swing.*; public class StopWatch extends JLabel implements MouseListener { private long startTime; // Start time of timer. // (Time is measured in milliseconds.) private boolean running; // True when the timer is running. public StopWatch() { // Constructor. super(" Click to start timer. ", JLabel.CENTER); addMouseListener(this); } public void mousePressed(MouseEvent evt) { // React when user presses the mouse by // starting or stopping the timer. if (running == false) { // Record the time and start the timer. running = true; startTime = evt.getWhen(); // Time when mouse was clicked. setText("Timing...."); } else { // Stop the timer. Compute the elapsed time since the // timer was started and display it. running = false; long endTime = evt.getWhen(); double seconds = (endTime - startTime) / 1000.0; setText("Time: " + seconds + " sec."); } } public void mouseReleased(MouseEvent evt) { } public void mouseClicked(MouseEvent evt) { } public void mouseEntered(MouseEvent evt) { } public void mouseExited(MouseEvent evt) { } } // end StopWatch Don't forget that since StopWatch is a subclass of JLabel, you can do anything with a StopWatch that you can do with a JLabel. You can add it to a container. You can set its font, foreground color, and background color. You can set the text that it displays (although this would interfere with its stopwatch function). You can even add a Border if you want. Let's look at one more example of defining a custom component. Suppose that -- for no good reason whatsoever -- I want a component that acts like a JLabel except that it displays its text in mirror-reversed form. Since no standard component does anything like this, the MirrorLabel class is defined as a subclass of JPanel. It has a constructor that specifies the text to be displayed and a setText() method http://math.hws.edu/eck/cs124/javanotes4/c7/s4.html (7 of 11) [6/11/2004 11:11:01 AM] Java Programming: Section 7.4 that changes the displayed text. The paintComponent() method draws the text mirror-reversed, in the center of the component. This uses techniques discussed in Section 1. Information from a FontMetrics object is used to center the text in the component. The reversal is achieved by using an off-screen image. The text is drawn to the off-screen image, in the usual way. Then the image is copied to the screen with the following command, where OSC is the variable that refers to the off-screen image: g.drawImage(OSC, widthOfOSC, 0, 0, heightOfOSC, 0, 0, widthOfOSC, heightOfOSC, this); This is the version of drawImage() that specifies corners of destination and source rectangles. The corner (0,0) in OSC is matched to the corner (widthOfOSC,0) on the screen, while (widthOfOSC,heightOfOSC) is matched to (0,heightOfOSC). This reverses the image left-to-right. Here is the complete class: import java.awt.*; import javax.swing.*; public class MirrorLabel extends JPanel { // Constructor and methods meant for use public use. public MirrorLabel(String text) { // Construct a MirrorLable to display the specified text. this.text = text; } public void setText(String text) { // Change the displayed text. Call revalidate // so that the layout of its container can be // recomputed. this.text = text; revalidate(); // Tells container that size might have changed. repaint(); } public String getText() { // Return the string that is displayed by this component. return text; } // Implementation. Not meant for public use. private String text; // The text displayed by this component. private Image OSC; // An off-screen image holding the non-reversed text. private int widthOfOSC, heightOfOSC; // Current size of the off-screen image, if one exists. public void paintComponent(Graphics g) { // The paint method makes a new OSC, if necessary. It writes // a non-reversed copy of the string to the the OSC, then // reverses the OSC as it copies it to the screen. // (Note: color or font might have changed since the // last time paintComponent() was called, so I can't just http://math.hws.edu/eck/cs124/javanotes4/c7/s4.html (8 of 11) [6/11/2004 11:11:01 AM] Java Programming: Section 7.4 // reuse the old image in the OSC.) if (OSC == null || getSize().width != widthOfOSC || getSize().height != heightOfOSC) { OSC = createImage(getSize().width, getSize().height); widthOfOSC = getSize().width; heightOfOSC = getSize().height; } Graphics OSG = OSC.getGraphics(); OSG.setColor(getBackground()); OSG.fillRect(0, 0, widthOfOSC, heightOfOSC); OSG.setColor(getForeground()); OSG.setFont(getFont()); FontMetrics fm = OSG.getFontMetrics(getFont()); int x = (widthOfOSC - fm.stringWidth(text)) / 2; int y = (heightOfOSC + fm.getAscent() - fm.getDescent()) / 2; OSG.drawString(text, x, y); OSG.dispose(); g.drawImage(OSC, widthOfOSC, 0, 0, heightOfOSC, 0, 0, widthOfOSC, heightOfOSC, null); } // end paintComponent() public Dimension getPreferredSize() { // Compute a preferred size that will hold the string plus // a border of 5 pixels. FontMetrics fm = getFontMetrics(getFont()); return new Dimension(fm.stringWidth(text) + 10, fm.getAscent() + fm.getDescent() + 10); } } // end class MirrorLabel This class defines the method "public Dimension getPreferredSize()". This method is called by a layout manager when it wants to know how big the component would like to be. Standard components come with a way of computing a preferred size. For a custom component based on a JPanel, it's a good idea to provide a custom preferred size. As I mentioned in Section 1, every component has a method setPrefferedSize() that can be used to set the preferred size of the component. For our MirrorLabel component, however, the preferred size depends the font and the text of the component, and these can change from time to time. We need a way to compute a preferred size on demand, based on the current font and text. That's what we do by defining a getPreferredSize() method. The system calls this method when it wants to know the preferred size of the component. In response, we can compute the preferred size based on the current font and text. The StopWatch and MirrorLabel class define components. Components don't stand on their own. You have to add them to an applet or other container. Here is an applet that demonstrates a MirrorLabel and a StopWatch component: (Applet "ComponentTest" would be displayed here if Java were available.) The source code for this applet is in the file ComponentTest.java. The applet uses a FlowLayout, so the components are not arranged very neatly. The applet also contains a button, which is there to illustrate another fine point of programming with components. If you click the button labeled "Change Text in this Applet", the text in all the components will be changed. You can also click on the "Timing..." label to start and stop the StopWatch. When you do any of these things, you will notice that the components will be rearranged to take the new sizes into account. This is known as "validating" the container. This is done http://math.hws.edu/eck/cs124/javanotes4/c7/s4.html (9 of 11) [6/11/2004 11:11:01 AM] Java Programming: Section 7.4 automatically when a standard component changes in some way that requires a change in preferred size or location. This may or may not be the behavior that you want. (Validation doesn't always cause as much disruption as it does in this applet. For example, in a GridLayout, where all the components are displayed at the same size, it will have no effect at all. I've chosen a FlowLayout for this example to make the effect more obvious.) A custom component such as MirrorLabel can call the revalidate() method to indicate that the container that contains the component should be validated. In the MirrorLabel class, revalidate() is called in the setText() method. A Null Layout Example As a final example, we'll look at an applet that does not use a layout manager. If you set the layout manager of a container to be null, then you assume complete responsibility for positioning and sizing the components in that container. For an applet, you can remove the layout manager with the command: getContentPane().setLayout(null); If comp is any component, then the statement comp.setBounds(x, y, width, height); puts the top left corner of the component at the point (x,y), measured in the coordinated system of the container that contains the component, and it sets the width and height of the component to the specified values. You should only set the bounds of a component if the container that contains it has a null layout manager. In a container that has a non-null layout manager, the layout manager is responsible for setting the bounds, and you should not interfere with its job. Assuming that you have set the layout manager to null, you can call the setBounds() method any time you like. (You can even make a component that moves or changes size while the user is watching.) If you are writing an applet that has a known, fixed size, then you can set the bounds of each component in the applet's init() method. That's what done in the following applet, which contains four components: two buttons, a label, and a panel that displays a checkerboard pattern. This applet doesn't do anything useful. The buttons just change the text in the label. (Applet "NullLayoutDemo" would be displayed here if Java were available.) In the init() method of this applet, the components are created and added to the applet. Then the setBounds() method of each component is called to set the size and position of the component: public void init() { getContentPane().setLayout(null); // I will do the layout myself! getContentPane().setBackground(new Color(0,150,0)); // Set a dark green background. /* Create the components and add them to the content pane. If you don't add them to the a container, they won't appear, even if you set their bounds! */ board = new Checkerboard(); // (Checkerboard is defined later in this class.) getContentPane().add(board); newGameButton = new JButton("New Game"); newGameButton.addActionListener(this); http://math.hws.edu/eck/cs124/javanotes4/c7/s4.html (10 of 11) [6/11/2004 11:11:01 AM] Java Programming: Section 7.4 getContentPane().add(newGameButton); resignButton = new JButton("Resign"); resignButton.addActionListener(this); getContentPane().add(resignButton); message = new JLabel("Click \"New Game\" to begin a game.", JLabel.CENTER); message.setForeground( new Color(100,255,100) ); message.setFont(new Font("Serif", Font.BOLD, 14)); getContentPane().add(message); /* Set the position and size of each component by calling its setBounds() method. */ board.setBounds(20,20,164,164); newGameButton.setBounds(210, 60, 120, 30); resignButton.setBounds(210, 120, 120, 30); message.setBounds(0, 200, 330, 30); /* Add a border to the content pane. Since the return type of getContentPane() is Container, not JComponent, getContentPane() must be type-cast to a JComponent in order to call the setBorder() method. Although I think the content pane is always, in fact, a JPanel, to be safe I test that the return value really is a JComponent. */ if (getContentPane() instanceof JComponent) { ((JComponent)getContentPane()).setBorder( BorderFactory.createEtchedBorder()); } } // end init(); It's reasonably easy, in this case, to get an attractive layout. It's much more difficult to do your own layout if you want to allow for changes of size. In that case, you have to respond to changes in the container's size by recomputing the sizes and positions of all the components that it contains. If you want to respond to changes in a container's size, you can register an appropriate listener with the container. Any component generates an event of type ComponentEvent when its size changes (and also when it is moved, hidden, or shown). You can register a ComponentListener with the container and respond to size change events by recomputing the sizes and positions of all the components in the container. Consult a Java reference for more information about ComponentEvents. However, my real advice is that if you want to allow for changes in the container's size, try to find a layout manager to do the work for you. [ Next Section | Previous Section | Chapter Index | Main Index ] http://math.hws.edu/eck/cs124/javanotes4/c7/s4.html (11 of 11) [6/11/2004 11:11:02 AM] Java Programming: Section 7.5 Section 7.5 Menus and Menubars ANY USER OF A GRAPHICAL USER INTERFACE is accustomed to selecting commands from menus, which can be found in a menu bar at the top of a window (or sometimes at the top of a screen). In Java, menu bars, menus, and the items in menus are JComponents, just like all the other Swing components. Java makes it easy to add a menu bar to a JApplet or, as we will see in Section 7, to a JFrame. Here is a sample applet that uses menus: (Applet "ShapeDrawWithMenus" would be displayed here if Java were available.) This is a much improved version of the ShapeDraw applet from Section 5.4. You can add shapes to the large white drawing area and drag them around. To add a shape, select one of the commands in the "Add" menu. The other menus allow you to control the properties of the shapes and set the background color of the drawing area. This applet illustrates many ideas related to menus. There is a menu bar. A menu bar serves as a container for menus. In this case, there are three menus in the menu bar. The menus have titles: "Add", "Color", and "Options". When you click on one of these titles, the menu items in the menu appear. Each menu has an associated mnemonic, which is a character that is underlined in the name. Instead of clicking on the menu, you can select it by pressing the mnemonic key while holding down the ALT key. (This assumes that the applet has the keyboard focus.) Once the menu has appeared, you can select an item in the menu by clicking on it, or by using the arrow keys to select the item and then pressing return. It is possible to assign mnemonics to individual items in a menu, but I haven't done that in this example. The commands in the "Add" menu and the "Clear" command do have accelerators. An accelerator is a key or combination of keys that can be pressed to invoke a menu item without ever opening the menu. The accelerator is shown in the menu, next to the name of the item. For example, the accelerator for the "Rectangle" command in the "Add" menu is "Ctrl-R". This means that you can invoke the command by holding down the Control key and pressing the R key. (Again, this assumes that the applet has the keyboard focus. The accelerators might not function at all in an applet. If so, you'll see how they work in Section 7) The commands in the "Color" menu act like a set of radio buttons. Only one item in the menu can be selected at a given time. The selected item in this menu determines the color of newly added shapes. Similarly, two of the commands in the "Options" menu act just like checkboxes. The first of these items determines whether newly added shapes will be large or small. The second determines whether newly added shapes will have a black border drawn around them. The last item in the "Options" menu is actually another menu. This is called a sub-menu. When you select this item, the sub-menu will appear. Select an item from the sub-menu to set the background color of the drawing area. This applet also demonstrates a pop-up menu. The pop-up menu appears when you click on one of the shapes in just the right way. The exact action you have to take depends on the look-and-feel and is called the pop-up trigger. The pop-up trigger is probably either clicking with the right mouse button, clicking with the middle mouse button, or clicking while holding down the Control key. The pop-up menu in this example contains commands for editing the shape on which you clicked. In the rest of this section, we'll look at how all this can be programmed. If you would like to see the complete source code of the applet, you will find it in the file ShapeDrawWithMenus.java. The source code is just over 600 lines long. The menus are created and configured in a very long init() method. http://math.hws.edu/eck/cs124/javanotes4/c7/s5.html (1 of 7) [6/11/2004 11:11:03 AM] Java Programming: Section 7.5 Menu Bars and Menus A menu bar is just an object that belongs to the class JMenuBar. Since JMenuBar is a subclass of JComponent, a menu bar could actually be used anywhere in any container. In practice though, a JMenuBar is generally added to a top-level container such as a JApplet. This can be done in the init() method of a JApplet using commands of the form JMenuBar menubar = new JMenuBar(); setMenuBar(menubar); The applet's setMenuBar() method does not add the menu bar to the applet's content pane. The menu bar appears in a separate area, above the content pane. A menu bar is a container for menus. The type of menu that can appear in a menu bar is an object belonging to the class JMenu. The constructor for a JMenu specifies a title for the menu, which appears in the menu bar. A menu is added to a menu bar using the menu bar's add() method. For example, the following commands will create a menu with title "Options" and add it to the JMenuBar, menubar: JMenu optionsMenu = new JMenu("Options"); menubar.add(optionsMenu); A mnemonic can be added to a JMenu using the menu's addMnemonic() method, which takes a parameter of type char. For example: optionsMenu.setMnemonic('O'); A mnemonic provides a keyboard shortcut for the menu. If a mnemonic has been set for a menu, then the menu can be opened by pressing the specified character key while holding down the ALT key. If the mnemonic character appears in the title of the menu, it will be underlined. The mnemonic does not have to be the first character in the title. In fact, it doesn't have to appear in the title at all. Uppercase and lowercase letters are equivalent for mnemonics. Note, by the way, that you can add a menu to a menu bar either before or after you have added menu items to the menu. You can add a menu to a menu bar even after it has appeared on the screen, but I found that I had to call menubar.validate() after adding the menu to get the menu to appear. You can remove a menu from a menubar by calling menubar.remove(menu), but again, I found it necessary to call menubar.validate() after doing this if the menubar is already on the screen. Menu Items, Sub-menus, and Separators Each of the items in a menu is an object belonging to the class JMenuItem. A JMenuItem can be created with a constructor that specifies the text that appears in the menu, and it can be added to the menu using the menu's add() method. For example: JMenuItem clear = new JMenuItem("Clear"); optionsMenu.add(clear); // where optionsMenu is of type JMenu You can specify a mnemonic for the menu item as the second parameter to the constructor: new JMenuItem("Clear",'C'). The JMenu class also has an add() method that takes a String as parameter. This version of add() creates a new menu item with the given string as its text, and it adds that menu item to the menu. Furthermore, the menu item that was created is returned as the value of the method. This means that the two commands shown above can be abbreviated to the single command: JMenuItem clear = optionsMenu.add("Clear"); A JMenuItem generates an ActionEvent when it is invoked by the user. If you want the menu item to http://math.hws.edu/eck/cs124/javanotes4/c7/s5.html (2 of 7) [6/11/2004 11:11:03 AM] Java Programming: Section 7.5 have some effect when it is invoked, you have to add an ActionListener to the menu item. For example, if listener is the object of type ActionListener that is to respond to the "Clear" command, you can say: clear.addActionListener(listener); Action events from JMenuItems can be processed in the same way as action events from JButtons: When the actionPerformed() method of the listener is called, the action command will be the text of the menu item, and the source of the event will be the menu item object itself. In many cases, the only things you want to do with a menu item are add it to a menu and add an action listener to it. It's possible to do both of these with one command. For example: optionsMenu.add("Clear").addActionListener(listener); This funny looking line does the following: optionsMenu.add("Clear") creates creates a menu item and adds it to the menu, optionsMenu, and it returns the menu item as the value of the method call. Then the addActionListener() method is applied to the return value, that is, to the menu item that was just created. The items in a menu are often separated into logical groups by horizontal lines drawn across the menu. The "Options" menu in the sample applet contains two such lines. You can add a separating line to the end of a JMenu by calling the menu's addSeparator() method. For example: optionsMenu.addSeparator(); The JMenu class is actually defined as a subclass of JMenuItem, which means that you can add one menu to another. The menu that is added appears as a sub-menu in the menu to which it is added. The title of the sub-menu appears as an item in the main menu. When the user selects this item, the sub-menu appears. For example, in the applet at the top of this page, the "Background Color" sub-menu of the "Options" menu is created with the commands: JMenu background = new JMenu("Background Color"); optionsMenu.add(background); // Add as sub-menu. background.add("Red").addActionListener(canvas); background.add("Green").addActionListener(canvas); background.add("Blue").addActionListener(canvas); background.add("Cyan").addActionListener(canvas); background.add("Magenta").addActionListener(canvas); background.add("Yellow").addActionListener(canvas); background.add("Black").addActionListener(canvas); background.add("Gray").addActionListener(canvas); background.add("White").addActionListener(canvas); Checkbox and Radio Button Menu Items The JMenuItem class has two subclasses, JCheckBoxMenuItem and JRadioButtonMenuItem, that can be used to create menu items that serve as check boxes and radio buttons. Check boxes and radio buttons were covered in Section 3 and just about everything that was said there applies here as well. A JCheckBoxMenuItem can be in one of two states, either selected or unselected. The user changes the state by selecting the menu item. Just as with a JCheckBox, you can determine the state of a JCheckBoxMenuItem by calling its isSelected() method. You can set the state by calling the item's setSelected(boolean) method. You can register an ActionListener with a JCheckBoxMenuItem, if you want to respond immediately when the user changes the state. In many cases, however, you can just check the state at the point in your program where you need to know it. For example, one of the JCheckBoxMenuItems in the sample applet determines the size of the shapes that http://math.hws.edu/eck/cs124/javanotes4/c7/s5.html (3 of 7) [6/11/2004 11:11:03 AM] Java Programming: Section 7.5 are added to the drawing area. If the "Add Large Shapes" box is checked when a shape is added, then the shape will be large; if not, the shape will be small. There is no action listener in this case because nothing happens when the user selects the item (except that it changes state). When the user adds a new shape, the program calls addLargeShapes.isSelected() to determine which size to use. (addLargeShapes is the instance variable that refers to the JCheckBoxMenuItem.) JRadioButtonMenuItems are almost always used in groups, where at most one of the radio buttons in the group can be selected at any given time. As with JRadioButtons, all the JRadioButtonMenuItems in a group are added to a ButtonGroup, which ensures that at most one of the items is selected. In the sample applet, for example, there are nine JRadioButtonMenuItems in the "Color" menu. These are represented by instance variables named red, green, blue, and so on. The code that creates the menu items and adds them both to the menu and to a button group look like this: ButtonGroup colorGroup = new ButtonGroup(); red = new JRadioButtonMenuItem("Red"); shapeColorMenu.add(red); // Add to menu. colorGroup.add(red); // Add to button group. green = new JRadioButtonMenuItem("Green"); shapeColorMenu.add(green); colorGroup.add(green); blue = new JRadioButtonMenuItem("Blue"); shapeColorMenu.add(blue); colorGroup.add(blue); . . . Initially, the "Red" item is selected. This is accomplished with the command red.setSelected(true). There are no ActionListeners for the JRadioButtonMenuItems. When a new shape is added to the drawing area, the program checks the items in the "Color" menu to see what color is selected, and that color is used as the color of the new shape. This is done in an addShape() method using code that look like: if (red.isSelected()) shape.setColor(Color.red); else if (green.isSelected()) shape.setColor(Color.green); else if (blue.isSelected()) shape.setColor(Color.blue); . . . Note that red, green, and the other variables that represent the menu items in the "Color" menu must be defined as instance variables since they are initialized in the init() method of the applet and are also used in another method. The variable that represents the ButtonGroup, on the other hand, is just a local variable in the init() method, since it is not used in any other method. http://math.hws.edu/eck/cs124/javanotes4/c7/s5.html (4 of 7) [6/11/2004 11:11:03 AM] Java Programming: Section 7.5 Accelerators A menu item in a JMenu can have an accelerator. The accelerator is a key, possibly with some modifiers such as ALT or Control, that the user can press to invoke the menu item without opening the menu. The menu item is processed in exactly the same way whether it is invoked with an accelerator, with a mnemonic, or with the mouse. An accelerator can be described by a string that specifies the key to be pressed and any modifiers that must be held down while the key is pressed. Modifiers are specified by the words shift, alt, ctrl, and meta. These must be lower case. The key is specified by an upper case letter or by the name of certain special keys including: HOME, END, DELETE, INSERT, LEFT, RIGHT, UP, DOWN, F1, F2, .... The string that describes an accelerator consists of as many modifiers as you want, followed by any one key specification. You don't use the string directly to create an accelerator. The string is passed as a parameter to the static method KeyStroke.getKeyStroke(String), which returns an object of type KeyStroke. The KeyStroke object can be used to add an accelerator to a JMenuItem. This is done by calling the JMenuItem's setAccelerator() method, which requires a parameter of type KeyStroke. For example, the first menu item in the "Add" menu of the sample applet was created with the commands: JMenuItem rect = new JMenuItem("Rectangle"); rect.setAccelerator( KeyStroke.getKeyStroke("ctrl R") ); The menu item will be invoked if the user holds down the Control key and presses the R key. Although it's unfortunate that you have to go through the KeyStroke class, it's really not all that complicated. Here are a few more examples of accelerators: menuitem.setAccelerator( KeyStroke.getKeyStroke("shift ctrl S") ); // User must hold down both SHIFT and // Control, while pressing the S key. menuitem.setAccelerator( KeyStroke.getKeyStroke("HOME") ); // User can invoke the menu item just by pressing // the HOME key. menuitem.setAccelerator( KeyStroke.getKeyStroke("alt F4") ); // Pressing the F4 key while holding down the ALT key // is equivalent to selecting the menu item. On a // Macintosh, ALT refers to the Command key. Under // Windows and Linux, this accelerator will probably // be non-functional, since the operating system will // intercept ALT-F4 and interpret it as a request to // close the window. Enabling and Disabling Menu Items Since a JMenuItem is a JComponent, you can call menuitem.setEnabled(false) to disable a menu item and menuitem.setEnabled(true) to enable a menu item. A menu item that is disabled will appear "grayed out," and the user will not be able to select it, either with the mouse or with an accelerator. It's always a good idea to give the user visual feedback about the state of a program. Disabling a menu item when it doesn't make any sense to select it is one good way of doing this. http://math.hws.edu/eck/cs124/javanotes4/c7/s5.html (5 of 7) [6/11/2004 11:11:03 AM] Java Programming: Section 7.5 Pop-up Menus A pop-up menu is an object belonging to the class JPopupMenu. It can be created with a constructor that has no parameters. Menu items, sub-menus, and separating lines can be added to a JPopupMenu in exactly the same way that they would be added to a JMenu. Menu items in a JPopupMenu generate ActionEvents, just as they would if they were in a JMenu. However, a JPopupMenu is not added to a menu bar. In fact, it is not added to any container at all. A JPopupMenu has a show() method that you can call to make it appear on the screen. Once it has appeared, the user can invoke an item in the menu in the usual way. When the user makes a selection, the menu disappears. The user can click outside the menu or hit the Escape key to dismiss the menu without making any selection from it. The show() method takes three parameters. The first parameter is a Component. Since a JPopupMenu is not contained in any component, you have to tell it which component it will be associated with when it appears on the screen. The next two parameters of show() are integers that specify where the popup should appear on the screen. The integers give the coordinates of the point where the upper left corner of the popup menu will be located. The coordinates are specified in the coordinate system of the component that is provided as the first parameter to show(). You can call show() any time you like. Usually, though, it's done in response to a mouse click. In that case, the first parameter to show() is generally the component on which the user clicked, and the next two parameters are the x and y coordinates where the mouse was clicked. (Actually, I usually use something like x-10 and y-2 so that the mouse position will be inside the popup menu, rather than exactly at the upper left corner. This tends to work better.) Suppose, for example, that you want to show the menu when the user right-clicks on a component. You need to set up a mouse listener for the component and call the popup menu's show() method in the mousePressed() method of the listener. Let's say that popup is the variable of type JPopupMenu that refers to the popup menu and that comp is the variable of type JComponent that refers to the component. Then the mousePressed() method could be written as: public void mousePressed(MouseEvent evt) { if (evt.isMetaDown()) { // This tests for a right-click int x = evt.getX(); // X-coord of mouse click int y = evt.getY(); // Y-coord of mouse click popup.show( comp, x-10, y-2 ); } } If the component is also serving as the mouse listener, you could replace popup.show(comp,x-10,y-2) with popup.show(this,x-10,y-2). Note that the only thing you do in response to the user's click is show the popup. The commands in the popup have to be handled elsewhere, such as in the actionPe