3.7. Null Object

3.7.1. Purpose

NullObject is not a GoF design pattern but a schema which appears frequently enough to be considered a pattern. It has the following benefits:

  • Client code is simplified

  • Reduces the chance of null pointer exceptions

  • Fewer conditionals require less test cases

Methods that return an object or null should instead return an object or NullObject. NullObjects simplify boilerplate code such as if (!is_null($obj)) { $obj->callSomething(); } to just $obj->callSomething(); by eliminating the conditional check in client code.

3.7.2. Examples

  • Null logger or null output to preserve a standard way of interaction between objects, even if the shouldn’t do anything

  • null handler in a Chain of Responsibilities pattern

  • null command in a Command pattern

3.7.3. UML Diagram

Alt NullObject UML Diagram

3.7.4. Code

You can also find this code on GitHub

Service.php

 1<?php
 2
 3declare(strict_types=1);
 4
 5namespace DesignPatterns\Behavioral\NullObject;
 6
 7class Service
 8{
 9    public function __construct(private Logger $logger)
10    {
11    }
12
13    /**
14     * do something ...
15     */
16    public function doSomething()
17    {
18        // notice here that you don't have to check if the logger is set with eg. is_null(), instead just use it
19        $this->logger->log('We are in ' . __METHOD__);
20    }
21}

Logger.php

 1<?php
 2
 3declare(strict_types=1);
 4
 5namespace DesignPatterns\Behavioral\NullObject;
 6
 7/**
 8 * Key feature: NullLogger must inherit from this interface like any other loggers
 9 */
10interface Logger
11{
12    public function log(string $str);
13}

PrintLogger.php

 1<?php
 2
 3declare(strict_types=1);
 4
 5namespace DesignPatterns\Behavioral\NullObject;
 6
 7class PrintLogger implements Logger
 8{
 9    public function log(string $str)
10    {
11        echo $str;
12    }
13}

NullLogger.php

 1<?php
 2
 3declare(strict_types=1);
 4
 5namespace DesignPatterns\Behavioral\NullObject;
 6
 7class NullLogger implements Logger
 8{
 9    public function log(string $str)
10    {
11        // do nothing
12    }
13}

3.7.5. Test

Tests/LoggerTest.php

 1<?php
 2
 3declare(strict_types=1);
 4
 5namespace DesignPatterns\Behavioral\NullObject\Tests;
 6
 7use DesignPatterns\Behavioral\NullObject\NullLogger;
 8use DesignPatterns\Behavioral\NullObject\PrintLogger;
 9use DesignPatterns\Behavioral\NullObject\Service;
10use PHPUnit\Framework\TestCase;
11
12class LoggerTest extends TestCase
13{
14    public function testNullObject()
15    {
16        $service = new Service(new NullLogger());
17        $this->expectOutputString('');
18        $service->doSomething();
19    }
20
21    public function testStandardLogger()
22    {
23        $service = new Service(new PrintLogger());
24        $this->expectOutputString('We are in DesignPatterns\Behavioral\NullObject\Service::doSomething');
25        $service->doSomething();
26    }
27}