3.6. Null Object

3.6.1. Purpose

NullObject is not a GoF design pattern but a schema which appearsfrequently enough to be considered a pattern. It has the followingbenefits:

• 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 orNullObject. NullObjects simplify boilerplate code such asif (!is_null($obj)) { $obj->callSomething(); } to just$obj->callSomething(); by eliminating the conditional check inclient code.

3.6.2. Examples

• Symfony2: null logger of profiler
• Symfony2: null output in Symfony/Console
• null handler in a Chain of Responsibilities pattern
• null command in a Command pattern

3.6.3. UML Diagram

3.6.4. Code

You can also find this code on GitHub

Service.php

<?php
 
namespace DesignPatterns\Behavioral\NullObject;
 
class Service
{
    /**
     * @var LoggerInterface
     */
    private $logger;
 
    /**
     * @param LoggerInterface $logger
     */
    public function __construct(LoggerInterface $logger)
    {
        $this->logger = $logger;
    }
 
    /**
     * do something ...
     */
    public function doSomething()
    {
        // notice here that you don't have to check if the logger is set with eg. is_null(), instead just use it
        $this->logger->log('We are in '.__METHOD__);
    }
}

LoggerInterface.php

<?php
 
namespace DesignPatterns\Behavioral\NullObject;
 
/**
 * Key feature: NullLogger must inherit from this interface like any other loggers
 */
interface LoggerInterface
{
    public function log(string $str);
}

PrintLogger.php

<?php
 
namespace DesignPatterns\Behavioral\NullObject;
 
class PrintLogger implements LoggerInterface
{
    public function log(string $str)
    {
        echo $str;
    }
}

NullLogger.php

<?php
 
namespace DesignPatterns\Behavioral\NullObject;
 
class NullLogger implements LoggerInterface
{
    public function log(string $str)
    {
        // do nothing
    }
}

3.6.5. Test

Tests/LoggerTest.php

<?php
 
namespace DesignPatterns\Behavioral\NullObject\Tests;
 
use DesignPatterns\Behavioral\NullObject\NullLogger;
use DesignPatterns\Behavioral\NullObject\PrintLogger;
use DesignPatterns\Behavioral\NullObject\Service;
use PHPUnit\Framework\TestCase;
 
class LoggerTest extends TestCase
{
    public function testNullObject()
    {
        $service = new Service(new NullLogger());
        $this->expectOutputString('');
        $service->doSomething();
    }
 
    public function testStandardLogger()
    {
        $service = new Service(new PrintLogger());
        $this->expectOutputString('We are in DesignPatterns\Behavioral\NullObject\Service::doSomething');
        $service->doSomething();
    }
}