add test for null object

Tests/NullObject/LoggerTest.php

@@ -0,0 +1,35 @@
+<?php
+
+/*
+ * DesignPatternPHP
+ */
+
+namespace DesignPatterns\Tests\NullObject;
+
+use DesignPatterns\NullObject\NullLogger;
+use DesignPatterns\NullObject\Service;
+use DesignPatterns\NullObject\PrintLogger;
+
+/**
+ * LoggerTest tests for different loggers
+ */
+class LoggerTest extends \PHPUnit_Framework_TestCase
+{
+
+    public function testNullObject()
+    {
+        // one can use a singleton for NullObjet : I don't think it's a good idea
+        // because the purpose behind null object is to "avoid special case".
+        $service = new Service(new NullLogger());
+        $this->expectOutputString(null);  // no output
+        $service->doSomething();
+    }
+
+    public function testStandardLogger()
+    {
+        $service = new Service(new PrintLogger());
+        $this->expectOutputString('We are in DesignPatterns\NullObject\Service::doSomething');
+        $service->doSomething();
+    }
+
+}