add a simple example with a null logger

NullObject/LoggerInterface.php

@@ -0,0 +1,16 @@
+<?php
+
+/*
+ * DesignPatternPHP
+ */
+
+namespace DesignPatterns\NullObject;
+
+/**
+ * LoggerInterface is a contract for logging something
+ */
+interface LoggerInterface
+{
+
+    public function log($str);
+}

NullObject/NullLogger.php

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+<?php
+
+/*
+ * DesignPatternPHP
+ */
+
+namespace DesignPatterns\NullObject;
+
+/**
+ * NullOutput is a example of NullObject pattern. It is not formely a Design
+ * Pattern by the GoF but it's a schema which appears frequently enough to
+ * be a pattern. Futhermore it is a really good pattern in my opinion :
+ * - the code in the client is simple
+ * - it reduces the chance of null pointer exception
+ * - less "if" => less test cases
+ * 
+ * The purpose : every time you have a method which returns an object or null,
+ * you should return an object or a "NullObject". With NullObject, you don't need
+ * statement like "if (!is_null($obj)) { $obj->callSomething(); }" anymore.
+ *  
+ * In this case, this a logger which does nothing. Other examples :
+ * - null logger of symfony profiler
+ * - null output in symfony/console 
+ * - null handler in a Chain of Responsiblities pattern
+ * - null command in a Command pattern
+ * 
+ * Performance concerns : ok there is a call for nothing but we spare an "if is_null"
+ * I didn't run a benchmark but I think it's equivalent.
+ */
+class NullLogger implements LoggerInterface
+{
+
+    public function log($str)
+    {
+        
+    }
+
+}

NullObject/PrintLogger.php

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+<?php
+
+/*
+ * DesignPatternPHP
+ */
+
+namespace DesignPatterns\NullObject;
+
+/**
+ * PrintLogger is a logger that prints the log entry to standard output
+ */
+class PrintLogger implements LoggerInterface
+{
+
+    public function log($str)
+    {
+        echo $str;
+    }
+
+}

NullObject/Service.php

@@ -0,0 +1,30 @@
+<?php
+
+/*
+ * DesignPatternPHP
+ */
+
+namespace DesignPatterns\NullObject;
+
+/**
+ * Service is dummy service that uses a logger
+ */
+class Service
+{
+
+    protected $logger;
+
+    // we inject the logger in ctor and it is mandatory
+    public function __construct(LoggerInterface $log)
+    {
+        $this->logger = $log;
+    }
+
+    public function doSomething()
+    {
+        // no more check "if (!is_null($this->logger))..." with the NullObject pattern
+        $this->logger->log('We are in ' . __METHOD__);
+        // something to do...
+    }
+
+}