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copied!<p>This is a callback. <a href="http://en.wikipedia.org/wiki/Callback_%28computer_programming%29">According to Wikipedia</a>:</p> <blockquote> <p>In computer programming, a callback is a reference to a piece of executable code that is passed as an argument to other code.</p> </blockquote> <p>So let's look at the executable code:</p> <pre><code>public void getHelp(HelpCallback callback){ //do something callback.call(OK); } </code></pre> <p>Here, the <code>callback</code> argument is a reference to an object of type <code>HelpCallback</code>. Since that reference is passed in as an argument, it is a callback.</p> <h1>An example of delegation</h1> <p>Delegation is done internally by the object - independent of how the method is invoked. If, for example, the <code>callback</code> variable wasn't an argument, but rather an instance variable:</p> <pre><code>class MyDriver { public static void main(String[] argv){ // definition of HelpStrategy omitted for brevity MyObject myObj = new MyObject(new HelpStrategy() { @Override public void getHelp() { System.out.println("Getting help!"); } }); myObj.getHelp(); } } class MyObject { private final HelpStrategy helpStrategy; public MyObject(HelpStrategy helpStrategy) { this.helpStrategy = helpStrategy; } public void getHelp(){ helpStrategy.getHelp(); } } </code></pre> <p>... then it would be delegation.</p> <p>Here, <code>MyObject</code> uses the <a href="http://en.wikipedia.org/wiki/Strategy_pattern">strategy pattern</a>. There are two things to note:</p> <ol> <li>The invocation of <code>getHelp()</code> doesn't involve passing a reference to executable code. i.e. this is not a callback.</li> <li>The fact that <code>MyObject.getHelp()</code> invokes <code>helpStrategy.getHelp()</code> is not evident from the public interface of the <code>MyObject</code> object or from the <code>getHelp()</code> invocation. This kind of <a href="http://en.wikipedia.org/wiki/Information_hiding">information hiding</a> is somewhat characteristic of delegation.</li> </ol> <p>Also of note is the lack of a <code>// do something</code> section in the <code>getHelp()</code> method. When using a callback, the callback does not do anything relevant to the object's behavior: it just notifies the caller in some way, which is why a <code>// do something</code> section was necessary. However, when using delegation the actual behavior of the method depends on the delegate - so really we could end up needing both since they serve distinct purposes:</p> <pre><code> public void getHelp(HelpCallback callback){ helpStrategy.getHelp(); // perform logic / behavior; "do something" as some might say if(callback != null) { callback.call(); // invoke the callback, to notify the caller of something } } </code></pre>

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