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copied!<p>The reason why you can add objects of both type <code>A</code> and <code>B</code> is due to the fact that you parametized your XClass with the interface, so there is nothing wrong with adding two different classes that implement that interface.</p> <p>If, on the other hand, you had defined XClass as:</p> <pre><code>XClass<A> xc = new XClass<A>(); </code></pre> <p>then the expression <code>xc.add(b);</code> would give a compilation error, since all the objects added must have the same type as was declared, in this case, A.</p> <p>If you declare you <code>xc</code> as, for instance: </p> <p><code>XClass<? extends AnInterface> xc = new XClass<AnInterface>();</code></p> <p>Then it's not legal anymore to add <code>a</code> or <code>b</code>, since the only thing we know is that xc is of some unknown <strong>but fixed</strong> subtype of <code>AnInterface</code>, and there is no way to know if that unknown type is <code>A</code> or <code>B</code> or anything else.</p> <p>But let's say you're writing a method to accept a XClass type that you can iterate over the elements that were added before. Your only restriction (for the sake of the example), is that the items extend <code>AnInterface</code>, you don't care what the actual type is.</p> <p>You can declare this method like:</p> <pre><code>public static void dummyMethod(XClass<? extends AnInterface> dummy){ //do stuff here, all the elements extend (implement in this case), AnInterface, go wild. } </code></pre> <p>And now you can pass into this method anything like <code>XClass<A></code>, <code>XClass<B></code> or <code>XClass<AnInterface></code>, and it will all be valid.</p> <p>Keep in mind that you can't add to the object you pass, for the same reason above. We don't know what the unknown type is!</p> <pre><code>public static void dummyMethod(XClass<? extends AnInterface> dummy){ //do stuff here, all the elements extend (implement in this case), AnInterface, go wild. dummy.add(new A()); //you can't do this, we have no idea what type ? stand for in this case } </code></pre>

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