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    <p>This is the way generics work in Java. Class String implements interface CharSequence, but you can't cast List&lt;CharSequence&gt; to List&lt;String&gt; (or vice versa). But you can cast ArrayList&lt;String&gt; to List&lt;String&gt; (and List&lt;String&gt; to ArrayList&lt;String&gt; - at least compiler won't complain, but you'll get a runtime exception if your List object isn't an instance of ArrayList).</p> <p>What if it were possible? Here's an example:</p> <pre><code>1. List&lt;String&gt; listOfStrings = new ArrayList&lt;String&gt;(); // piece of cake! 2. List&lt;Object&gt; listOfObjects = (List&lt;Object&gt;) listOfString; // String is a subclass ofObject, so why not? 3. listOfObjects.add(new Object()); // obviously correct 4. String s = listOfString.get(0); // ouch! We've just put an Object into this list, but generic type promises there are only Strings! </code></pre> <p>This is why there's a compilation error in line 2. Of course, you can fool the compiler and try this:</p> <pre><code>A&lt;I&gt; myobj = null; A genericA = new B(); myobj = (A&lt;I&gt;) genericA; </code></pre> <p>This will work because generic types are removed in the runtime. You can do this also with lists:</p> <pre><code>List&lt;String&gt; listOfStrings = new ArrayList&lt;String&gt;(); List genericList = listOfStrings; genericList.add(new Object()); String s = listOfStrings.get(0); </code></pre> <p>This code will compile (with warnings), but you will get an exception in the runtime:</p> <pre><code>Exception in thread "main" java.lang.ClassCastException: java.lang.Object cannot be cast to java.lang.String </code></pre> <p>This code:</p> <pre><code>myobj = (A&lt;I&gt;) Class.forName("B").newInstance(); </code></pre> <p>works, because method <em>newInstance()</em> returns an instance of java.lang.Object. Compiler has no idea what it will return in the runtime, so it allows casting it to everything. Because of the <a href="http://docs.oracle.com/javase/tutorial/java/generics/erasure.html">Type Erasure</a>, in the runtime A&lt;I&gt; is just A, and since B is a subclass of A, you can cast an instance of B to A.</p>
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