title: Java Generic List<List<? extends Number>>
body: <p>How come in java we cannot do: </p>
  
  <pre><code>List&lt;List&lt;? extends Number&gt;&gt; aList = new ArrayList&lt;List&lt;Number&gt;&gt;();
  </code></pre>
  
  <p>Even though this is OK:</p>
  
  <pre><code>List&lt;? extends Number&gt; aList = new ArrayList&lt;Number&gt;();
  </code></pre>
  
  <p>Compiler error message is:</p>
  
  <blockquote>
    <p><code>Type mismatch: cannot convert from ArrayList&lt;List&lt;Number&gt;&gt; to List&lt;List&lt;? extends Number&gt;&gt;</code></p>
  </blockquote>