title: Java generics, nested collection of wildcard
body: <p>This compiles (1.6)</p>
  
  <pre><code>List&lt;? extends Object&gt; l = new ArrayList&lt;Date&gt;();
  </code></pre>
  
  <p>But this does not</p>
  
  <pre><code>List&lt;List&lt;? extends Object&gt;&gt; ll = new ArrayList&lt;List&lt;Date&gt;&gt;();
  </code></pre>
  
  <p>with the error of </p>
  
  <pre><code>Type mismatch: cannot convert from ArrayList&lt;List&lt;Date&gt;&gt; to List&lt;List&lt;? extends Object&gt;&gt;
  </code></pre>
  
  <p>Could someone explain why?
  Thanks</p>
  
  <p>EDIT: edited for being consequent</p>