title: Creating new generic object with wildcard
body: <p>Please explain this generic code wildcard compile time error:</p>
  
  <pre><code>//no compile time error.
  List&lt;? extends Number&gt; x = new ArrayList&lt;&gt;(); 
  
  //compile time error.  
  List&lt;? extends Number&gt; x = new ArrayList&lt;? extends Number&gt;();
  </code></pre>