title: Incompatible types List of List and ArrayList of ArrayList
body: <p>The below line gives me error : </p>
  
  <pre><code>Incompatible Types.
  
  List&lt;List&lt;Integer&gt;&gt; output = new ArrayList&lt;ArrayList&lt;Integer&gt;&gt;();
  </code></pre>
  
  <p>What is the reason?</p>
  
  <p><strong>EDIT</strong></p>
  
  <p>I understand if I change my second ArrayList to List, it does not give me error. I want to know the reason of error though. Thanks</p>