title: Java Generic Collection of Generic Type with Bounded Wildcard
body: <p>Please help me with this:</p>
  
  <p>If <code>Lion IS-A Animal</code> and given <code>Cage&lt;T&gt;</code>:</p>
  
  <pre><code>Cage&lt;? extends Animal&gt; c = new Cage&lt;Lion&gt;(); // ok,
  </code></pre>
  
  <p>but</p>
  
  <pre><code>Set&lt;Cage&lt;? extends Animal&gt;&gt; cc = new HashSet&lt;Cage&lt;Lion&gt;&gt;(); // not ok
  </code></pre>
  
  <p>What I don't see here?</p>