title: Can't add value to the Java collection with wildcard generic type
body: <p>Why this code does not compile (<code>Parent</code> is an interface)?</p>
  
  <pre><code>List&lt;? extends Parent&gt; list = ...
  Parent p = factory.get();   // returns concrete implementation
  list.set(0, p);   // fails here: set(int, ? extends Parent) cannot be applied to (int, Parent)
  </code></pre>