title: what does List<? extends SomeClass> mean?
body: <p>I saw that definition in a protobuf generated java file:</p>
  
  <pre><code>java.util.List&lt;? extends xxx.yyy.zzz.proto.BasicMessage.DestInfoOrBuilder&gt; foo();
  </code></pre>
  
  <p>But what dose <code>&lt;?</code> and <code>extends</code> mean?  I can understand <code>List&lt;SomeClass&gt;</code> I can't understand <code>List&lt;? extends SomeClass&gt;</code>..</p>