title: java protocol buffers - construct sub message with extension field
body: <p>How do we construct a sub message by adding the extension field in protocol buffers. In C++, one way of doing it is by using MutableMessage(Message, FieldDescriptor) with reflection interface. How do we do the similar thing in Java? </p>
  
  <p>I'm a newbie so don't know a lot about protocol buffers.</p>