title: Should I use a Listener or Observer?
body: <p>I have a dropdown box in my GUI which shows the contents of an ArrayList in another class.
  New objects can be added to the ArrayList elsewhere in the GUI, so I need to know when it is updated, so I can refresh the dropdown menu. From what I can gather, my two options are to extend the ArrayList class to allow me to add my own changeListener to it, or to make the class which contains the ArrayList in question extend observable.</p>
  
  <p>Which would be a more appropriate solution?</p>