title: DefaultListModel.addElement() doesn't update JList
body: <p>I'm using Firebase with my Java project, I've added a listener to query data then add the elements to <code>DefaultListModel</code> but the <code>JList</code> only removes the old items and doesn't show the new data. <br>
  My <code>JList</code></p>
  
  <pre><code>usersData = new DefaultListModel&lt;&gt;();
  JList&lt;String&gt; users = new JList&lt;&gt;(usersData);
  users.setSelectionMode(ListSelectionModel.SINGLE_SELECTION);
  users.setSelectedIndex(0);
  users.setVisibleRowCount(5);
  JScrollPane fruitListScrollPane = new JScrollPane(users);
  add(fruitListScrollPane, BorderLayout.SOUTH);
  </code></pre>
  
  <p>and my firebase value listener</p>
  
  <pre><code>firebaseRef.addListenerForSingleValueEvent(new ValueEventListener() {
  
      @Override
      public void onDataChange(DataSnapshot snapshot) {
          // remove all old elements from DefaultListModel
          usersData.removeAllElements();
          // get the data from snapshot
          for (DataSnapshot userSnapshot : snapshot.getChildren()) {
              User user = userSnapshot.getValue(User.class);
              String userInfo = "Username: " + user.getUsername() + " &amp; Password: " + user.getPassword();
              // add the info to the model
              usersData.addElement(userInfo);
              // print the info
              System.out.println(userInfo);
          }
      }
  
      @Override
      public void onCancelled(FirebaseError error) {
      }
  });
  </code></pre>