code: public synchronized void addListener(CollectionProxyListener listener)
      {
          if (_listeners == null)
          {
              _listeners = new ArrayList();
          }
          // to avoid multi-add of same listener, do check
          if(!_listeners.contains(listener))
          {
              _listeners.add(listener);
          }
      }
docs: Adds a listener to this collection.
  
  @param listener The listener to add