public void addPopupListener(final PopupListener listener){
  if (!popupListeners.contains(listener)) {
    popupListeners.add(listener);
  }
}

public void addPopupListener ( final PopupListener listener ) { if ( ! popupListeners . contains ( listener ) ) { popupListeners . add ( listener ) ; } }
adds a listener which will be notified when the popup is opened .
