public void addListener(RefreshListener listener){
  if (listener == null) {
    throw new NullPointerException("Listener must not be null");
  }
  refreshListeners.add(listener);
}

public void addListener ( RefreshListener listener ) { if ( listener == null ) { throw new NullPointerException ( "Listener must not be null" ) ; } refreshListeners . add ( listener ) ; }
adds a listener , to be notified when a reference is refreshed / swapped .
