public CheckBox addListener(Listener listener){
  if (listener != null && !listeners.contains(listener)) {
    listeners.add(listener);
  }
  return this;
}

public CheckBox addListener ( Listener listener ) { if ( listener != null && ! listeners . contains ( listener ) ) { listeners . add ( listener ) ; } return this ; }
adds a listener to this check box so that it will be notificed on certain user actions
