if (listener == null) return;  if (changeSupport == null) changeSupport = new PropertyChangeSupport (this);  changeSupport.addPropertyChangeListener (listener);
super.addPropertyChangeListener(listener);
  public void addPropertyChangeListener (PropertyChangeListener listener)  {    if (listener == null)      return;    if (changeSupport == null)      changeSupport = new PropertyChangeSupport (this);    changeSupport.addPropertyChangeListener (listener);  }