SE450: Change Listeners [63/72]

• Use standard ChangeListener interface type
  public interface ChangeListener
  {
     void stateChanged(ChangeEvent event);
  }
  
• Invoice collects ArrayList of change listeners
• When the invoice changes, it notifies all listeners:
  ChangeEvent event = new ChangeEvent(this);
  for (ChangeListener listener : listeners)
     listener.stateChanged(event);