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Change Listeners

• 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 (int i = 0; i < listeners.size(); i++)
    {
      ChangeListener listener = (ChangeListener)listeners.get(i);
      listener.stateChanged(event);
    }
  

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