public interface PropertyListener extends EventListener {
   public void propertyChanged(PropertyEvent propertyEvent);
} Webjx.Com

import java.util.*;

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public class Exam {
  private int property;
  //listeners用来存放已注册的监听对象
  private Set listeners= new HashSet();
  .....
  public void addListener(PropertyListener propertyListener){
    //listeners必须保证只能被一个线程访问
    synchronized(listeners){
      listeners.add(propertyListener);
    }
  }
  public void firePropertyChange(){
    Iterator iterator;
    synchronized(listeners){
      //将listeners中的类名放到iterator
      iterator = new HashSet(listeners).iterator();
    }

    //创建事件类
    PropertyEvent propertyEvent = new PropertyEvent();
    while(iterator.hasNext()){
      PropertyListener propertyListener = (propertyListener) iterator.next();
      //调用用户的事件处理程序
      propertyListener.propertyChanged(propertyEvent);
    }
  }
}