观察者模式--java jdk中提供的支持

public interface Observer {
    /**
     * 该方法当主题对象发生改变的时候被调用. An
     * 一个应用调用主题对象的notifyObservers方法来application calls an <tt>Observable</tt> object's
     * <code>notifyObservers</code> method to have all the object's
     * observers notified of the change.
     *
     * @param   o     the observable object.
     * @param   arg   an argument passed to the <code>notifyObservers</code>
     *                 method.
     */
    void update(Observable o, Object arg);
}

public class Observable {
    private boolean changed = false;
    private Vector obs;//用于存放观察者对象的引用

    public Observable() {
        obs = new Vector();
    } 
    
    //添加一个观察者
    public synchronized void addObserver(Observer o) {
        if (o == null)
            throw new NullPointerException();
        if (!obs.contains(o)) {
            obs.addElement(o);
        }
    } 

    //移除一个观察者
    public synchronized void deleteObserver(Observer o) {
        obs.removeElement(o);
    } 
    
    //通知所有观察者
    public void notifyObservers() {
        notifyObservers(null);
    }
    
    //通知指定的观察者
    public void notifyObservers(Object arg) { 
        Object[] arrLocal;

        synchronized (this) { 
            if (!changed)//当状态没有发生改变的时候，直接返回
                return;
            arrLocal = obs.toArray();
            clearChanged();
        }
    //通知并调用观察者的update方法
        for (int i = arrLocal.length-1; i>=0; i--)
            ((Observer)arrLocal[i]).update(this, arg);
    } 
    
    //移除所有观察者
    public synchronized void deleteObservers() {
        obs.removeAllElements();
    } 

    //设置状态改变
    protected synchronized void setChanged() {
        changed = true;
    } 

    //重置状态
    protected synchronized void clearChanged() {
        changed = false;
    }

    //判断是否状态是否发生改变
    public synchronized boolean hasChanged() {
        return changed;
    }

    返回观察者的个数
    public synchronized int countObservers() {
        return obs.size();
    }
}

具体主题对象
package com.observer;

import java.util.Observable;
/**
 * 具体主题对象
 *
 */
public class ConcreteSubject extends Observable {

    private String info;

    public String getInfo() {
        return info;
    }

    public void setInfo(String info) {
        this.info = info;
    }

    public void doSomething(String str) {
        System.out.println("subject: " + str);
        setInfo(str);
        setChanged();//改变主题对象的状态，这里如果不改变它的状态，则不会对观察者对象调用update方法
//        notifyObservers();//也可以不传递参数
        notifyObservers("subject is changed");//这里传递的参数是观察者对象中update方法参数列表中的第二个参数
    }
}

package com.observer;

import java.util.Observable;
import java.util.Observer;
/**
 * 具体观察者对象
 *
 */
public class ConcreteObserver implements Observer {

    /**
     * @param subject  具体主题对象
     * @param arg notifyObservers(String str) 传递的参数
     */
    public void update(Observable o, Object arg) {
        System.out.println(arg);
        
        ConcreteSubject subject = (ConcreteSubject)o;
        System.out.println("observer: " + subject.getInfo());
    }
}　

package com.observer;

import java.util.Observer;

public class Test {
    public static void main(String[] args) {
        //观察者对象
        Observer observer = new ConcreteObserver();
        Observer observer2 = new ConcreteObserver();
        Observer observer3 = new ConcreteObserver();

        //主体对象
        ConcreteSubject observable = new ConcreteSubject();
        
        //观察者加入主题对象汇总的观察者列表
        observable.addObserver(observer);
        observable.addObserver(observer2);
        observable.addObserver(observer3);

        //主体对象发生改变
        observable.doSomething("happy");
        observable.doSomething("sad");
    }
}