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");}
}