public abstract class Subject
{/** 用来保存注册的观察者对象 */private List<Observer> list = new ArrayList<Observer>();/** 注册观察者对象 */public void attch(Observer observer){list.add(observer);System.out.println("Attached an observer");}/** 删除观察者对象 */public void detach(Observer observer){list.remove(observer);System.out.println("Detached an observer");}/** 通知所有注册的观察者对象 */public void notifyObservers(String newState){for (int i = 0; i < list.size(); i++){list.get(i).update(newState);}}
}

public class ConcreteSubject extends Subject
{private String state;public String getState(){return state;}public void change(String newState){state = newState;System.out.println("主题状态为：" + state);// 状态发生改变时，通知各个观察者this.notifyObservers(state);}
}

public interface Observer
{void update(String state);
}

public class ConcreteObserver implements Observer
{/** 观察者的状态 */private String observerState;public void update(String state){/** 更新观察者的状态 */observerState = state;System.out.println("状态为：" + observerState);}
}

public static void main(String[] args)
{/** 创建主题角色 */ConcreteSubject subject = new ConcreteSubject();/** 创建观察者对象 */Observer observer = new ConcreteObserver();/** 将观察者注册到主题对象上 */subject.attch(observer);/** 改变主题对象的状态 */subject.change("new state");
}

Attached an observer
主题状态为：new state
状态为：new state

public interface Observer {/*** This method is called whenever the observed object is changed. An* 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;/** Construct an Observable with zero Observers. */public Observable() {obs = new Vector();}/*** Adds an observer to the set of observers for this object, provided * that it is not the same as some observer already in the set. * The order in which notifications will be delivered to multiple * observers is not specified. See the class comment.** @param   o   an observer to be added.* @throws NullPointerException   if the parameter o is null.*/public synchronized void addObserver(Observer o) {if (o == null)throw new NullPointerException();if (!obs.contains(o)) {obs.addElement(o);}}...
}

public class Watched extends Observable
{private String data = "";public String getData(){return data;}public void setData(String data){if (!this.data.equals(data)){this.data = data;setChanged();}notifyObservers();}
}

public class Watcher implements Observer
{String data;public Watcher(Observable o){o.addObserver(this);}public String getData(){return data;}public void update(Observable o, Object arg){this.data = ((Watched)o).getData();System.out.println("状态发生改变：" + ((Watched)o).getData());}
}

public static void main(String[] args)
{/** 创建被观察者对象 */Watched watched = new Watched();/** 创建观察者对象，并将被观察者对象登记 */Watcher watcher = new Watcher(watched);/** 给被观察者状态赋值 */watched.setData("start");watched.setData("run");watched.setData("stop");
}

状态发生改变：start
状态发生改变：run
状态发生改变：stop