http://www.cnblogs.com/java-my-life/archive/2012/05/16/2502279.html

import java.util.ArrayList;
import java.util.List;
abstract class Subject {
/**
* 用来保存注册的观察者对象
*/
private List list = new ArrayList();
/**
* 注册观察者对象
*
* @param observer
* 观察者对象
*/
public void attach(Observer observer) {
list.add(observer);
System.out.println("Attached an observer");
}
/**
* 删除观察者对象
*
* @param observer
* 观察者对象
*/
public void detach(Observer observer) {
list.remove(observer);
}
/**
* 通知所有注册的观察者对象
*/
public void nodifyObservers(String newState) {
for (Observer observer : list) {
observer.update(newState);
}
}
}
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，通知各个观察者
this.nodifyObservers(state);
}
}
interface Observer {
/**
* 更新接口
*
* @param state
* 更新的状态
*/
public void update(String state);
}
class ConcreteObserver implements Observer {
// 观察者的状态
private String observerState;
@Override
public void update(String state) {
/**
* 更新观察者的状态，使其与目标的状态保持一致
*/
observerState = state;
System.out.println("状态为：" + observerState);
}
}
class Client {
public static void main(String[] args) {
// 创建主题对象
ConcreteSubject subject = new ConcreteSubject();
// 创建观察者对象
Observer observer = new ConcreteObserver();
// 将观察者对象登记到主题对象上
subject.attach(observer);
// 改变主题对象的状态
subject.change("new state");
}
}

JDK 观察者支持

import java.util.ArrayList;
import java.util.List;
import java.util.Observable;
import java.util.Observer;
class ConcreteSubject extends Observable {
private String state;
public void setState(String state) {
this.state = state;
}
@Override
public synchronized void addObserver(Observer arg0) {
// TODO Auto-generated method stub
super.addObserver(arg0);
}
@Override
public synchronized int countObservers() {
// TODO Auto-generated method stub
return super.countObservers();
}
@Override
protected synchronized void setChanged() {
// TODO Auto-generated method stub
super.setChanged();
System.out.println("主题状态为：" + state);
super.notifyObservers(state);
}
}
class ConcreteObserver implements Observer {
// 观察者的状态
private String observerState;
@Override
public void update(Observable arg0, Object arg1) {
String observerState = (String) arg1;
System.out.println("观察者状态为：" + observerState);
}
}
class Client {
public static void main(String[] args) {
// 创建主题对象
ConcreteSubject subject = new ConcreteSubject();
// 创建观察者对象
Observer observer1 = new ConcreteObserver();
// 将观察者对象登记到主题对象上
subject.addObserver(observer1);
Observer observer2 = new ConcreteObserver();
subject.addObserver(observer2);
// 改变主题对象的状态
subject.setState("new state");
subject.setChanged();
System.out.println(subject.countObservers());
}
}

ITPub博客

观察者模式

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