1．概念

        定义一种一对多的依赖关系，当一个对象的状态发生改变时，所有依赖于它的对象都得到通知并被自动更新。

2．UML图

3．代码

interface Subject {    public void attach(Observer observer);    public void detach(Observer observer);    void notifyObservers(); } public class ConcreteSubject implements Subject {
      private Vector observersVector = new Vector();    public void attach(Observer observer) {        observersVector.addElement(observer);    }    public void detach(Observer observer){        observersVector.removeElement(observer);    }    public void notifyObservers(){        Enumeration enumeration = observersVector.clone()).elements();        while (enumeration.hasMoreElements()){         ((Observer)enumeration.nextElement()).update();     }   } } public interface Observer {    void update(); } public class ConcreteObserver1 implements Observer{    public void update() {      System.out.println("Obsver1 execute");    } } public class ConcreteObserver2 implements Observer{    public void update() {      System.out.println("Obsver2 execute");    } } public class Main {       public static void main(String[] args) {           ConcreteSubject concreteSubject = new ConcreteSubject ();           ConcreteObserver1 observer1 = new ConcreteObserver1();           ConcreteObserver2 observer2 = new ConcreteObserver2();           generator.addObserver(observer1);           generator.addObserver(observer2);           concreteSubject.notifyObservers();      }   }

4．应用场景

   当对一个对象的改变需要同时改变其它对象,依赖于它的对象都需要改变时。

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