观察者模式（Observer Pattern）

观察者模式是对象的行为模式，又叫做发布-订阅模式、模型-视图模式、源-监听器模式或从属者模式。观察者模式定义了一种一对多的依赖关系，让多个观察者对象同时监听某一个主题对象。这个主题对象在状态上发生变化时，会通知所有观察者对象，使它们能够自动更新自己。
类图：

示例一：
1、结构

2、java代码

Subject接口：

package observer.subject;

import observer.observer.Observer;
public interface Subject {
public void attach(Observer o);
public void detach(Observer o);
public void notifyObservers();
}

ConcreteSubject类：

package observer.subject;

import java.util.Enumeration;
import java.util.Vector;
import observer.observer.Observer;
public class ConcreteSubject implements Subject{
private Vector observersVector = new Vector();
public void attach(Observer o) {
  observersVector.addElement(o);
}
public void detach(Observer o) {
  observersVector.removeElement(o);
}
public void notifyObservers() {
  Enumeration enmu = observes();
  while(enmu.hasMoreElements()){
   ((Observer)enmu.nextElement()).update();
  }
}
public Enumeration observes(){
  return ((Vector) observersVector.clone()).elements();
}
}

Observer接口：

package observer.observer;

public interface Observer {
public void update();
}

ConcreteObserver类：

package observer.observer;

public class ConcreteObserver implements Observer{
public void update() {
System.out.println("I am noticed......");
}
}

Client类：

package observer;

import observer.observer.ConcreteObserver;
import observer.observer.Observer;
import observer.subject.ConcreteSubject;
import observer.subject.Subject;
public class Client {
public static void main(String args[]){
  Subject subject = new ConcreteSubject();
  Observer o = new ConcreteObserver();
  subject.attach(o);
  subject.notifyObservers();
     subject.detach(o);
     subject.notifyObservers();
}
}

示例二：

1、结构

2、java代码

Subject接口：

package observer1.subject;

import java.util.Enumeration;
import java.util.Vector;
import observer1.observer.Observer;
abstract public class Subject {
private Vector observersVector = new Vector();
public void attach(Observer o) {
  observersVector.addElement(o);
}
public void detach(Observer o) {
  observersVector.removeElement(o);
}
public void notifyObservers() {
  Enumeration enmu = observes();
  while(enmu.hasMoreElements()){
   ((Observer)enmu.nextElement()).update();
  }
}
public Enumeration observes(){
  return ((Vector) observersVector.clone()).elements();
}

}

ConcreteSubject类：

package observer1.subject;

public class ConcreteSubject extends Subject{
private String state;
public void change(String state){
  this.state = state;
  System.out.println("State is " + this.state);
  this.notifyObservers();
}
}

Observer接口：

package observer1.observer;

public interface Observer {
public void update();
}

ConcreteObserver类：

package observer1.observer;

public class ConcreteObserver implements Observer{
public void update() {
System.out.println("I am noticed......");
}
}

Client类：

package observer1;

import observer1.observer.ConcreteObserver;
import observer1.observer.Observer;
import observer1.subject.ConcreteSubject;
public class Client {
private static ConcreteSubject subject;
private static Observer observer;
public static void main(String args[]) {
  subject = new ConcreteSubject();
  observer = new ConcreteObserver();
  subject.attach(observer);
  subject.change("new state");
}
}

观察者模式（Observer Pattern）

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