观察者模式：定义对象间的一种一对多的依赖关系。当一个对象的状态发生改变的时候，所有依赖于它的对象都得到通知并被自动更新。

简单例子：比如有两个人张三和李四订阅了气象台的天气推送消息。

 

1 public class Subject   2 {  3     private List
     
       observers = new ArrayList
      
       ();  4   5     public void attach(Observer observer)  6     {  7         observers.add(observer);  8         observer.update(this);  9     } 10      11     public void detach(Observer observer) 12     { 13         observers.remove(observer); 14     } 15      16     protected void notifyObservers() 17     { 18         for(Observer observer: observers) 19         { 20             observer.update(this); 21         } 22     } 23 } 24  25 public interface Observer  26 { 27     public void update(Subject subject); 28 } 29  30 public class ConcreteSubject extends Subject  31 { 32     private String subjectState; 33  34     public String getSubjectState()  35     { 36         return subjectState; 37     } 38  39     public void setSubjectState(String subjectState)  40     { 41         this.subjectState = subjectState; 42         this.notifyObservers(); 43     } 44 } 45  46 import static java.lang.System.out; 47 public class ConcreteObserver implements Observer 48 { 49     private String observerState; 50  51     public void update(Subject subject)  52     { 53         observerState = ((ConcreteSubject)subject).getSubjectState(); 54     } 55      56     public void info() 57     { 58         out.println("我是：" + observerName); 59         out.println("信息：" + observerState); 60         out.println(); 61     } 62      63     private String observerName; 64  65     public void setObserverName(String observerName)  66     { 67         this.observerName = observerName; 68     } 69 } 70  71 public class Client  72 { 73     public static void main(String[] args)  74     { 75         ConcreteSubject weatherSubject = new ConcreteSubject(); 76         weatherSubject.setSubjectState("天气晴朗，25°"); 77          78         ConcreteObserver zhangsan = new ConcreteObserver(); 79         zhangsan.setObserverName("张三"); 80          81         ConcreteObserver lisi = new ConcreteObserver(); 82         lisi.setObserverName("李四"); 83          84         weatherSubject.attach(zhangsan); 85         weatherSubject.attach(lisi); 86          87         zhangsan.info(); 88         lisi.info(); 89          90         weatherSubject.detach(zhangsan); 91         weatherSubject.setSubjectState("多云，20°"); 92          93         zhangsan.info(); 94         lisi.info(); 95     } 96 } 97  98 我是：张三 99 信息：天气晴朗，25°100 101 我是：李四102 信息：天气晴朗，25°103 104 我是：张三105 信息：天气晴朗，25°106 107 我是：李四108 信息：多云，20°