设计模式之观察者模式

1 观察者模式

当一个对象被修改时，则会自动通知它的依赖对象。观察者模式属于行为型模式。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时，所有依赖于它的对象都得到通知并被自动更新

2 类图

3 代码实现

1）Observer.java

  

   

    

     

    
    

     

      public abstract class Observer {
     
    
   

    

     

    
    

     
   protected Subject subject;
     
    
   

    

     

    
    

     
   public abstract void update();
     
    
   

    

     

    
    

     

      }
     
    
  
 

2）AddObserver.java

  

   

    

     

    
    

     

      public class AddObserver extends Observer{
     
    
   

    

     

    
    

     
 
     
    
   

    

     

    
    

     

         public AddObserver(Subject subject){
     
    
   

    

     

    
    

     
 this.subject = subject;
     
    
   

    

     

    
    

     
 this.subject.attach(this);
     
    
   

    

     

    
    

     

         }
     
    
   

    

     

    
    

     
 
     
    
   

    

     

    
    

     
   @Override
     
    
   

    

     

    
    

     

         public void update() {
     
    
   

    

     

    
    

     
 System.out.println( "add value is: " 
     
    
   

    

     

    
    

     

       + (subject.getState() + subject.getState())); 
     
    
   

    

     

    
    

     

         }
     
    
   

    

     

    
    

     

      }
     
    
  
 

3）SubObserver.java

  

   

    

     

    
    

     

      public class SubObserver extends Observer{
     
    
   

    

     

    
    

     
 
     
    
   

    

     

    
    

     

         public SubObserver(Subject subject){
     
    
   

    

     

    
    

     
 this.subject = subject;
     
    
   

    

     

    
    

     
 this.subject.attach(this);
     
    
   

    

     

    
    

     

         }
     
    
   

    

     

    
    

     
 
     
    
   

    

     

    
    

     
   @Override
     
    
   

    

     

    
    

     

         public void update() {
     
    
   

    

     

    
    

     
 System.out.println( "sub value is: " 
     
    
   

    

     

    
    

     

       + (subject.getState() - 5)); 
     
    
   

    

     

    
    

     

         }
     
    
   

    

     

    
    

     

      }
     
    
  
 

4）Subject.java

  

   

    

     

    
    

     

      import java.util.ArrayList;
     
    
   

    

     

    
    

     

      import java.util.List;
     
    
   

    

     

    
    

     
 
     
    
   

    

     

    
    

     

      public class Subject {
     
    
   

    

     

    
    

     
   
     
    
   

    

     

    
    

     
   private List<Observer> observers 
     
    
   

    

     

    
    

     

       = new ArrayList<Observer>();
     
    
   

    

     

    
    

     
   private int state;
     
    
   

    

     

    
    

     
 
     
    
   

    

     

    
    

     
   public int getState() {
     
    
   

    

     

    
    

     
 return state;
     
    
   

    

     

    
    

     

         }
     
    
   

    

     

    
    

     
 
     
    
   

    

     

    
    

     
   public void setState(int state) {
     
    
   

    

     

    
    

     
 this.state = state;
     
    
   

    

     

    
    

     

       notifyAllObservers();
     
    
   

    

     

    
    

     

         }
     
    
   

    

     

    
    

     
 
     
    
   

    

     

    
    

     
   public void attach(Observer observer){
     
    
   

    

     

    
    

     

       observers.add(observer); 
     
    
   

    

     

    
    

     

         }
     
    
   

    

     

    
    

     
 
     
    
   

    

     

    
    

     
   public void notifyAllObservers(){
     
    
   

    

     

    
    

     
 for (Observer observer : observers) {
     
    
   

    

     

    
    

     

       observer.update();
     
    
   

    

     

    
    

     

       }
     
    
   

    

     

    
    

     

         }  
     
    
   

    

     

    
    

     

      }
     
    
  
 

5）Test.java

  

   

    

     

    
    

     

      public class Test {
     
    
   

    

     

    
    

     
   public static void main(String[] args) {
     
    
   

    

     

    
    

     

       Subject subject = new Subject();
     
    
   

    

     

    
    

     
 
     
    
   

    

     

    
    

     
 new AddObserver(subject);
     
    
   

    

     

    
    

     
 new SubObserver(subject);
     
    
   

    

     

    
    

     
 
     
    
   

    

     

    
    

     

       System.out.println("add value state to 5");   
     
    
   

    

     

    
    

     

       subject.setState(5);
     
    
   

    

     

    
    

     

       System.out.println("add value state to 8");  
     
    
   

    

     

    
    

     

       subject.setState(10);
     
    
   

    

     

    
    

     

         }
     
    
   

    

     

    
    

     

      }
     
    
  
 

4 运行结果

  

   

    

     

    
    

     

      add value state to 5
     
    
   

    

     

    
    

     

      add value is: 10
     
    
   

    

     

    
    

     

      sub value is: 0
     
    
   

    

     

    
    

     

      add value state to 5
     
    
   

    

     

    
    

     

      add value is: 16
     
    
   

    

     

    
    

     

      sub value is: 3
     
    
  
 

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原文链接：chenyu.blog.csdn.net/article/details/103725191