23種設計模式（15）觀察者模式

當一個對象變化時，其它依賴該對象的對象都會收到通知，並且隨着變化！對象之間是一種一對多的關係

1、創建一個觀察者接口

public interface Observer {  
   public void update();  
}  

2、爲接口創建兩個實現類

public class Observer1 implements Observer {  
  
    public void update() {  
        System.out.println("observer1 has received!");  
    }  
}

public class Observer2 implements Observer {  
  
    public void update() {  
        System.out.println("observer2 has received!");  
    }  	  
}  

3、創建Subject接口及實現類

public interface Subject {  
      
    /*增加觀察者*/  
    public void add(Observer observer);  
      
    /*刪除觀察者*/  
    public void del(Observer observer);  
      
    /*通知所有的觀察者*/  
    public void notifyObservers();  
      
    /*自身的操作*/  
    public void operation();  
}  

public abstract class AbstractSubject implements Subject {  
  
    private Vector<Observer> vector = new Vector<Observer>();  

    public void add(Observer observer) {  
        vector.add(observer);  
    }  
  
    public void del(Observer observer) {  
        vector.remove(observer);  
    }  
  
    public void notifyObservers() {  
        Enumeration<Observer> enumo = vector.elements();  
        while(enumo.hasMoreElements()){  
            enumo.nextElement().update();  
        }  
    }  
}  

public class MySubject extends AbstractSubject {  
  
    public void operation() {  
        System.out.println("update self!");  
        notifyObservers();  
    }    
}  

3、測試類

public class ObserverTest {  
  
    public static void main(String[] args) {  
        Subject sub = new MySubject();  
        sub.add(new Observer1());  
        sub.add(new Observer2());  
          
        sub.operation();  
    }  
}  

輸出：	
update self!
observer1 has received!
observer2 has received!