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

 

作者：chen.yu
深信服三年半工作经验，目前就职游戏厂商，希望能和大家交流和学习,
微信公众号：编程入门到秃头 或扫描下面二维码
零基础入门进阶人工智能(链接)