观察者模式——java实现

问题描述：

当股票的价格上涨或下降5%时，会通知持有该股票的股民，当股民听到价格上涨的消息时会买股票，当价格下降时会大哭一场。

类图：

Java源代码：

//Observer.java
package shiyan21;

public interface Observer {
    public void buy();
    public void cry();
}  
//Subject.java
package shiyan21;

import java.util.ArrayList;

public abstract class Subject {
    protected ArrayList<Observer> observers;
    public Subject(){
        observers=new ArrayList<>();
    }
    public void attach(Observer a){
        observers.add(a);
    } 
    public void detach(Observer a){
        observers.remove(a);
    }
    public abstract void up();
    public abstract void down();
}
//Gufen.java
package shiyan21;

import java.util.ArrayList;
public class Gufen extends Subject{
    public void up(){
        System.out.println("****股份上涨5%****");
        for(Observer guan:observers){
            guan.buy();
            }
    }
    public void down(){
        System.out.println("****股份下跌5%****");
        for(Observer guan:observers){
            guan.cry();
            }
    }
}
//Gumin.java
package shiyan21;

public class Gumin implements Observer{
    private String name;
    public Gumin(String a){name=a;}
    public void buy(){
        System.out.println("股民"+name+"买了一些股票");
    }
    public void cry(){
        System.out.println("股民"+name+"哭了");
    }
}  
//main.java
package shiyan21;
public class main {
    public static void main(String[] args) {
        Gufen f=new Gufen();
        Gumin g1=new Gumin("小张");
        Gumin g2=new Gumin("小王");
        f.attach(g1);
        f.attach(g2);
        f.up();
        f.down();
    }
}

运行结果：