观察者模式(Java实现)

import java.util.ArrayList;
import java.util.Iterator;

/* 抽象观察者类 */

abstract class Observer {
    
    public abstract void update();
}

/* 抽象被观察主题类 */

abstract class Subject{
    
    private ArrayList obs_list = new ArrayList();
    
    public void attach(Observer obs) {
        obs_list.add(obs);
    }
    
    public void Detach(Observer obs) {
        obs_list.remove(obs);
    }
    
    public void notifyObs() {
        for (Iterator iter = obs_list.iterator(); iter.hasNext();) {
            Observer obs = (Observer) iter.next();
            obs.update();
        }
    }
}

/* 具体观察者类 */
class ConcreteObserver extends Observer {
    
    private String obs_name;
    private ConcreteSubject cc_subj;
    
    public ConcreteObserver(String obs_name, ConcreteSubject cc_subj) {
        this.obs_name = obs_name;
        this.cc_subj = cc_subj;
    }

    public void update() {
        if (cc_subj.getStatus() == 1) {
            System.out.println("Boss come back, " + obs_name + " turn off screen!");
        } else {
            System.out.println("Boss go out, " + obs_name + " turn on screen!");
        }
    }
}

/* 具体观察主题类 */
class ConcreteSubject extends Subject {
    private int status;

    public ConcreteSubject(int status) {
        this.status = status;
    }
    
    public int getStatus() {
        return status;
    }

    public void setStatus(int status) {
        
        if (status != this.status) {
            this.status = status;
            super.notifyObs();
        }
    }
    
}

public class Observer_Test{

    public static void main(String[] args) {
        
        ConcreteSubject cc_sub = new ConcreteSubject(0);
        
        ConcreteObserver cc_obs1 = new ConcreteObserver("E1", cc_sub);
        ConcreteObserver cc_obs2 = new ConcreteObserver("E2", cc_sub);
        ConcreteObserver cc_obs3 = new ConcreteObserver("E3", cc_sub);
        cc_sub.attach(cc_obs1);
        cc_sub.attach(cc_obs2);
        cc_sub.attach(cc_obs3);
        
        cc_sub.setStatus(1); //status changed to 1
        System.out.println("---------------------------------------");
        cc_sub.setStatus(0); //status changed to 0

    }

}

result：

Boss come back, E1 turn off screen!
Boss come back, E2 turn off screen!
Boss come back, E3 turn off screen!
---------------------------------------
Boss go out, E1 turn on screen!
Boss go out, E2 turn on screen!
Boss go out, E3 turn on screen!