觀察者模式

public interface Observer {
    public void update(String state);
}

public class Subject {
    //保存註冊的觀察者對象
    private List<Observer> mObserver = new ArrayList<>();

    //註冊觀察者對象
    public void attach(Observer observer){
        mObserver.add(observer);
        System.out.println("註冊"+observer);
    }

    //註銷觀察者對象
    public void detach(Observer observer){
        mObserver.remove(observer);
        System.out.println("註銷"+observer);
    }

    //通知所有註冊的觀察者對象
    public void notifyEvenOne(String newStade){
         for (Observer observer:mObserver){
             observer.update(newStade);
         }
    }
}

public class ConcreteSubject extends Subject {
    private String state;

    public String getState() {
        return state;
    }

    public void change(String newState){
        this.state = newState;
        System.out.println("通知狀態發生改變");
        notifyEvenOne(state);
    }
}

public class ObserverA implements Observer {

    //觀察者狀態
    private String observerState;

    @Override
    public void update(String newState) {
        //更新觀察者狀態，讓它與目標狀態一致
        observerState = newState;
        System.out.println( "接收到消息：" + newState + "；我是A模塊，快來搶吧！！");
    }
}
public class ObserverB implements Observer {
    //觀察者狀態
    private String observerState;

    @Override
    public void update(String newState) {
        //更新觀察者狀態，讓它與目標狀態一致
        observerState = newState;
        System.out.println( "接收到消息：" + newState + "；我是B模塊，快來搶吧！！");
    }
}
public class ObserverC implements Observer {
    //觀察者狀態
    private String observerState;

    @Override
    public void update(String newState) {
        //更新觀察者狀態，讓它與目標狀態一致
        observerState = newState;
        System.out.println( "接收到消息：" + newState + "；我是C模塊，快來搶吧！！");
    }
}

    public static void main(String [] arg){

        ConcreteSubject concreteSubject = new ConcreteSubject();
        concreteSubject.attach(new ObserverA());
        concreteSubject.attach(new ObserverB());
        concreteSubject.attach(new ObserverC());
        int s = 0;
        while (true){

            try {
                Thread.sleep(3000);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            concreteSubject.change("ccc"+s);
            s++;
        }
        
    }

6.輸出結果
通知狀態發生改變
接收到消息：ccc210；我是A模塊，快來搶吧！！
接收到消息：ccc210；我是B模塊，快來搶吧！！
接收到消息：ccc210；我是C模塊，快來搶吧！！
通知狀態發生改變
接收到消息：ccc211；我是A模塊，快來搶吧！！
接收到消息：ccc211；我是B模塊，快來搶吧！！
接收到消息：ccc211；我是C模塊，快來搶吧！！