定義：

對象之間存在一對多的關系，一的一方（被依賴對象，一般為Subject）變化，通知多的一方（依賴對象，一般為ObServer）。

UML：

代碼實現例子：

public interface ISubject {
    public void registerObServer(IObserver iObserver);
    public void removeObServer(IObserver iObserver);
    public void notifyObServers();
}


public class ConcreteSubject implements ISubject {
    List
 
<IObserver> data = new ArrayList<IObserver>();
    Message msgObj = null;

    @Override
    public void registerObServer(IObserver iObserver) {
        data.add(iObserver);
    }

    @Override
    public void removeObServer(IObserver iObserver) {
        if (data.contains(iObserver)) {
            data.remove(iObserver);
        }
    }

    @Override
    
 
public void notifyObServers() {
        for (int i = 0; i < data.size(); i++) {
            IObserver ob = data.get(i);
            ob.update(msgObj);
        }
    }

    public void createMessage(String msg) {
        SimpleDateFormat df = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
        msgObj 
 
= new Message(msg, df.format(new Date()));
    }

    public class Message {
        String message = null;
        String time = null;

        public Message(String msg, String time) {
            this.message = msg;
            this.time = time;
        }

        public String getMessage() {
            return this.message;
        }

        public String getTime() {
            return this.time;
        }
    }
}


public interface IObserver {
    void update(Object obj);//訂閱端獲取到通知後處理邏輯

}

public class ObserverA implements IObserver {

    @Override
    public void update(Object obj) {
        Message msg = (Message) obj;
        doThings(msg);

    }

    public void doThings(Message msg) {
      System.out.println("我是A，我在時間："+msg.getTime()+",接收到訂閱消息："+msg.getMessage());
    }

}

public class ObserverB implements IObserver {

    @Override
    public void update(Object obj) {
        Message msg = (Message) obj;
        doThings(msg);

    }

    public void doThings(Message msg) {
      System.out.println("我是B，我在時間："+msg.getTime()+",接收到訂閱消息："+msg.getMessage());
    }

}



public class test {
    public static void main(String[] args) {
        IObserver A=new ObserverA();
        IObserver B=new ObserverB();
        ConcreteSubject subject=new ConcreteSubject();
        subject.registerObServer(A);
        subject.registerObServer(B);
        subject.createMessage("這是第一條消息");
        subject.notifyObServers();

      
    }

}

觀察者模式