观察者模式

//被观察者接口
public interface Subject {
    
    void add(Observer observer);
    
    void remove(Observer observer);
    
    void notifyObserver();
}

//具体被观察者类
public class ConcreteSubject implements Subject{
    
    private List<Observer> list = new ArrayList<>();

    @Override
    public void add(Observer observer) {
        
        list.add(observer);
    }

    @Override
    public void remove(Observer observer) {
        list.remove(observer);
        
    }

    @Override
    public void notifyObserver() {
        
        for (Observer observer : list) {
            
            observer.update();
        }
    }
}

//观察者接口
public interface Observer {
    
    void update();
}

//具体观察者类1
public class ConcreteObserver1 implements Observer {

    @Override
    public void update() {
        
        System.out.println("观察者1已更新!");
        System.out.println("--------------------");
    }
}

//具体观察者类2
public class ConcreteObserver2 implements Observer {

    @Override
    public void update() {
        
        System.out.println("观察者2已更新!");
        System.out.println("--------------------");
    }
}

//测试类
public class Test {
    
    public static void main(String[] args) {
        
        Subject subject = new ConcreteSubject();
        Observer ob1 = new ConcreteObserver1();
        Observer ob2 = new ConcreteObserver2();
        subject.add(ob1);
        subject.add(ob2);
        subject.notifyObserver();
    }
}

输出结果：
观察者1已更新!
--------------------
观察者2已更新!
--------------------