设计模式（观察者模式）

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

public class ConcreteObserver extends Observer {

    @Override
    public void doSomething() {
        System.out.println("ConcreteObserver");
    }
}

public class Target {
    private  List<Observer> list = new ArrayList<Observer>();
    
    public void addObserver(Observer obv){
        list.add(obv);
    }
    
    public void noticeObserver(){
        for(Observer observer : list){
            observer.doSomething();
        }
    }
}

public class App {

    public static void main(String[] args) {
        Target target = new Target();
        target.addObserver(new ConcreteObserver());
        target.addObserver(new ConcreteObserver());
        target.noticeObserver();
    }
}