import java.util.ArrayList;
import java.util.List;

interface Observer {
    void update();
}

class ConcreteObserver implements Observer {
    private String name;

    public ConcreteObserver(String name) {
        this.name = name;
    }

    public void update() {
        System.out.println("ConcreteObserver " + name + " received an update!");
    }
}

class Subject {
    private List<Observer> observers = new ArrayList<>();

    public void attach(Observer observer) {
        observers.add(observer);
    }

    public void detach(Observer observer) {
        observers.remove(observer);
    }

    public void notifyObservers() {
        for (Observer observer : observers) {
            observer.update();
        }
    }
}

public class ObserverMain {
    public static void main(String[] args) {
        Subject subject = new Subject();
        ConcreteObserver observer1 = new ConcreteObserver("observer1");
        ConcreteObserver observer2 = new ConcreteObserver("observer2");

        subject.attach(observer1);
        subject.attach(observer2);

        subject.notifyObservers();

        subject.detach(observer1);

        subject.notifyObservers();
    }
}