在 Java 中，什么叫观察者设计模式（observer design pattern）？

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

// Subject（主题）
interface Subject {
    void registerObserver(Observer o);
    void removeObserver(Observer o);
    void notifyObservers();
}

// Observer（观察者）
interface Observer {
    void update(String message);
}

// ConcreteSubject（具体主题）
class ConcreteSubject implements Subject {
    private List<Observer> observers;
    private String state;

    public ConcreteSubject() {
        this.observers = new ArrayList<>();
    }

    @Override
public void registerObserver(Observer o) {
        observers.add(o);
    }

    @Override
    public void removeObserver(Observer o) {
        observers.remove(o);
    }

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

    public void setState(String state) {
        this.state = state;
        notifyObservers();
    }
}

// ConcreteObserver（具体观察者）
class ConcreteObserver implements Observer {
    private String name;

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

    @Override
    public void update(String message) {
        System.out.println(name + " received: " + message);
    }
}

// 测试类
public class ObserverPatternDemo {
    public static void main(String[] args) {
        ConcreteSubject subject = new ConcreteSubject();

        Observer observer1 = new ConcreteObserver("Observer 1");
        Observer observer2 = new ConcreteObserver("Observer 2");

        subject.registerObserver(observer1);
        subject.registerObserver(observer2);

        subject.setState("State 1");
        subject.removeObserver(observer1);
        subject.setState("State 2");
    }
}