Java Observer

The Observer pattern is a behavioral design pattern that defines a one-to-many dependency between objects so that when one object changes state, all its dependents are notified and updated automatically.

定义观察者接口

java

        public interface Observer {
    void update(String message);
}

定义主题接口

java

        public interface Subject {
    void registerObserver(Observer observer);
    void removeObserver(Observer observer);
    void notifyObservers();
}

实现具体的主题

java

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

public class ConcreteSubject implements Subject {
    private List<Observer> observers;
    private String message;

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

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

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

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

    public void setMessage(String message) {
        this.message = message;
        notifyObservers();
    }
}

实现具体的观察者

java

        public 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: " + message);
    }
}

使用观察者模式

java

        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.setMessage("Hello, Observers!");

        subject.removeObserver(observer1);

        subject.setMessage("Hello, Observer 2!");
    }
}