二十三种设计模式全面解析-当你的对象需要知道其他对象的状态变化时，观察者模式是你的救星！

import java.util.ArrayList;
import java.util.List;
interface Observer {
    public void update(String message);
}
interface Subject {
    public void attach(Observer observer);
    public void detach(Observer observer);
    public void notifyObservers();
}
class ConcreteSubject implements Subject {
    private List<Observer> observers = new ArrayList<Observer>();
    private String message;
    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(message);
        }
    }
    public void setMessage(String message) {
        this.message = message;
        notifyObservers();
    }
}
class ConcreteObserver implements Observer {
    private String name;
    public ConcreteObserver(String name) {
        this.name = name;
    }
    public void update(String message) {
        System.out.println(name + " received message: " + message);
    }
}
public class ObserverPatternDemo {
    public static void main(String[] args) {
        ConcreteSubject subject = new ConcreteSubject();
        ConcreteObserver observer1 = new ConcreteObserver("Observer 1");
        ConcreteObserver observer2 = new ConcreteObserver("Observer 2");
        ConcreteObserver observer3 = new ConcreteObserver("Observer 3");
        subject.attach(observer1);
        subject.attach(observer2);
        subject.setMessage("Hello World!");
        subject.detach(observer2);
        subject.attach(observer3);
        subject.setMessage("Goodbye World!");
    }
}