一、概述：

1、定义：观察者模式定义了一种一对多的依赖关系，让多个观察者对象同时监听某一个主题对象。这个主题对象在状态发生变化时,会通知所有观察者对象，使它们能够自动更新自己。

2、结构图：

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

public interface Observer {
    void update();
}

实现

import java.util.ArrayList;
import java.util.List;
 
public class ConcreteSubject implements Subject {
    private List<Observer> observers = new ArrayList<>();
    private int state;
 
    public int getState() {
        return state;
    }
 
    public void setState(int state) {
        this.state = state;
        notifyObservers();
    }
 
    @Override
    public void registerObserver(Observer observer) {
        observers.add(observer);
    }
 
    @Override
    public void unregisterObserver(Observer observer) {
        observers.remove(observer);
    }
 
    @Override
    public void notifyObservers() {
        for (Observer observer : observers) {
            observer.update();
        }
    }
}

public class ConcreteObserver implements Observer {
    private int observerState;
    private ConcreteSubject subject;
 
    public ConcreteObserver(ConcreteSubject subject) {
        this.subject = subject;
        this.subject.registerObserver(this);
    }
 
    @Override
    public void update() {
        observerState = subject.getState();
        System.out.println("Observer state updated: " + observerState);
    }
}

 调用

public class Main {
    public static void main(String[] args) {
        ConcreteSubject subject = new ConcreteSubject();
 
        ConcreteObserver observer1 = new ConcreteObserver(subject);
        ConcreteObserver observer2 = new ConcreteObserver(subject);
 
        subject.setState(10);
        // 输出：
        // Observer state updated: 10
        // Observer state updated: 10
 
        subject.setState(20);
        // 输出：
        // Observer state updated: 20
        // Observer state updated: 20
 
        subject.unregisterObserver(observer2);
 
        subject.setState(30);
        // 输出：
        // Observer state updated: 30
    }
}