多个对象间存在一对多关系，当一个对象发生改变时，把这种改变通知给其他多个对象，从而影响其他对象的行为。

package com.strife.pattern.observer;

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

/**
 * 观察者模式
 *
 * @author mengzhenghao
 * @date 2022/6/2
 */
public class Observer {

    public static void main(String[] args) {
        WeatherData weatherData = new WeatherData();

        AbstractObserver observer1 = new ConcreteObserver();
        AbstractObserver observer2 = new BaiduSite();

        weatherData.registerObserver(observer1);
        weatherData.registerObserver(observer2);

        weatherData.setData(21, 10, 10);

        System.out.println("--------------------------------------");

        weatherData.removeObserver(observer2);
        weatherData.setData(22.0f, 130.1f, 15);
    }
}

/** 抽象主题 */
interface Subject {

    /** 注册观察者 */
    void registerObserver(AbstractObserver observer);

    /** 删除观察者 */
    void removeObserver(AbstractObserver observer);

    /** 通知所有观察者 */
    void notifyObservers();
}

/** 具体主题：被观察者 */
class WeatherData implements Subject {

    /** 温度 */
    private float temperature;

    /** 气压 */
    private float pressure;

    /** 湿度 */
    private float humidity;

    /** 观察者集合 */
    private List<AbstractObserver> observers;

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

    public float getTemperature() {
        return temperature;
    }

    public float getPressure() {
        return pressure;
    }

    public float getHumidity() {
        return humidity;
    }

    public void setData(float temperature, float pressure, float humidity) {
        this.temperature = temperature;
        this.pressure = pressure;
        this.humidity = humidity;
        //通知观察者
        notifyObservers();
    }

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

    @Override
    public void removeObserver(AbstractObserver observer) {
        if (observers.contains(observer)) {
            observers.remove(observer);
        }
    }

    @Override
    public void notifyObservers() {
        for (AbstractObserver observer : observers) {
            observer.update(this.temperature, this.pressure, this.humidity);
        }
    }
}

/** 抽象观察者 */
abstract class AbstractObserver {

    public abstract void update(float temperature, float pressure, float humidity);
}

class ConcreteObserver extends AbstractObserver {

    /** 温度 */
    private float temperature;

    /** 气压 */
    private float pressure;

    /** 湿度 */
    private float humidity;

    public void update(float temperature, float pressure, float humidity) {
        this.temperature = temperature;
        this.pressure = pressure;
        this.humidity = humidity;
        display();
    }

    public void display() {
        System.out.println("----------当前天气情况----------");
        System.out.println("temperature: " + this.temperature);
        System.out.println("pressure: " + this.pressure);
        System.out.println("humidity: " + this.humidity);
    }
}

class BaiduSite extends AbstractObserver {

    /** 温度 */
    private float temperature;

    /** 气压 */
    private float pressure;

    /** 湿度 */
    private float humidity;

    public void update(float temperature, float pressure, float humidity) {
        this.temperature = temperature;
        this.pressure = pressure;
        this.humidity = humidity;
        display();
    }

    public void display() {
        System.out.println("----------当前天气情况----------");
        System.out.println("百度气温: " + this.temperature);
        System.out.println("百度气压: " + this.pressure);
        System.out.println("百度湿度: " + this.humidity);
    }
}