具体代码

//Observer.java
package org.example.test019;

public interface Observer {
    public void buy();
    public void cry();
}

//Subject.java
package org.example.test019;

import java.util.ArrayList;

public abstract class Subject {
    protected ArrayList<Observer> observers = new ArrayList();

    public Subject() {
        observers = new ArrayList<>();
    }
    public void attach(Observer observer) {
        observers.add(observer);
    }

    public void detach(Observer observer) {
        observers.remove(observer);
    }

    public abstract void up();
    public abstract void down();
}

//GuFen.java
package org.example.test019;

import javax.swing.*;

public class GuFen extends Subject {

    @Override
        public void up() {
        System.out.println("股份上涨5%");
        for (Observer guan : observers) {
            guan.buy();
        }
    }

    @Override
        public void down() {
        System.out.println("股份下降5%");
        for (Observer guan : observers) {
            guan.cry();
        }
    }
}

//GuMin.java
package org.example.test019;

public class GuMin implements Observer {
    private String name;

    public GuMin(String name) {
        this.name = name;
    }
    @Override
        public void buy() {
        System.out.println("股民" + name + "买了一些股票");
    }

    @Override
        public void cry() {
        System.out.println("股民" + name + "大哭一场");
    }
}

//Main.java
package org.example.test019;

public class Main {
    public static void main(String[] args) {
        GuMin g1 = new GuMin("张三");
        GuMin g2 = new GuMin("李四");

        GuFen f = new GuFen();
        f.attach(g1);
        f.attach(g2);

        f.up();
        f.down();
    }
}