JPanel仅显示第一个添加的图像

Question

我正在尝试为2D游戏制作基本布局，每个人都说将对象添加到JPanel然后将面板添加到JFrame但是仍然只有一个对象一次显示在这里是我的代码

我有一个矩形点和球类，但他们都在工作的问题是JPanel只显示添加的第一个对象

import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
import javax.swing.Timer;

public class test{
    public static void main (String[] args) throws InterruptedException {
        JFrame frame = new JFrame("PONG.s'aight");
        JPanel panel = new JPanel(new FlowLayout());
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setBounds(0,0,800,500);
        frame.setLocationRelativeTo(null);
        frame.setVisible(true);
        Ball a = new Ball (100,100,5,5);
        Ball b = new Ball(200,200,15,15);
        panel.add(a);
        panel.add(b);
        frame.getContentPane().add(panel, BorderLayout.CENTER);

        /*if(b.isCollide(paddle)) b.reverse(Direction.x);*/

        while(true){
            if(b.x==770)b.reverse(Direction.x);
            if(b.x==0)b.reverse(Direction.x);
            if(b.y==450)b.reverse(Direction.y);
            if(b.y==0)b.reverse(Direction.y);
            if(b.x==770)a.reverse(Direction.x);
            if(b.x==0)a.reverse(Direction.x);
            if(b.y==450)a.reverse(Direction.y);
            if(b.y==0)a.reverse(Direction.y);
            b.moveGen();
            a.moveGen();
            frame.repaint();
            Thread.sleep(2);
        }
    }
}