在面板上绘图闪一下就没了

Java code

import javax.swing.*;import java.awt.*;import java.awt.event.*;import java.net.*;import java.io.*;public class FiveChess extends JFrame{    private JPanel gamePanel = new JPanel();        public FiveChess(){        super();        this.setTitle("五子棋游戏");        this.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);        this.setBounds(200,20,800,700);        this.setResizable(false);        this.getContentPane().setLayout(new BorderLayout());        this.getContentPane().add(gamePanel, BorderLayout.CENTER);                        gamePanel.setBackground(new Color(112,122,12));                        this.setVisible(true);                initGamePanel(gamePanel.getGraphics());    }            public void initGamePanel(Graphics g){        g.setColor(Color.red);        g.fill3DRect(0, 0, 200, 200, true);        //gamePanel.repaint();    }        public static void main(String[] args) {        // TODO Auto-generated method stub        new FiveChess();    }}