Java 为什么鼠标移动到按钮的位置时才会显示按钮

package 窗口;

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

public class TestFrame1 extends JFrame
{

private static final long serialVersionUID = 1L;
private Image image = null;
private ImageIcon icon = new ImageIcon("images/1.jpg");
JPanel jp1,jp2,jp3;
JButton jb1,jb2,jb3;
JLabel jl1,jl2;

public TestFrame1()
{
super("测试窗口");
jl1=new JLabel("");
jl2=new JLabel("");
jp1=new JPanel();
jp2=new JPanel();
jp3=new JPanel();
jb1=new JButton(" 借书 ");
jb2=new JButton(" 还书 ");
jb3=new JButton(" 租还查询 ");
jp1.add(jb1);
jp2.add(jb2);
jp3.add(jb3);

this.setLayout(new GridLayout(4, 1));
this.add(jl1);
this.add(jp1);
this.add(jp2);
this.add(jp3);

this.setSize(500,360);
this.setVisible(true);
this.setDefaultCloseOperation(DISPOSE_ON_CLOSE);
}

public static void main(String[] args)
{
new TestFrame();
}
public void paint(Graphics g)
{
super.paint(g);
image = icon.getImage();
g.drawImage(image, 0, 0, null);
}
}

绘背景后，再绘一下
jb1.repaint();
jb2.repaint();
jb3.repaint();

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