JButton里有没有更改按钮上图片的set方法？该怎么解决

import java.awt.*;import java.awt.event.*;import javax.swing.*;public class TestFrame extends JFrame{    private JButton btn = new JButton();        public TestFrame()    {        this.setLayout(new GridBagLayout());    //貌似需要GridBagLayout布局，不然btn旁边会有东西                btn.setIcon(new ImageIcon("up.jpg"));    //初始时的图片        btn.setPressedIcon(new ImageIcon("down.jpg")); //按下去的图片        btn.setBorder(BorderFactory.createEmptyBorder());    //必须把边界设为空，否则btn四周会有东西        add(btn);    }        public static void main(String[] args)    {        JFrame frame = new TestFrame();        frame.setSize(400, 300);        frame.setLocationRelativeTo(null);        frame.setVisible(true);        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);    }}