java更换窗口背景颜色

package events;

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

public class SimpleChangeBKG {
public static void main(String[] args) {
SimpleChangeFrame frame = new SimpleChangeFrame();
frame.setVisible(true);
}
}

class SimpleChangeFrame extends JFrame {
private SimpleChangePanel p;

public SimpleChangeFrame() {
setTitle("Change BackGround");
setSize(400, 300);
setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
Container contentPane = getContentPane();
//add panel to frame
p = new SimpleChangePanel();
contentPane.add(p, "Center");
}
}

class SimpleChangePanel extends JPanel implements ActionListener {
private JButton yellowButton;