import javax.swing.*;import java.awt.*;import java.awt.event.ActionEvent;import java.awt.event.ActionListener;public class MyJFrame implements ActionListener {    JFrame f;    JPanel p;    JButton b;    public MyJFrame() {        f = new JFrame();        p = new JPanel();        b = new JButton("变色");        b.addActionListener(this);        f.setSize(600,400);        f.add(p);        p.add(b);        f.setVisible(true);        }          public static void main(String[] args) {        new MyJFrame();        }    @Override    public void actionPerformed(ActionEvent e) {        p.setBackground(Color.blue);            }}