编写一个java程序，当按键盘的r、g、b键时，分别设置窗体背景色为红、绿、蓝三种色

import java.awt.Color;
import java.awt.Container;
import java.awt.event.KeyAdapter;
import java.awt.event.KeyEvent;
import java.awt.event.WindowAdapter;
import java.awt.event.WindowEvent;

import javax.swing.JDialog;

public class MyTest {
	static JDialog dialog = new JDialog();
	Container cp = new Container();

	public static void main(String[] args) {
		MyTest mytest = new MyTest();
		dialog.setSize(300, 200);
		dialog.setLocationRelativeTo(null);
		dialog.setTitle("测试");
		dialog.addKeyListener(mytest.new MyKeyListener());
		dialog.setDefaultCloseOperation(JDialog.DO_NOTHING_ON_CLOSE);
		dialog.addWindowListener(new WindowAdapter() {
			public void windowClosing(WindowEvent e) {
				dialog.dispose();
			}
		});
		dialog.setVisible(true);
	}

	class MyKeyListener extends KeyAdapter {
		public void keyPressed(KeyEvent e) {
			char key = e.getKeyChar();
			cp = dialog.getContentPane();
			cp.setLayout(null);
			switch (key) {
			case 'r':
				cp.setBackground(Color.red);
				break;
			case 'g':
				cp.setBackground(Color.green);
				break;
			case 'b':
				cp.setBackground(Color.blue);
				break;
			}
			System.out.println("你按了【" + key + "】键");
		}
	}

}