JAVA的GUI 如何实现按钮退出程序

package GUI;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JOptionPane;
import javax.swing.JPanel;
public class TestButtonExit {
private JButton exitButton;
private JFrame frame;
private JPanel panel;
public TestButtonExit() {
    //初始化组件
    frame = new JFrame();
    panel = new JPanel();
    exitButton = new JButton("测试退出程序");
    
    //添加组件
    frame.add(panel);
    panel.add(exitButton);
    frame.setVisible(true);//设置窗口可见
    frame.setSize(400,400);//设置大小
    frame.setLocationRelativeTo(null);//相对居中显示...
    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);//设置关闭动作

    //为按钮添加监听器
    exitButton.addActionListener(new ActionListener() {
@Override
public void actionPerformed(ActionEvent e) {
// TODO Auto-generated method stub
JOptionPane.showMessageDialog(null,"程序即将退出...");
System.exit(0);
}
});
}
public static void main(String[] args) {
new TestButtonExit();
}
}