请教:为什么不能弹出窗口

import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
import javax.swing.border.*;public class DemoUI extends JFrame implements ActionListener
{
private JPanel toolBar;
private JPanel actionPanel;
private JPanel contentPanel;
private JButton about;
private JButton help;

public DemoUI() 
{
this.setSize(600,400);
this.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
this.setResizable(false);
this.setTitle("Demo");

toolBar = new JPanel();
toolBar.setBackground(new Color(45,112,162));
toolBar.setPreferredSize(new Dimension(600,40));

    actionPanel = new JPanel();
    actionPanel.setBackground(Color.yellow);
    actionPanel.setPreferredSize(new Dimension(150,360));   

    contentPanel = new JPanel();
    contentPanel.setBackground(Color.blue);
    contentPanel.setPreferredSize(new Dimension(450,360));
    
    
    this.getContentPane().add(toolBar, BorderLayout.NORTH);
    this.getContentPane().add(actionPanel,BorderLayout.EAST);
    this.getContentPane().add(contentPanel,BorderLayout.CENTER);
    
    about = new JButton("关于");
    help = new JButton("帮助");
    toolBar.add(help);
    toolBar.add(about);
}
public void actionPerformed(ActionEvent e)
{
if(e.getSource() == about)
{

JOptionPane.showMessageDialog(this,"作者","关于",JOptionPane.INFORMATION_MESSAGE);
}
if(e.getSource() == help)
{
JOptionPane.showMessageDialog(this,"帮助","帮助",JOptionPane.INFORMATION_MESSAGE);
}
} public static void main(String[] args) throws HeadlessException
{
DemoUI demoUI = new demoUI();
demoUI.show(); }}请教高人,当点击"关于"或者"帮助"的时候不能弹出对话框,是为什么啊