java 关联已有的窗口

import java.awt.BorderLayout;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JPanel;

public class Main extends JFrame {
public Main() {
super();

setVisible(true);
setSize(200, 300);
final JPanel panel = new JPanel();
getContentPane().add(panel, BorderLayout.CENTER);

final JButton button = new JButton();
button.addActionListener(new ActionListener() {
public void actionPerformed(ActionEvent arg0) {
about a=new about(null,"关于",true);
a.setVisible(true);
}
});
button.setText("关于");
panel.add(button);
}

public static void main(String[] arg){
new Main();
}
}
下面是about。java
import java.awt.Dialog;

import javax.swing.JDialog;

public clas