Creating a JFormattedTextField that only accepts telephone numbers

Main.java:

import javax.swing.text.*;
import java.awt.event.*;
import javax.swing.*;
import java.text.*;
import java.awt.*;
 
public class Main extends JFrame 
{
   public Main() throws Exception
   {
      addWindowListener(new WindowAdapter() {
         public void windowClosing(WindowEvent we) {
            System.exit(1);
         }
      });
 
      final JFormattedTextField formattedTf =
                   new JFormattedTextField(new MaskFormatter("(###) ###-####"));
 
      final JTextField normalTf = new JTextField(25);
      JButton button = new JButton("Get value");
      button.addActionListener(new ActionListener() {
         public void actionPerformed(ActionEvent ae) { 
            normalTf.setText(""+formattedTf.getValue());
         }
      });
 
      getContentPane().setLayout(new FlowLayout(FlowLayout.LEFT));
      getContentPane().add(formattedTf);
      getContentPane().add(button);
      getContentPane().add(normalTf);
 
      formattedTf.setPreferredSize(normalTf.getPreferredSize());
   }
  
   public static void main(String args[]) throws Exception 
   {
      Main main = new Main();
      main.setSize(300, 150);
      main.setVisible(true);
   }
}