Comment fonctionne un MaskFormatter java

import java.awt.Color;
import java.awt.Dimension;
import java.awt.Font;
import java.text.ParseException;

import javax.swing.JFormattedTextField;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JPanel;
import javax.swing.JTextField;
import javax.swing.text.MaskFormatter;

public class Field
{
    private static final Font POLICE = new Font("Arial", Font.BOLD, 14);
    
    private static JTextField fromMask(String mask,int w,int h) throws ParseException
    {
        JTextField jtf = new JFormattedTextField(new MaskFormatter(mask));
        jtf.setFont(POLICE);
        jtf.setPreferredSize(new Dimension(w, h));
        jtf.setForeground(Color.BLUE);
        return jtf;
    }
    
    public static void main(String...args) throws ParseException 
    {
        JPanel top = new JPanel();
            top.add(new JLabel("Téléphone FR"));
            top.add(fromMask("UUUUU",100,30));        
            top.add(new JLabel("Téléphone FR"));
            top.add(fromMask("[U.U.U.U.U]",100,30));
        
        JFrame field = new JFrame();        
            field.setTitle("Animation");
            field.setSize(250, 200);
            field.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
            field.setLocationRelativeTo(null);
            field.setContentPane(top);
            field.setVisible(true);
    }
}