Controlling content of text field

Hello,

I need to control content of text field in Swing.
I mean:
- user may enter only digits (0-9) and no other characters,
- there may by no more than 4 digits in text field,
- if value in text field is smaller than 50 or greater than 500,
   background of text field should become red -- if value
   it correct, background should be green.
All checkings should take place "on-line" (after each new
character(s) or after deleting of character(s)).

I created two working examples with use of a) JTextField
and b) JFormattedTextField.
I just wonder if there is some other (better) way to solve
my problem...?
Since I am new to Swing, I wish to learn correct methods
of solving basic problems. So, please tell me if my code is
good, and what are other possibilities. Thanks in advance!

Regards,
Robbo

------------------------
a)

import javax.swing.*;
import javax.swing.event.*;
import javax.swing.text.*;
import java.awt.*;
import java.awt.event.*;

public class Swing {
    public static void main(String[] args) {
        new Swing();
    }

    private static final int MIN = 50;
    private static final int MAX = 500;
    private static final int MAX_CHARS = 4;

    JTextField tf1 = null;

    public Swing() {
        JFrame f = new JFrame("Swing...");
        f.setSize(300, 120);
        Container c = f.getContentPane();
        c.setLayout(new FlowLayout());
        f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        tf1 = new JTextField(10);
        MyDocument md = new MyDocument();
        tf1.setDocument(md);
        md.addDocumentListener(new MyDocumentListener());
        c.add(tf1);
        f.setVisible(true);
    }

    class MyDocument extends PlainDocument {
        @Override
        public void insertString(int ofs, String str,
javax.swing.text.AttributeSet a)
                                            throws
javax.swing.text.BadLocationException {
            if (str == null)
                return;

            char[] t = new char[str.length()];
            int l = 0;
            for (int i = 0; i < str.length(); i++) {
                char c = str.charAt(i);
                if (Character.isDigit(c))
                   t[l++] = c;
            }

            if ((str.length() != l) || (ofs >= MAX_CHARS)) {
                Toolkit.getDefaultToolkit().beep();
                l = 0;
            }

            super.insertString(ofs, new String(t, 0, l), a);
        }
    }

    class MyDocumentListener implements DocumentListener {
        public void changedUpdate(DocumentEvent e) { }

        public void insertUpdate(DocumentEvent e) {
            funct();
        }

        public void removeUpdate(DocumentEvent e) {
            funct();
        }

        private void funct() {
            int i = 0;
            try {
                i = Integer.valueOf(tf1.getText());
            } catch (NumberFormatException e) {
                tf1.setBackground(Color.red);
                return;
            }

            if ((i < MIN) || (i > MAX)) {
                tf1.setBackground(Color.red);
            } else
                tf1.setBackground(Color.green);
        }
    }
}
------------------------
b)

import javax.swing.*;
import javax.swing.event.*;
import javax.swing.text.*;
import java.awt.*;
import java.awt.event.*;

public class Swing {
    public static void main(String[] args) {
        new Swing();
    }

    private static final int MIN = 50;
    private static final int MAX = 500;

    JFormattedTextField ftf1 = null;

    public Swing() {
        JFrame f = new JFrame("Swing...");
        f.setSize(250, 100);
        Container c = f.getContentPane();
        c.setLayout(new BorderLayout());
        f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        try {
            ftf1 = new JFormattedTextField(new MaskFormatter("####"));
        } catch (java.text.ParseException e) {
            System.exit(1);
        }

        ftf1.getDocument().addDocumentListener(new MyDocumentListener());
        c.add(ftf1);
        f.setVisible(true);
    }

    class MyDocumentListener implements DocumentListener {
        public void changedUpdate(DocumentEvent e) { }

        public void insertUpdate(DocumentEvent e) {
            funct();
        }

        public void removeUpdate(DocumentEvent e) {
            funct();
        }

        private void funct() {
            int i = 0;
            try {
                i = Integer.valueOf(ftf1.getText().trim());
            } catch (NumberFormatException e) {
                ftf1.setBackground(Color.red);
                return;
            }

            if ((i < MIN) || (i > MAX)) {
                ftf1.setBackground(Color.red);
            } else
                ftf1.setBackground(Color.green);
        }
    }
}