Validating number data entry.

Question

Hi there,
I want to validate the data entered (numeric) by the user. I have a customdocument to achieve the same. (the code is below).
What I want to know is - if I have a constructor which accepts the JTextfield as one of the arguments - how do I pass the jTextfield while invoking the class?
I tried using "this" (minus quotes) - but it gave me an error .
Any help is greatly appreciated.
Thanks
class CustomLongStyledDocument extends javax.swing.text.DefaultStyledDocument {
int maxCharacters;
JTextField textfield = null;
public CustomLongStyledDocument(int maxChars) {
maxCharacters = maxChars;
public CustomLongStyledDocument(int maxChars, JTextField txtfld) {
maxCharacters = maxChars;
textfield = txtfld;
public void insertString(int offs, String str, javax.swing.text.AttributeSet a) throws javax.swing.text.BadLocationException {
//This rejects the entire insertion if it would make
//the contents too long. Another option would be
//to truncate the inserted string so the contents
//would be exactly maxCharacters in length.
if ((getLength() + str.length()) <= maxCharacters) {
//super.insertString(offs, str, a);
     if(offs>=maxCharacters) {
return;     
long ltemp = 0;
          try     {          
               ltemp = Long.parseLong(str);
if (ltemp < 1940 || ltemp > 2003) {
textfield.setText("");
return;
          catch(NumberFormatException nfe) {            
java.awt.Toolkit.getDefaultToolkit().beep();
               return;     
          } // string is a number so insert in to textfield     
          super.insertString(offs,str,a);
else
java.awt.Toolkit.getDefaultToolkit().beep();

Answer 1

The code would look something like:
JTextfield textField = new JTextField(4);
CustomLongStyledDocument doc = new CustomLongStyledDocument(4, textField);
Other comments about you class:
1) JTextField uses a PlainDocument so normally you would extend PlainDocument
2) the insertString() method is called for each character typed. You are attempting to do a range check for a date between 1940 - 2003. This test will alway fail because the variable 'str' will only contain a single digit.