Re: Keystroke validation in JTextField

Knute Johnson <>
Mon, 01 Oct 2007 13:18:00 -0700
icogs wrote:

Simple one I hope:

How can I intercept keystrokes to a JTextField so I could, for
example, implement a digit-only text field.

You don't want to do it that way with JComponents. Use a Document to
control those sorts of things. Look at PlainDocument in the docs and
see an example below of how to implement a document that only allows
upper case letters.

// UpperCaseDocument

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

public class UpperCaseDocument extends PlainDocument {
     int length = 0;

     public UpperCaseDocument() {

     public UpperCaseDocument(int length) {
         this.length = length;

     public void insertString(int offs, String str, AttributeSet a)
      throws BadLocationException {
         if (str == null) {
         if (length > 0)
             if (str.length() + getLength() > length)
                 str = str.substring(0,length - getLength());
         char[] upper = str.toCharArray();
         for (int i = 0; i < upper.length; i++)
             upper[i] = Character.toUpperCase(upper[i]);
         super.insertString(offs, new String(upper), a);


Knute Johnson
email s/nospam/knute/

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