Re: Keystroke validation in JTextField

Lew <>
Mon, 01 Oct 2007 16:26:58 -0400
icogs wrote:

Simple one I hope:

Please do not multipost.

It causes answers to appear in only one of the multiple groups to which you
posted your message. You need to keep the answers together, e.g., by
cross-posting, or better, by sticking with only one group for your post.

How can I intercept keystrokes to a JTextField so I could, for
example, implement a digit-only text field [question mark omitted in orig.]

Knute Johnson's answer for the other group who missed it:

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);

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