Re: Keystroke validation in JTextField

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) {
            return;
        }
        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);
    }
}