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Re: keypad

"Jason R. Kretzer" wrote:
I have created a keypad using a JTextfield, 12 JButtons, and a JDialog window. Is there a way to set a Listener on the JTextField that won't allow any more than four characters to be typed in it?




Sun recommends that you handled this type of thing by modifying the document model which the field uses.   In the following example, the "BoundedDocument" model both limits the data entry to a certain length and optionally forces the entry to upper case.

Cynthia Jeness


package org.ajug.gui;

import javax.swing.*;
import javax.swing.text.*;

import java.awt.Toolkit;
import java.text.NumberFormat;
import java.text.ParseException;
import java.util.Locale;

 * This class adds additional functionality to the JTextField so that the
 * number of characters entered can be limited and optionally converted
 * to upper case.
public class TextField extends JTextField {

   /** Toolkit to use for reporting errors through "beep" */
   private Toolkit toolkit;

   /** Maximum length */
   private int maxLength;

   /** Flag indicating that upper case only */
   boolean upperOnly;

    * This constructs the text entry field based on the upper case
    * option, the data entry width and the display width.
    * @param   upOnly    True if upper case only.
    * @param   elength   Data entry length.
    * @param   dlength   Display length.
   public TextField(boolean upOnly, int elength, int dlength) {

      toolkit = Toolkit.getDefaultToolkit();
      this.upperOnly = upOnly;
      maxLength = elength;

    * This method overrides the method of the parent class in order
    * trim spaces from the string.
    * @return   Trimmed text contents.
   public String getText() {

      String tmp = super.getText();
      return tmp.trim();

    * This creates the default document model to use the
    * "BoundedDocument" model.
    * @return   WholeNumberDocumentModel object.
   protected Document createDefaultModel() {
      return new BoundedDocument();

    * This class provides a document which will limit the
    * number of characters that a user can enter and optionally
    * force them to upper case.
   protected class BoundedDocument extends PlainDocument {

       * This method limits the entry to the maximum number of
       * characters specified and optionally forces upper case.
       * @param   offs  Offset into the entry.
       * @param   str   Entry string
       * @param   a     Attribute
       * @exception     BadLocationException
      public void insertString(int offs,
                               String str,
                               AttributeSet a)
         throws BadLocationException {

         // Check that the maximum length is not exceeded.
         if (maxLength > 0) {
            if ((getLength()+str.length() ) > maxLength) {

         if (upperOnly) {
            char[] source = str.toCharArray();
            char[] result = new char[source.length];
            for (int i=0; i < source.length; i++) {
               result[i] = Character.toUpperCase(source[i]);
            super.insertString(offs, new String(result, 0, result.length), a);
            super.insertString(offs, str, a);