You'll need to implement a custom Document that refuses to accept any more
input once the max length has been entered (this only works for Swing
though).
Something like this would do:

public class LimitedLengthDocument extends javax.swing.text.PlainDocument {

 private int maxLength;

 /** Creates a new instance of LimitedLengthDocument */
 public LimitedLengthDocument( int maxLength) {
   this.maxLength =maxLength;
 }

 public LimitedLengthDocument() {
   this.maxLength = 3;
 }

 void setMaxLength(int maxLength) {
   this.maxLength = maxLength;
 }

public void insertString(int offset, String str, AttributeSet a) throws
BadLocationException {
   if ((getLength() + str.length()) <= maxLength) {
     super.insertString(offset, str, a);
   }else{
     java.awt.Toolkit.getDefaultToolkit().beep();
   }
 }
}

You'll need to give an instance of this document to your JTextField.

Hi Gregg,

Another way to do this...

...if you are using Java 1.4+ is with a JFormattedTextField.  You can
do all sorts of cool input validation with it.

--g