This week's book giveaway is in the OCPJP forum. We're giving away four copies of OCA/OCP Java SE 7 Programmer I & II Study Guide and have Kathy Sierra & Bert Bates on-line! See this thread for details.
You can create a customized Textfield extends JTextField. In the constructor use the class which extends PlainDocument.
By overriding the method insertString in PlainDocument interface you can allow the user to enter only the specified number of character.