This week's book giveaway is in the OO, Patterns, UML and Refactoring forum. We're giving away four copies of Refactoring for Software Design Smells: Managing Technical Debt and have Girish Suryanarayana, Ganesh Samarthyam & Tushar Sharma 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.