Have you ever tried to make a textfield that accepts only 20 characters? Or a text area that automatically converts the user typed letters to upper case? Swing’s Document Listener is the most accurate and hassle-free way to achieve this in Swing. This articles explains you what you need to get started with Document Filter. Step by Step.
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Max Length on JTextField Using DocumentFilter: A Step by Step tutorial

Let’s consider a common coding situation. Suppose you have a text field in your form that accepts an email address. Suppose you need to code such that if the email address typed by the user is not valid, then you shouldn’t let the focus out of this textfield. Where would you put such a code? In the focus lost of the email-text-field? In the focus gained for all other text-fields? I don’t know what’s the best way to do this in JavaScript but there’s a super-elegant way in Swing!