Jtextfield that accepts only two letters

Question

How do I make it in a JTextField, I can just type two letters of the keyboard and not use any other letter, number or character? For example: in a field I just want it accepted either the letter T or the letter F (uppercase).

Answer 1

Events of KeyListener only detect, but do not prevent text from being typed within the field, and use KeyListener to filter characters and remove them from the field is not a good practice, due to the low level that these methods work to monitor keyboard actions.

Not to mention, if you copy and paste(Control+C and Control+V) any text that has the two letters or not, the KeyListener will not identify.

The ideal is to use PlainDocument or DocumentFilter, which do not rely exclusively on keyboard actions such as keyPress and directly monitor the string typed in the field.

In the example of the duplicate, In addition to allowing you to limit the amount of characters, you are only filtering numbers, so to allow only two unique letters, simply change the regex. Following the example of the given letters, it would look like this:

super.insertString(offset, str.replaceAll("([^FT])", ""), attr);

I am allowing to be typed only the letters mentioned in their versions in high box, not allowing any other characters, even if try to paste from the Clipboard.

I made an executable example that is found on github, just compile and run.