Can't figure out what's wrong with my DocumentFilter

Question

I tried putting together custom PlainDocument made by others for one I needed, but since I don't know the mechanics of PlainDocument, I failed and it didn't work. I need something that will make sure my textfield only allows 2 letters, so anything a-zA-Z that occurs only twice. I tried this first:

    public class LetterDocument extends PlainDocument {

    private String text = "";

    @Override
    public void insertString(int offset, String txt, AttributeSet a) {
        try {
            text = getText(0, getLength());
            if ((text + txt).matches("^[a-zA-Z]{2}$")) {
                super.insertString(offset, txt, a);
            }
         } catch (Exception ex) {
            Logger.getLogger(LetterDocument.class.getName()).log(Level.SEVERE, null, ex);
         }

        }
    }

This doesn't even let me type in anything. I then tried this, which I tried putting together from two other threads, one of them letting only letters be typed in, and the other limiting the characters:

    public class LetterDocument extends PlainDocument {
    private int limit;
    private String text = "";

    LetterDocument(int limit) {
        super();
        this.limit = limit;
    }

    @Override
    public void insertString(int offset, String txt, AttributeSet a)
            throws BadLocationException {
        if (txt == null)
            return;
        try {
            text = getText(0, getLength());

            if (((text + txt).matches("[a-zA-Z]"))
                    && (txt.length()) <= limit) {
                super.insertString(offset, txt, a);
            }
        } catch (Exception ex) {
            Logger.getLogger(LetterDocument.class.getName()).log(Level.SEVERE,
                    null, ex);
        }

    }
}

I have no idea what's wrong.

Answer 1

Don't use a custom Document.

Instead use a DocumentFilter. Read the section from the Swing tutorial on Implementing a Document Filter for a working example that limits the number of characters you can enter in a Document.

Then just add some additional logic to make sure only letters are added.

Or an easier option is to use a JFormatttedTextField with a character mask. Again see the tutorial on Using a Formatted Text Field.