Question

So I’m writing the TextValueChanged handler for a JTextField that takes a long as it’s input. It needs to only allow the user to type in valid values between 0 and Long.MAX. If the user types an invalid character, it should strip it, so that the value in the JTextField is always a valid long.

My current code looks like this, but it seems ugly. With no external libs (including Apache Commons), is there a cleaner/simpler way to do this?

public void textValueChanged(TextEvent e) {
    if (e.getSource() instanceof JTextField) {
        String text = ((JTextField) e.getSource()).getText();
        try {
            // Try to parse cleanly
            long longNum = Long.parseLong(text);
            // Check for < 1 
            if (longNum < 1) throw new NumberFormatException();
            // If we pass, set the value and return
            setOption("FIELDKEY", longNum);
        } catch (NumberFormatException e1) {
            // We failed, so there's either a non-numeric or it's too large.
            String s = ((JTextField) e.getSource()).getText();
            // Strip non-numeric characters
            s = s.replaceAll("[^\\d]", "");
            long longNum = -1;
            if (s.length() != 0) {
                /* Really ugly workaround for the fact that a
                 * TextValueChanged event can capture more than one
                 * keystroke at a time, if it's typed fast enough,
                 * so we might have to strip more than one
                 * character. */
                Exception e3;
                do {
                    e3 = null;
                    try {
                        // Try and parse again
                        longNum = Long.parseLong(s);
                    } catch (NumberFormatException e2) {
                        // We failed, so it's too large.
                        e3 = e2;
                        // Strip the last character and try again.
                        s = s.substring(0, s.length() - 1);
                    }
                    // Repeat
                } while (e3 != null);
            }
            // We parsed, so add it (or blank it if it's < 1) and return.
            setOption("FIELDKEY", (longNum < 1 ? 0 : longNum));
        }
    }
}

The field is constantly repopulated from a getOption(key) call, so to ‘store’ the value, just pass it to that call.

Answer 1

DocumentFilter is a good solution for your case.

The Definitive Guide to Java Swing by John Zukowski has an excellent example on pages 542-546 showing a custom document filter for limiting an integer range.

Providing custom DocumentFilter should easily accommodate the requirement to strip off the unwanted characters upon copy/pase as you have specified in one of the comments.