Java: check if string contains more than one character

Question

I would like to check my inputted string on a comma, which is pretty easy ( just used this: How can I check if a single character appears in a string?). Now I want to check if a character is present more than once. (in this case a comma (/u002C)) I have created this code:

public static void addTextLimiterDouble(final TextField tf, final int maxLength, final Button submitButton) {
    tf.textProperty().addListener(new ChangeListener<String>() {
        @Override
        public void changed(final ObservableValue<? extends String> ov, final String oldValue, final String newValue) {

            if (oldValue.contains("\u002C") && newValue.endsWith("\u002C")) {
                tf.setText(oldValue);
            }
    }
});

This gives a StackOverflow error, because it seems it can not get out of the loop. Can anyone explain why it can't?

Note: I know there are maybe more ways to check for more than one character, but I am just curious why this gives me an error.

Any help is greatly appreciated!

Answer 1

Imagine the situation (actually, that's most likely the situation where you are in) where you are editing a string that ends in a comma. When you change something else in that string, then your if statement (if (oldValue.contains("\u002C") && newValue.endsWith("\u002C"))) returns true and you try to revert to the old value by calling tf.setText()

However- that changes the text! And your ChangeListener is invoked again. The new value ends in a comma (you were reverting back to a value that ends in a comma) and the old value also contains a comma, so again, your listener calls tf.setText().

After that, it invokes your ChangeListener because the text was changed. And again... ad nauseum