public int getCheckedButtonIndex() {
        final List<Integer> checkedButtonIndexes = getCheckedButtonIndexes();
        return checkedButtonIndexes.size() == 1 ? checkedButtonIndexes.get(0) : View.NO_ID;
    }
Like {@link MaterialButtonToggleGroup#getCheckedButtonId()}, but returns the INDEX of the view instead of it's id
main/src/main/java/cgeo/geocaching/ui/ButtonToggleGroup.java