como agrego que un int sea para palabras

String numberToWords(int num) {
        if (num == 0)
            return "Zero";

        int billion = num / 1000000000;
        int million = (num - billion * 1000000000) / 1000000;
        int thousand = (num - billion * 1000000000 - million * 1000000) / 1000;
        int rest = num - billion * 1000000000 - million * 1000000 - thousand * 1000;

        String result = "";
        if (billion != 0)
            result = threeDigit(billion) + " Billion";
        if (million != 0) {
            if (!result.isEmpty())
                result += " ";
            result += threeDigit(million) + " Million";
        }
        if (thousand != 0) {
            if (!result.isEmpty())
                result += " ";
            result += threeDigit(thousand) + " Thousand";
        }
        if (rest != 0) {
            if (!result.isEmpty())
                result += " ";
            result += threeDigit(rest);
        }
        return result