Check whether a credit card number is valid or not? Valid credit card Number. Luhns algorithm

CreditCardValid.java

//http://rosettacode.org/wiki/Luhn_test_of_credit_card_numbers#Java

       public static boolean isEmptyString(String stringValue) {
		return stringValue == null || stringValue.trim().length() == 0;
	}

	public static boolean isCreditCardValid(String number) {
		if (isEmptyString(number))
			return false;
		int s1 = 0, s2 = 0;
		String reverse = new StringBuffer(number).reverse().toString();
		for (int i = 0; i < reverse.length(); i++) {
			int digit = Character.digit(reverse.charAt(i), 10);
			if (i % 2 == 0) {// this is for odd digits, they are 1-indexed in
								// the algorithm
				s1 += digit;
			} else {// add 2 * digit for 0-4, add 2 * digit - 9 for 5-9
				s2 += 2 * digit;
				if (digit >= 5) {
					s2 -= 9;
				}
			}
		}
		return (s1 + s2) % 10 == 0;
	}