creditCarda

private static boolean creditCardInput(String cardInput)
    {
        boolean value = false;
        //credit card processing logic
        if(cardInput.length() == 16)
        {
            int s1 = 0, s2 = 0;
            String reverse = new StringBuffer(cardInput).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;
                    }
                }
            }
           value = ((s1 + s2) % 10 == 0);
        }
        else
        {
            value = false;
        }

        return value;
    }