private static boolean isValid(String e) {
        int sum = 0;
        for (int i = 0; i < e.length() - 1; i++) {
            sum += (e.charAt(i) - '0') * 2;
        }

        return sum % 10 == e.charAt(e.length() - 1) - '0';
    }
Why Did you create this class? what does it do?
src/codeforces/ValidateCard.java