How do I implement the Luhn algorithm?

Question

I am trying to create a program to validate 10 to 12 digit long number sequences based on the luhn algorithm, but my program keeps on telling me that every number is invalid

Answer 1

If you use Java 10 or higher, you can use the following code:

public static boolean luhn(String s) {
    IntUnaryOperator sumDigits = n -> n / 10 + n % 10;
    var digits = s.chars()
                  .map(Character::getNumericValue)
                  .toArray();
    return IntStream.rangeClosed(1, digits.length)
                    .map(i -> digits.length - i)
                    .map(i -> i % 2 == 0 ? digits[i] : sumDigits.applyAsInt(digits[i] * 2))
                    .sum() % 10 == 0;
}

It's the functional approach to this algorithm.