Check Credit Card Validity using Luhn Algorithm

Question

I tried to check the validation of credit card using Luhn algorithm, which works as the following steps:

1. Double every second digit from right to left. If do

Answer 1

I took a stab at this with Java 8:

public static boolean luhn(String cc) {
    final boolean[] dbl = {false};
    return cc
            .chars()
            .map(c -> Character.digit((char) c, 10))
            .map(i -> ((dbl[0] = !dbl[0])) ? (((i*2)>9) ? (i*2)-9 : i*2) : i)
            .sum() % 10 == 0;
}

Add the line

            .replaceAll("\\s+", "")

Before

            .chars()

If you want to handle whitespace.

Seems to produce identical results to

return LuhnCheckDigit.LUHN_CHECK_DIGIT.isValid(cc);

From Apache's commons-validator.