Can produce wrong result

[clayjh]

The luhn algorithm has to start from the furthest right of the card number. I would reverse the string before you set n

[eix128]

Try this :

package com.journaldev.util;

public class JavaLuhnAlgorithm {

public static void main(String[] args) {
validateCreditCardNumber("12345678903555");
String imei = "012850003580200";
validateCreditCardNumber(imei);
}

private static void validateCreditCardNumber(String str) {

int[] ints = new int[str.length()];
for (int i = 0; i < str.length(); i++) {
    ints[i] = Integer.parseInt(str.substring(i, i + 1));
}
for (int i = ints.length - 2; i >= 0; i = i - 2) {
    int j = ints[i];
    j = j * 2;
    if (j > 9) {
        j = j % 10 + 1;
    }
    ints[i] = j;
}
int sum = 0;
for (int i = 0; i < ints.length; i++) {
    sum += ints[i];
}
if (sum % 10 == 0) {
    System.out.println(str + " is a valid credit card number");
} else {
    System.out.println(str + " is an invalid credit card number");
}

}
}

[eix128]

Or try this:

public static boolean checkCreditCard(String cc) {
final boolean[] dbl = {false};
return cc
.replaceAll("\s+", "")
.chars()
.map(c -> Character.digit((char) c, 10))
.map(i -> ((dbl[0] = !dbl[0])) ? (((i2)>9) ? (i2)-9 : i*2) : i)
.sum() % 10 == 0;
}