// To generate bank card checksum that satisfies Luhn algoritm specificaiton:

private int generateChecksum(String num) {

    char[] binAccId = num.toCharArray();

    int sum = 0;

    for (int i = 0; i < binAccId.length; i++) {

        int no = Character.getNumericValue(binAccId[i]);

        if (i % 2 == 0) {

            binAccId[i] = Character.forDigit(no * 2 > 9 ? no * 2 - 9 : no * 2, RADIX);

            sum += Character.getNumericValue(binAccId[i]);

            continue;

        }

        sum += no;

    }

​

    return 10 - (sum % 10) == 10 ? 0 : 10 - (sum % 10);

 }