Credit Card Check Digit Generator Java Program

In the last post, we saw that Credit Card numbers are not random and it can be validated using Luhn Algorithm and I wrote a java program for credit card number validation.

A credit card number last digit is called Check Digit and its appended to a partial credit card number to generate the complete valid credit card number. For example, if we have a partial card number of 15 digits as 123456789012345 then using Luhn algorithm, we find that check digit should be 2, so the valid credit card number will be 1234567890123452.

Here I am providing a method in java to generate the check digit for a partial credit card number.

package com.journaldev.design.test;

public class GenerateCheckDigit {

	public static void main(String[] args) {
		long l = 123456789012345L;
		int cd = generateCheckDigit(l);
		System.out.println("Valid card number="+l+cd);
	}


	private static int generateCheckDigit(long l) {
		String str = Long.toString(l);
		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){
			return 0;
		}else return 10-(sum%10);
	}

}

Once the valid account number is generated by adding check digit to the partial credit card number, you can validate it using my last program to validate credit card number.