Luhn algorithm implementation in Java

The Luhn algorithm ("modulus 10" or "mod 10" algorithm, Luhn formula) is a simple checksum formula used to validate a variety of identification numbers, such as credit card numbers (PAN) or IMEI numbers. The algorithm is specified in ISO/IEC 7812-1. The algorithm was designed to protect against accidental errors. The Luhn algorithm will detect any single-digit error, as well as almost all transpositions of adjacent digits. It will not, however, detect transposition of the two-digit sequence 09 to 90 (or vice versa).

Description

1. Calculate the check digit

The check digit is obtained by computing the sum of the non-check digits then computing 9 times that value modulo 10. In algorithm form:

• From the rightmost digit, which is the check digit, moving left, double the value of every second digit;
• If the product of this doubling operation is greater than 9, then sum the digits of the products or alternatively subtract 9 from the product.
• Take the sum of all the digits.
• Multiply by 9.
• The last digit is the check digit.

Example

Initial PAN

Step 1 - double the values

Step2 - sum the digits

Step3 - sum all digits

Step4 - multiply by 9

Result

The check digit is 1.

If you want to calculate online the check digit you can try this calculator.

2. Validate the check digit

• From the rightmost digit, which is the check digit, moving left, double the value of every second digit;
• if the product of this doubling operation is greater than 9,then sum the digits of the products or alternatively subtract 9 from the product.
• Take the sum of all the digits.
• If the total modulo 10 is equal to 0 then the number is valid according to the Luhn formula; else it is not valid.

Example

Initial pan

Step 1 - double the values

Step2 - sum the digits

Step3 - sum all digits

Step4 - modulo 10

Result

The pan number is valid.

If you want to validate online a PAN number you can try this validator.

Implementation

Here is the Luhn algorithm Java implementation with in-line comments.

package com.admfactory.cards;

public class Cards {
	/**
	 * Checks if the card is valid
	 * 
	 * @param card
	 *           {@link String} card number
	 * @return result {@link boolean} true of false
	 */
	public static boolean luhnCheck(String card) {
		if (card == null)
			return false;
		char checkDigit = card.charAt(card.length() - 1);
		String digit = calculateCheckDigit(card.substring(0, card.length() - 1));
		return checkDigit == digit.charAt(0);
	}
	
	/**
	 * Calculates the last digits for the card number received as parameter
	 * 
	 * @param card
	 *           {@link String} number
	 * @return {@link String} the check digit
	 */
	public static String calculateCheckDigit(String card) {
		if (card == null)
			return null;
		String digit;
		/* convert to array of int for simplicity */
		int[] digits = new int[card.length()];
		for (int i = 0; i < card.length(); i++) {
			digits[i] = Character.getNumericValue(card.charAt(i));
		}
		
		/* double every other starting from right - jumping from 2 in 2 */
		for (int i = digits.length - 1; i >= 0; i -= 2)	{
			digits[i] += digits[i];
			
			/* taking the sum of digits grater than 10 - simple trick by substract 9 */
			if (digits[i] >= 10) {
				digits[i] = digits[i] - 9;
			}
		}
		int sum = 0;
		for (int i = 0; i < digits.length; i++) {
			sum += digits[i];
		}
		/* multiply by 9 step */
		sum = sum * 9;
		
		/* convert to string to be easier to take the last digit */
		digit = sum + "";
		return digit.substring(digit.length() - 1);
	}
	
	public static void main(String[] args)	{
		String pan1 = "37828224631000";
		System.out.println("Cards luhn calculator");
		System.out.println();
		System.out.println("Calculate check digit for: " + pan1);
		String digit = calculateCheckDigit(pan1);
		System.out.println("Check digit: " + digit);
		System.out.println();
		
		String pan2 = "4012888888881881";
		System.out.println("Validate pan number '" + pan2 + "': " + luhnCheck(pan2));
	}
	
}

For testing, I used cards from PaypalObjects website.The output for the tests is:

Cards luhn calculator

Calculate check digit for: 37828224631000
Check digit: 5

Validate pan number '4012888888881881': true

Also, you can try with different cards or even with your own cards.

References

• Luhn Algorithm
• PaypalObjects
• DataGeneratorTools - Luhn Digit Calculator
• DataGeneratorTools - Card Number Validator