Java Source Code

The following simple Java class is free for you to use as you wish, without any restrictions or guarantees.

//---------------------------------------------------------
// Checks for valid credit card number using Luhn algorithm
//---------------------------------------------------------

public abstract class LuhnCheck {

 //--------------------------------
 // Filter out non-digit characters
 //--------------------------------

 private static String getDigitsOnly (String s) {
   StringBuffer digitsOnly = new StringBuffer ();
   char c;
   for (int i = 0; i < s.length (); i++) {
     c = s.charAt (i);
     if (Character.isDigit (c)) {
       digitsOnly.append (c);
     }
   }
   return digitsOnly.toString ();
 }

 //-------------------
 // Perform Luhn check
 //-------------------

 public static boolean isValid (String cardNumber) {
   String digitsOnly = getDigitsOnly (cardNumber);
   int sum = 0;
   int digit = 0;
   int addend = 0;
   boolean timesTwo = false;

   for (int i = digitsOnly.length () - 1; i >= 0; i--) {
     digit = Integer.parseInt (digitsOnly.substring (i, i + 1));
     if (timesTwo) {
       addend = digit * 2;
       if (addend > 9) {
         addend -= 9;
       }
     }
     else {
       addend = digit;
     }
     sum += addend;
     timesTwo = !timesTwo;
   }

   int modulus = sum % 10;
   return modulus == 0;

 }

 //-----
 // Test
 //-----

 public static void main (String[] args) {
   String cardNumber = "4408 0412 3456 7890";
   boolean valid = LuhnCheck.isValid (cardNumber);
   System.out.println (cardNumber + ": " + valid);
   cardNumber = "4408 0412 3456 7893";
   valid = LuhnCheck.isValid (cardNumber);
   System.out.println (cardNumber + ": " + valid);
   cardNumber = "4417 1234 5678 9112";
   valid = LuhnCheck.isValid (cardNumber);
   System.out.println (cardNumber + ": " + valid);
   cardNumber = "4417 1234 5678 9113";
   valid = LuhnCheck.isValid (cardNumber);
   System.out.println (cardNumber + ": " + valid);
 }

}