Modulus 10

[Ray Marsh]

Not sure where this question should go.
Has anyone heard of a mod10 check in Java? Perhaps a class already exists that handles the logic.
Modulus 10 for those who are unfamiliar is a format check for credit card numbers. It does not authorize dollar amounts or even check if the account is active. All it does is perform a complicated algorithm on the credit card number to see if it is valid. Example '1111 1111 1111 1111' would fail. There are certain patterns that must exist in a CC number for it to pass mod10.
Thanks in advance.

[Glen Tanner]

There's a post on the Sun site with a code example, but I'm not sure if that is what you're looking for.
http://forum.java.sun.com/forum?14@@.eef01e5

[Ray Marsh]

Thanks Glen. I'll check it out.

[Frank Carver]

I've just tried the code from that site. It's a bit of a mess, but it does work.

[Glen Tanner]

Sorry, I should have at least tried the code!

[Ray Marsh]

Originally posted by Frank Carver:
I've just tried the code from that site. It's a bit of a mess, but it does work.

Thanks Frank.
How did you test the code? I'm having some trouble with it.

[Frank Carver]

Well I reformatted it, wrapped it up in a class and tried it with the numbers on my credit cards, and a few other known wrong ones.

/**
 verify the digits of a supplied credit card number
*/
public class CC
{
  public static boolean verify(String card) 
  { 
    int cnum[] = new int[25]; 
    int csum = 0; 
    // this will parse out all non-digit chars.
    //
    int digits = 0;
    for (int i = 0; i < card.length(); ++i) 
    {
      char c = card.charAt(i);
      if (Character.isDigit(c))
      { 
        cnum[digits++] = c - '0';
      } 
    }
    // this verifies that the numbers are MOD 10 
    //
    int weight = 1; 
    for(int i = digits - 1; i >= 0; --i) 
    { 
      int val = cnum[i] * weight; 
      if(val > 9)
      {
        val -= 9;
      }
      csum += val; 
      weight = (weight == 1) ? 2 : 1; 
    } 

    return csum % 10 == 0;
  } 
                
  /** 
   usage: java CC <cardnumber>
  */
  public static void main(String[] args)
  {
    String card = args.length > 0 ? args[0] : "0123456789012";
    System.out.println("card number '" + card + " => " + verify(card));
  }
}

For obvious reasons I'm reluctant to give you my test data!

[Ray Marsh]

Thanks Frank.
I don't need ALL your test data. Just your CC number, expiration date and credit limit. But I promise not to tell anyone!

[Manju Swamy]

Here is an improvised version. This does not have a card number digits limitation. Also when use isDigit() function it reruns true for various other language (like Indian, Arabic, etc.) numbers. Then the whole logic would fail.

/**
 *             Credit Card number verification.
 *
 * @author      Robert Suppenbach
 * @author      Frank Carver
 * @author      Manju Swamy
 * @version     %I%, %G% %U%
 *
 */
public class CreditCard {
    private int cardNumber[];
    private byte totalDigits = -1;
    // add other attributes like expiration dates etc., to the class
    // Constructor
    CreditCard() {}
    CreditCard(String cardNumber) {
        setCardNumber(cardNumber);
    }
    // Convert the string card numbers into an array of integer
    // elements weeding out the non-numeric characters.
    int setCardNumber(String cardNum) {
        char digit;
        int returnCode = 1;
        // Check for valid string object reference?
        if ((cardNum != null) && (cardNum.length() > 0)) {
            // Allocate the cardNumber array
            cardNumber = new int[cardNum.length()];
            // Parse the card number string
            for (int i = 0; i < cardNum.length(); i++) {
                digit = cardNum.charAt(i);
                // isDigit?
                if (digit >= '0' && digit <= '9') {
                    totalDigits++;
                    cardNumber[totalDigits] = digit - '0';
                }
            }
            // Did we get valid card number?
            returnCode = (totalDigits == -1)? -1: 1;
        }
        else {
            returnCode = -2;        // Invalid string reference
        }
        return returnCode;
    }
    // Verify the card number
    boolean verifyCard() {
        int result = 0;
        int weight = 1;
        int checkSum = 0;
        if (totalDigits == -1) {
            return false;
        }
        // Verify that the numbers are MOD 10.
        for (int i = totalDigits; i >= 0; i--) {
            result = (cardNumber[i] * weight);
            if (result >=  10) {
                result = (result % 10) + 1;
            }
            checkSum += result;
            weight = (weight == 1) ? 2 : 1;
        }
        // Returns true for valid card number.
        return ((checkSum % 10) == 0);
    }
    // Print the credit card number
    void print() {
        if (totalDigits == -1) {
            return;
        }
        for (int i = 0; i <= totalDigits; i++) {
            System.out.print(cardNumber[i]);
            if (((i + 1) % 4) == 0) {
                System.out.print(" ");
            }
        }
    }
    // add other methods related to CreditCard class
    /**
     *     usage:    java CreditCard <credit card number>
     *     example: java CreditCard "9876 5432 1023 4567"
     *                  java CreditCard 9876-5432-1023-4567
     *                  java CreditCard 9876543210234567
     */
    public static void main(String args[]) {
        boolean cardStatus;
        // Any credit card number passed in the argument?
        String card = (args.length > 0) ? args[0] : "1111-1111-1111-1111";
        // Create a new instance of CreditCard class
        CreditCard customerCreditCard = new CreditCard(card);
        // Verify the credit card number
        cardStatus = customerCreditCard.verifyCard();
        // Print the verification status
        System.out.print("Credit Card number: ");
        customerCreditCard.print();
        System.out.println(cardStatus? " is valid.": " is invalid.");
        // enable for garbage collection
        customerCreditCard = null;
    }
}

[This message has been edited by Manju Swamy (edited March 03, 2000).]
[This message has been edited by Manju Swamy (edited March 03, 2000).]

[Ray Marsh]

Thank you, Manju.
I'll give that one a test drive as well.
I have a modulus 10 routine in RPG that has been in production for a few years. The specs for the logic is on IBM's AS400 book shelf web-site in their DDS reference manual.
Once I have one or two working Java programs, I plan to run some parallel testing. It will be interesting to see what happens.

[Anonymous]

Why are you doing all that math? Putting the results of the calculations in a static array would make it much faster/easier to read. Pseudocode follows.
public class CCThingy
{
public static int[] results = {0, 2, 4, 6, 8, 1, 3, 5, 7, 9};
public boolean checkIt(int ccarray[])
{
// Sanity checking removed, you can figure
// that out for yourselves.
boolean flip = false;
int sum = 0;
for(int i = ccarray.length; i >= 0; i--)
{
if(flip)
{
sum += results[ccarray[i]];
} else {
sum += ccarray.length[i];
}
}
return (sum % 10 == 0);
}

This way you don't have to calculate values that aren't going to ever change. And you don't have to multiply something by 1.


------------------
--I'm Hugh Jass.

[Mark Barell]

But is there a way to check the expire date too ?