Check Credit Card Validity using Luhn Algorithm

Question

I tried to check the validation of credit card using Luhn algorithm, which works as the following steps:

1. Double every second digit from right to left. If doubling of a digit results in a two-digit number, add up the two digits to get a single-digit number.

   2 * 2 = 4

   2 * 2 = 4

   4 * 2 = 8

   1 * 2 = 2

   6 * 2 = 12 (1 + 2 = 3)

   5 * 2 = 10 (1 + 0 = 1)

   8 * 2 = 16 (1 + 6 = 7)

   4 * 2 = 8

2. Now add all single-digit numbers from Step 1.

   4 + 4 + 8 + 2 + 3 + 1 + 7 + 8 = 37

3. Add all digits in the odd places from right to left in the card number.

   6 + 6 + 0 + 8 + 0 + 7 + 8 + 3 = 38

4. Sum the results from Step 2 and Step 3.

   37 + 38 = 75

5. If the result from Step 4 is divisible by 10, the card number is valid; otherwise, it is invalid. For example, the number 4388576018402626 is invalid, but the number 4388576018410707 is valid.

Simply, my program always displays valid for everything that I input. Even if it's a valid number and the result of sumOfOddPlace and sumOfDoubleEvenPlace methods are equal to zero.
Any help is appreciated.

import java.util.Scanner;
public class CreditCardValidation {
      public static void main(String[] args) {
     Scanner in = new Scanner(System.in);
        int count = 0;
        long array[] = new long [16];
       do
       {
        count = 0;
       array = new long [16];
        System.out.print("Enter your Credit Card Number : ");
        long number = in.nextLong();
        for (int i = 0; number != 0; i++) {
        array[i] = number % 10;
        number = number / 10;
        count++;
        }
       }
        while(count < 13); 
        if ((array[count - 1] == 4) || (array[count - 1] == 5) || (array[count - 1] == 3 && array[count - 2] == 7)){
            if (isValid(array) == true) {
                System.out.println("\n The Credit Card Number is Valid. ");
        } else {
            System.out.println("\n The Credit Card Number is Invalid. ");
        }
        } else{
          System.out.println("\n The Credit Card Number is Invalid. ");
        }
    }

    public static boolean isValid(long[] array) {
        int total = sumOfDoubleEvenPlace(array) + sumOfOddPlace(array);        
        if ((total % 10 == 0)) {
         for (int i=0; i< array.length; i++){
            System.out.println(array[i]);}
            return true;
        } else {
          for (int i=0; i< array.length; i++){
            System.out.println(array[i]);}
            return false;
        }
    }

    public static int getDigit(int number) {
        if (number <= 9) {
            return number;
        } else {
            int firstDigit = number % 10;
            int secondDigit = (int) (number / 10);
            return firstDigit + secondDigit;
        }
    }

    public static int sumOfOddPlace(long[] array) {
        int result = 0;
        for (int i=0; i< array.length; i++)
        {
        while (array[i] > 0) {
            result += (int) (array[i] % 10);
            array[i] = array[i] / 100;
         }}
         System.out.println("\n The sum of odd place is " + result);
        return result;
    }

    public static int sumOfDoubleEvenPlace(long[] array) {
        int result = 0;
        long temp = 0;
        for (int i=0; i< array.length; i++){
        while (array[i] > 0) {
             temp = array[i] % 100;
             result += getDigit((int) (temp / 10) * 2);
            array[i] = array[i] / 100;
           }
        }
        System.out.println("\n The sum of double even place is " + result);
        return result;
    }
     }

Answer 1

There are two ways to split up your int into List<Integer>

1. Use %10 as you are using and store it into a List
2. Convert to a String and then take the numeric values

Here are a couple of quick examples

public static void main(String[] args) throws Exception {
    final int num = 12345;
    final List<Integer> nums1 = splitInt(num);
    final List<Integer> nums2 = splitString(num);
    System.out.println(nums1);
    System.out.println(nums2);
}

private static List<Integer> splitInt(int num) {
    final List<Integer> ints = new ArrayList<>();
    while (num > 0) {
        ints.add(0, num % 10);
        num /= 10;
    }
    return ints;
}

private static List<Integer> splitString(int num) {
    final List<Integer> ints = new ArrayList<>();
    for (final char c : Integer.toString(num).toCharArray()) {
        ints.add(Character.getNumericValue(c));
    }
    return ints;
}

Answer 2

I'll use 5 digit card numbers for simplicity. Let's say your card number is 12345; if I read the code correctly, you store in array the individual digits:

array[] = {1, 2, 3, 4, 5}

Since you already have the digits, in sumOfOddPlace you should do something like

public static int sumOfOddPlace(long[] array) {
    int result = 0;
    for (int i = 1; i < array.length; i += 2) {
        result += array[i];
    }
    return result;
}

And in sumOfDoubleEvenPlace:

public static int sumOfDoubleEvenPlace(long[] array) {
    int result = 0;
    for (int i = 0; i < array.length; i += 2) {
        result += getDigit(2 * array[i]);
    }
    return result;
}

Answer 3

Google and Wikipedia are your friends. Instead of long-array I would use int-array. On Wikipedia following java code is published (together with detailed explanation of Luhn algorithm):

   public static boolean check(int[] digits) {
     int sum = 0;
     int length = digits.length;
     for (int i = 0; i < length; i++) {

       // get digits in reverse order
       int digit = digits[length - i - 1];

       // every 2nd number multiply with 2
       if (i % 2 == 1) {
           digit *= 2;
       }
       sum += digit > 9 ? digit - 9 : digit;
     }
     return sum % 10 == 0;
   }

You should work on your input processing code. I suggest you to study following solution:

public static void main(String[] args) {
    Scanner in = new Scanner(System.in);
    boolean repeat;
    List<Integer> digits = new ArrayList<Integer>();

    do {
        repeat = false;
        System.out.print("Enter your Credit Card Number : ");
        String input = in.next();

        for (int i = 0; i < input.length(); i++) {
            char c = input.charAt(i);
            if (c < '0' || c > '9') {
                repeat = true;
                digits.clear();
                break;
            } else {
                digits.add(Integer.valueOf(c - '0'));
            }
        }
    } while (repeat);

    int[] array = new int[digits.size()];
    for (int i = 0; i < array.length; i++) {
        array[i] = Integer.valueOf(digits.get(i));
    }
    boolean valid = check(array);
    System.out.println("Valid: " + valid);
}

Answer 4

You can freely import the following code:

public class Luhn
{
    public static boolean Check(String ccNumber)
    {
            int sum = 0;
            boolean alternate = false;
            for (int i = ccNumber.length() - 1; i >= 0; i--)
            {
                    int n = Integer.parseInt(ccNumber.substring(i, i + 1));
                    if (alternate)
                    {
                            n *= 2;
                            if (n > 9)
                            {
                                    n = (n % 10) + 1;
                            }
                    }
                    sum += n;
                    alternate = !alternate;
            }
            return (sum % 10 == 0);
    }
}

Link reference: https://github.com/jduke32/gnuc-credit-card-checker/blob/master/CCCheckerPro/src/com/gnuc/java/ccc/Luhn.java

Answer 5

I took a stab at this with Java 8:

public static boolean luhn(String cc) {
    final boolean[] dbl = {false};
    return cc
            .chars()
            .map(c -> Character.digit((char) c, 10))
            .map(i -> ((dbl[0] = !dbl[0])) ? (((i*2)>9) ? (i*2)-9 : i*2) : i)
            .sum() % 10 == 0;
}

Add the line

            .replaceAll("\\s+", "")

Before

            .chars()

If you want to handle whitespace.

Seems to produce identical results to

return LuhnCheckDigit.LUHN_CHECK_DIGIT.isValid(cc);

From Apache's commons-validator.

Answer 6

this is the luhn algorithm implementation which I use for only 16 digit Credit Card Number

if(ccnum.length()==16){
    char[] c = ccnum.toCharArray();
    int[] cint = new int[16];
    for(int i=0;i<16;i++){
        if(i%2==1){
            cint[i] = Integer.parseInt(String.valueOf(c[i]))*2;
            if(cint[i] >9)
                cint[i]=1+cint[i]%10;
        }
        else
            cint[i] = Integer.parseInt(String.valueOf(c[i]));
    }
    int sum=0;
    for(int i=0;i<16;i++){
        sum+=cint[i];
    }
    if(sum%10==0)
        result.setText("Card is Valid");
    else
        result.setText("Card is Invalid");
}else
    result.setText("Card is Invalid");

If you want to make it use on any number replace all 16 with your input number length.

It will work for Visa number given in the question.(I tested it)

Answer 7

Here's my implementation of the Luhn Formula.

/**
 * Runs the Luhn Equation on a user inputed CCN, which in turn
 * determines if it is a valid card number.
 * @param c A user inputed CCN.
 * @param cn The check number for the card.
 * @return If the card is valid based on the Luhn Equation.
 */
public boolean luhn (String c, char cn)
{
    String card = c;
    String checkString = "" + cn;
    int check = Integer.valueOf(checkString);

    //Drop the last digit.
    card = card.substring(0, ( card.length() - 1 ) );

    //Reverse the digits.
    String cardrev = new StringBuilder(card).reverse().toString();

    //Store it in an int array.
    char[] cardArray = cardrev.toCharArray();
    int[] cardWorking = new int[cardArray.length];
    int addedNumbers = 0;

    for (int i = 0; i < cardArray.length; i++)
    {
        cardWorking[i] = Character.getNumericValue( cardArray[i] );
    }

    //Double odd positioned digits (which are really even in our case, since index starts at 0).

    for (int j = 0; j < cardWorking.length; j++)
    {
        if ( (j % 2) == 0)
        {
            cardWorking[j] = cardWorking[j] * 2;
        }
    }

    //Subtract 9 from digits larger than 9.

    for (int k = 0; k < cardWorking.length; k++)
    {
        if (cardWorking[k] > 9)
        {
            cardWorking[k] = cardWorking[k] - 9;
        }
    }

    //Add all the numbers together.
    for (int l = 0; l < cardWorking.length; l++)
    {
        addedNumbers += cardWorking[l];
    }

    //Finally, check if the number we got from adding all the other numbers
    //when divided by ten has a remainder equal to the check number.
    if (addedNumbers % 10 == check)
    {
        return true;
    }
    else
    {           
        return false;
    }
}

I pass in the card as c which I get from a Scanner and store in card, and for cn I pass in checkNumber = card.charAt( (card.length() - 1) );.

Answer 8

Here is the implementation of Luhn algorithm.

public class LuhnAlgorithm {

/**
 * Returns true if given card number is valid
 *
 * @param cardNum Card number
 * @return true if card number is valid else false
 */
private static boolean checkLuhn(String cardNum) {
    int cardlength = cardNum.length();
    int evenSum = 0, oddSum = 0, sum;
    for (int i = cardlength - 1; i >= 0; i--) {
        System.out.println(cardNum.charAt(i));
        int digit = Character.getNumericValue(cardNum.charAt(i));
        if (i % 2 == 0) {
            int multiplyByTwo = digit * 2;
            if (multiplyByTwo > 9) {
                /* Add two digits to handle cases that make two digits after doubling */
                String mul = String.valueOf(multiplyByTwo);
                multiplyByTwo = Character.getNumericValue(mul.charAt(0)) + Character.getNumericValue(mul.charAt(1));
            }
            evenSum += multiplyByTwo;
        } else {
            oddSum += digit;
        }
    }
    sum = evenSum + oddSum;
    if (sum % 10 == 0) {
        System.out.println("valid card");
        return true;
    } else {
        System.out.println("invalid card");
        return false;
    }

}

public static void main(String[] args) {
    String cardNum = "4071690065031703";
    System.out.println(checkLuhn(cardNum));
}

}

Answer 9

public class LuhnAlgorithm {

/**
 * Returns true if given card number is valid
 *
 * @param cardNum Card number
 * @return true if card number is valid else false
 */
private static boolean checkLuhn(String cardNum) {
    int cardlength = cardNum.length();
    int evenSum = 0, oddSum = 0, sum;
    for (int i = cardlength - 1; i >= 0; i--) {
        System.out.println(cardNum.charAt(i));
        int digit = Character.getNumericValue(cardNum.charAt(i));
        if (i % 2 == 0) {
            int multiplyByTwo = digit * 2;
            if (multiplyByTwo > 9) {
                /* Add two digits to handle cases that make two digits after doubling */
                String mul = String.valueOf(multiplyByTwo);
                multiplyByTwo = Character.getNumericValue(mul.charAt(0)) + Character.getNumericValue(mul.charAt(1));
            }
            evenSum += multiplyByTwo;
        } else {
            oddSum += digit;
        }
    }
    sum = evenSum + oddSum;
    if (sum % 10 == 0) {
        System.out.println("valid card");
        return true;
    } else {
        System.out.println("invalid card");
        return false;
    }

}

public static void main(String[] args) {
    String cardNum = "8112189875";
    System.out.println(checkLuhn(cardNum));
}

}

Hope it may works.

Answer 10

const options = {
  method: 'GET',
  headers: {Accept: 'application/json', 'X-Api-Key': '[APIkey]'}
};

fetch('https://api.epaytools.com/Tools/luhn?number=[CardNumber]&metaData=true', options)
  .then(response => response.json())
  .then(response => console.log(response))
  .catch(err => console.error(err));

Answer 11

private static int luhnAlgorithm(String number){
    int n=0;
    for(int i = 0; i<number.length(); i++){
        int x = Integer.parseInt(""+number.charAt(i));
        n += (x*Math.pow(2, i%2))%10;
        if (x>=5 && i%2==1) n++;
    }
    return n%10;
}

Answer 12

public class Creditcard {

    public static void main(String args[]){
        Scanner sc=new Scanner(System.in);
        String cardno = sc.nextLine();

        if(checkType(cardno).equals("U")) //checking for unknown type
          System.out.println("UNKNOWN");
        else
          checkValid(cardno); //validation 
}

private static String checkType(String S)
{
    int AM=Integer.parseInt(S.substring(0,2));
    int D=Integer.parseInt(S.substring(0,4)),d=0;
    for(int i=S.length()-1;i>=0;i--)
    {
        if(S.charAt(i)==' ')
            continue;
        else
            d++;
    }
    if((AM==34 || AM==37) && d==15)
        System.out.println("AMEX");
    else if(D==6011 && d==16)
        System.out.println("Discover");
    else if(AM>=51 && AM<=55 && d==16)
        System.out.println("MasterCard");
    else if(((S.charAt(0)-'0')==4)&&(d==13 || d==16)) 
        System.out.println("Visa");
    else
        return "U";
    return "";
}

private static void checkValid(String S) // S--> cardno
{
    int i,d=0,sum=0,card[]=new int[S.length()];

    for(i=S.length()-1;i>=0;i--)
    {
        if(S.charAt(i)==' ')
            continue;
        else
            card[d++]=S.charAt(i)-'0';
    }

    for(i=0;i<d;i++)
    {
        if(i%2!=0)
        {
            card[i]=card[i]*2;
            if(card[i]>9)
                sum+=digSum(card[i]);
            else
                sum+=card[i];
        }
        else
            sum+=card[i];
    }
    if(sum%10==0)
        System.out.println("Valid");
    else    
        System.out.println("Invalid");

}

public static int digSum(int n)
{
    int sum=0;
    while(n>0)
    {
        sum+=n%10;
        n/=10;
    }
    return sum;
}
}

Answer 13

Okay, this can be solved with a type conversions to string and some Java 8 stuff. Don't forget numbers and the characters representing numbers are not the same. '1' != 1

public static int[] longToIntArray(long cardNumber){

return Long.toString(cardNumber).chars()
    .map(x -> x - '0') //converts char to int 
    .toArray();  //converts to int array
}

You can now use this method to perform the luhn algorithm:

  public static int luhnCardValidator(int cardNumbers[]) {
                int sum = 0, nxtDigit;
                for (int i = 0; i<cardNumbers.length; i++) {
                    if (i % 2 == 0) 
                      nxtDigit  = (nxtDigit > 4) ? (nxtDigit * 2 - 10) + 1 : nxtDigit * 2;
                    sum += nxtDigit;
                }
                return (sum % 10);
            }