Recently I came to know that **Credit Card numbers** are not random and passes **Luhn Algorithm** test.

Table of Contents

## Java Credit Card Validation

Any credit card number should pass following test:

- From the rightmost digit, we should double every second digit. If the double is greater than 9, then add the both digits so that final number is of single digit.
- Now sum all the digits in the number, the unchanged numbers and the doubled numbers.
- The final sum should be multiple of 10 or mod 10 of the number should be 0. If it’s not then its not a valid credit card number.

Let’s check it with an example credit card number 12345678903555.

Digits are : 1,2,3,4,5,6,7,8,9,0,3,5,5,5

After doubling : 2,2,6,4,1,6,5,8,9,0,6,5,1,5

Sum of digits : 2+2+6+4+1+6+5+8+9+0+6+5+1+5 = 60 = 6*10 and hence a valid credit card number.

### Luhn Algorithm in Java

Here I am providing java Luhn Algorithm program to validate credit card numbers.

```
package com.journaldev.util;
public class JavaLuhnAlgorithm {
public static void main(String[] args) {
validateCreditCardNumber("12345678903555");
String imei = "012850003580200";
validateCreditCardNumber(imei);
}
private static void validateCreditCardNumber(String str) {
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) {
System.out.println(str + " is a valid credit card number");
} else {
System.out.println(str + " is an invalid credit card number");
}
}
}
```

Here I am taking input to the validation method as String, so that it will work also where the first digit is 0.

Output of the above program is:

```
12345678903555 is a valid credit card number
012850003580200 is a valid credit card number
```

Note that mobile phone IMEI number also follows the Luhn algorithm, go ahead and test it for your credit card numbers. 馃檪

This did not cover the part when the doubling of number leads to 2 digit number.

So I would say the program is half complete.

I think you didn’t read it properly. I have clearly mentioned that if the sum is greater than 9, then add them to get the single digit number. Also, see the code in

`if (j > 9)`

block where it’s implemented.This is very clear and concise. Thank you!