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

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## 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. 🙂

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!