Question

Validating Credit Card Numbers

Write a program named Creditcard.java that prompts the user for a credit card number and determines whether it is valid or not. (Much of this assignment is taken from exercise 6.31 in the book)

Credit card numbers follow certain patterns. A credit card number must have between 13 and 16 digits, and must start with:

• 4 for Visa cards
• 5 for Master cards
• 6 for Discover cards
• 37 for American Express cards

The algorithm for determining whether a card number is entered correctly as developed by Hans Luhn of IBM in 1954. The Luhn check or Mod 10 check works as follows. Consider the card number 5842016792622547.

1. Double every second digit from right to left. If the doubling of a digit results in a two-digit number, add up the digits to get a single-digit number. The digits are shown in bold and italic here:

   5842016792622547

   This gives us (from right to left):

   • 4 * 2 = 8
   • 2 * 2 = 4
   • 6 * 2 = 12 → (1 + 2) = 3
   • 9 * 2 = 18 → (1 + 8) = 9
   • 6 * 2 = 12 → (1 + 2) = 3
   • 0 * 2 = 0
   • 4 * 2 = 8
   • 5 * 2 = 10 → (1 + 0) = 1
2. Add up those numbers (8 + 4 + 3 + 9 + 3 + 0 + 8 + 1 = 36)
3. Now add up all the digits in the odd positions from right to left: (7 + 5 + 2 + 2 + 7 + 1 + 2 + 8 = 34)
4. Add those two numbers (36 + 34 = 70). If this sum is divisible by 10, the number is valid. If it is not divisible by 10, the number is not valid.

Your program will prompt the user to enter a credit card number as a long integer and display whether the number is valid or not. Design your program to use the following methods:

/** Return true if the card number is valid */
public static boolean isValid(long number)

/** Return the sum of the doubled even-place digits */
public static int sumOfDoubleEvenPlace(long number)

/** Return the given number if it is a single digit,
 *  otherwise return the sum of the two digits */
public static int getDigit(int n)

/** Return the sum of the odd-place digits */
public static int sumOfOddPlace(long number)

/** Return the number of digits in the given number */
public static int getSize(long number)

/** Return the first k number of digits from number. If the
 *  number of digits in number is less than k, return number. */
public static long getPrefix(long number, int k)

You may also implement this function if you feel it is useful for your solution:

/** Return true if d is a prefix for number */
public static boolean prefixMatched(long number, int d)

Here is the result of running the program a few times:

Enter credit card number: 412345678
That is not a valid credit card number.

Enter credit card number: 5842016792428358
That is a valid credit card number.

Enter credit card number: 5782077482835719
That is not a valid credit card number.

Extra Challenges

Challenge 1: If the card is valid, display which type of card it is. For example:

Enter credit card number: 5842016792428358
That is a valid Master card number.

Challenge 2: Repeatedly ask for credit card numbers until the user just presses ENTER. (Hint: while)

Answer 1

Code:

import java.util.*;
class Creditcard{
   static int sw;
   public static void main(String args[]){
       Scanner s=new Scanner(System.in);
       String si[]={"Visa","Master","Discover","American Express"};
       while(true){
           System.out.print("Enter a credit card number:");
           String xi;
           xi=s.nextLine();
           if(xi.isEmpty()){
               break;
           }
           else{
               long x=Long.parseLong(xi);
               if(isvalid(x)){
                   if(sw==4){
                       System.out.println("That is a valid "+si[0]+"card number.");
                   }
                   else if(sw==5){
                       System.out.println("That is a valid "+si[1]+"card number.");
                   }
                   else if(sw==6){
                       System.out.println("That is a valid "+si[2]+"card number.");
                   }
                   else{
                       System.out.println("That is a valid "+si[3]+"card number.");
                   }
               }
               else{
                   System.out.println("That is not a valid credit card number.");
               }
           }
           System.out.println();
          
       }
   }
   public static boolean isvalid(long number){
       int i=getSize(number);
       long sa,sb;
       if(i>12 && i<17){
           if(prefixMatched(number,4) || prefixMatched(number,5) || prefixMatched(number,6) || prefixMatched(number,37)){
               sa=sumOfDoubleEvenPlace(number);
               sb=sumOfOddPlace(number);
               if((sa+sb)%10==0){
                   return true;
               }
               else{
                   return false;
               }
           }
           else{
               return false;
           }          
       }
       else{
           return false;
       }
   }
   public static long sumOfDoubleEvenPlace(long number){
       int i=1;
       long r,sum=0;
       while(number>0){
           if(i%2==0){
               r=number%10;
               sum=sum+getDigit(2*r);
           }          
           number=number/10;
           i++;
       }
       return sum;
   }
   public static long getDigit(long n){
       long s=0;
       if(n<10){
           return n;
       }
       else{
           while(n>0){
               s=s+n%10;
               n=n/10;
           }
           return s;
       }
   }
   public static int getSize(long number){
       int i=0;
       while(number>0){
           number=number/10;
           i++;
       }
       return i;
   }
   public static long sumOfOddPlace(long number){
       int i=1;
       long d,sum=0;
       while(number>0){
           if(i%2!=0){
               d=number%10;
               sum=sum+d;
           }          
           number=number/10;
           i++;
       }
       return sum;      
   }
   public static long getPrefix(long number,int k){
       long s=1,i;
       for(i=1;i<getSize(number)-1;i++){
               s=s*10;
       }
       if(k==1){
           return number/(s*10);
       }
       else{
           return number/(s-1)*10;
       }
   }
   public static boolean prefixMatched(long number, int d){
       int i=(int) getPrefix(number,getSize(d));
       if(i==d){
           sw=d;
           return true;
       }
       else{
           return false;
       }
   }

}

Output: