Question

Credit card numbers adhere to certain constraints. First, a valid credit card number must have between 13 and 16 digits. Second, it must start with one of a fixed number of valid prefixes : 1 • 4 for Visa • 5 for MasterCard • 37 for American Express • 6 for Discover cards In 1954, Hans Peter Luhn of IBM proposed a “checksum” algorithm for validating credit card numbers . 2 The algorithm is useful to determine whether a card number is entered correctly or whether a credit card is scanned correctly by a scanner. Credit card numbers are generated following this validity check, commonly known as the Luhn check or the “Mod 10” check, which can be described as follows (for illustration, consider the card number 4388576018402626):

I need help with these two classes

/**
   * Returns true if the supplied string
   * begins with a valid prefix
   *
   * @param s credit card number (assumed to be comprised only of digits)
   * @return true if begins with a valid CC prefix
   */
   public static boolean validPrefix(String s) {

return false; //replace with your own code

}

public static int numDigits(int num) {

return 0;//replace with your own code

}

Answer 1

As the question lacks many relevant information, I have only completed the two methods you asked for, according to my understandings. Comments are included, go through it, learn how things work and let me know if you have any doubts or if you need anything to change. If you are satisfied with the solution, please rate the answer. Thanks

/**

* Returns true if the supplied string begins with a valid prefix

*

* @param s

*            credit card number (assumed to be comprised only of digits)

* @return true if begins with a valid CC prefix

*/

public static boolean validPrefix(String s) {

              // creating a String array containing valid prefixes

              String validPrefixes[] = new String[] { "4", "5", "37", "6" };

              // looping through each element in the above array

              for (int i = 0; i < validPrefixes.length; i++) {

                           // checking if String s starts with current prefix

                           if (s.startsWith(validPrefixes[i])) {

                                         // valid

                                         return true;

                           }

              }

              return false; // invalid

}

/**

* returns the number of digits in a number

*

* @param num

*            input number

* @return number of digits

*/

public static int numDigits(int num) {

              int c = 0; // initializing count of digits to 0

              // looping until all digits are counted

              do {

                           // removing last digit

                           num = num / 10;

                           // incrementing count

                           c++;

              } while (num > 0); // loop ends when num becomes 0

              return c;// returning count

}