ITEC 120 2008spring ibarland, jdymacek

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hw-ec02-cc
credit card numbers
representing and verifying

due 2008.Apr.06 (Mon)

Credit card numbers are always 16 digits long. To make the number more readable for humans, they they are written as four block of 4 digits, each block separated by a space (or, sometimes, a hyphen).

Frustratingly, many user-unfriendly websites ask users to type in a credit card number without any spaces or hyphens. (This makes it difficult for humans to look at what they've just typed and check for typos, or even if they have the correct number of digits.)

1. (up to 5pts)

   Your task is to write a static method which takes in a String typed into a website, and cooperatively return a credit card number (exactly 16 digits). That is, for each of the following three inputs, your method should return "6011000012345678"

   • 6011 0000 1234 5678
   • 6011-0000-1234-5678
   • 6011000012345678
   You can presume that the string will be in one of the three forms above. The String methods length, substring, and charAt are all likely to be helpful.

   For an extra 5 points, do some error checking on the input: make sure that it is a valid length, and that the separator (if used) is either a space or a hyphen, and is always the same (that is, 6011-0000 1234 5678 is not valid). In case of such an invalid input, your method should print a message to System.err, and return something reasonable.

2. (up to 20pts; requires part A)

   There is more information associated with a credit card than just the account number:

   • the owner's name,
   • the account number (16 digits, above),
   • the company (Visa, Discover, etc.),
   • an expiration date (specified as month and year only; the card will always expire after the last day of the indicated month).
   ¹ You are working for a small business who wants to store customer's credit-card information in their computers.

   Make a class to represent all the above information.
   (Stop here and compile, before proceeding!) Include the public methods (writing signature and test cases before the code, of course):

   • Include getters for all fields, but no setters.
     (Stop here and compile, before proceeding!)
   • A constructor which can takes all the required information; it has to accept the number either as 16 straight digits, or as four groups of 4 digits, as above.
     Create at least two instances and store them in the text fixture; write a test method which verifies the getters are working.
   • String toString(), which returns all of this card's associated information in an easy-to-read format.
     (For example, your company's web server might call this method when they are creating a web page for a customer who is updating their credit-card info.) Use newline characters '\n' as necessary. Test cases are unnecessary for this one method, as there is no one exact correct answer.
   • String number(), which returns the 16-digit account number (no spaces or hyphens);
     (… writing test cases before the body… okay, I'll stop saying that now.)
   • String numberWSpaces(), which returns the account number as four 4-digit blocks separated by spaces;
   • String numberWSHyphens(), which returns the account number as four 4-digit blocks separated by hyphens;
   • String getBlock(int), which takes in a number (0,1,2,3) and returns either the 1st, 2nd, 3rd, or 4th 4-digit block of digits. (This is useful when your company has a customer's credit-card number on file, and you want to set up a web-form which has four already-filled-in boxes for them.)
   • boolean validOn( Date d ) Is this credit card valid on the given date?
   • boolean validNow() Is this credit card valid on right now?
     (Note that code provided for the Date lab has a method today() which looks at the system clock and returns the current day's Date.)

3. (up to 15pts; requires part (A) but not necessarily (B))

   The first digit(s) of a credit card account number actually encode what type of card it is (Visa, Discover, etc.), as described here. Write a static method which takes a credit card number and returns the type of card.

   If you did part (B): Modify your constructor so that it double-checks the stated company against what the account number says it should be. If there is a discrepancy then your constructor should throw new java.lang.IllegalArgumentException( msg ), where you replace msg with some particular String describing the mistake. (This causes the program to crash.²) When comparing Strings, remember η from the homework grading guide.

   Credit card account numbers actually contain a check digit: The first 15 digits are “real” information, but the last digit is contrived so that adding up the 16 digits (with an extra twist described here⁴) always results in a multiple of 10. (You only have to worry about the types of cards which have exactly 16 digits.)

   Write a static method which tells whether or not a (purported) credit card number has a correct check-digit. If you did part (B), modify your constructor so that it throws an exception if the the customer accidentally provided an invalid account number.

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©2008, Ian Barland, Radford University Last modified 2008.Mar.20 (Thu)

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