Homework 2 Details for MPCS 5141

Each homework assignment will consist of a small problem in some OO code, which is somehow problematic, given your developing understanding of good OO best practices and patterns, you will improve, thus eliminating any negative issues in the code you are given. Each homework assignment is intended to give every student hands-on experience with the core concepts covered during the course.  You may code your solution to the problem in any legitimate OO language of your choice.  You need to submit labs to the TAs for grading--see submission instructions below.  Generally, unless otherwise specified, you will have one week to complete each assigned homework problem.

See the syllabus for information on grading.  Turning in lab assignments on time is required, without exception, and all late deliveries will be penalized, regardless of cause.  Submit your assignments to the subversion repository according to the directions on the syllabus page.

You may write these solutions in any programming language of your choice.  Our suggestion is now is not the time to learn a new programming language along with the concepts themselves.  So our suggestion is to use whatever programming language you  know best.

Homework 2   Due: 5:00 pm, Monday, February 15, 2021

Coding Problem: 

BACKGROUND:

Like all programming-related problems, learning a new programming language or style is not an exercise in reading but rather an exercise in thinking and typing.  This homework is designed to give you  hands-on experience in some fundamental skills involved in object-oriented design and coding.  You will generally find the References section below helpful in addition to the required and recommended reading. 

WHAT YOU NEED TO DO:

 See the code here.  You are to read and understand the code, what it is intending to accomplish, and what the responsibilities are of the class or classes.  Nonetheless, there are one or more problems in the code which require your insight in their amelioration.  Your mission, should you choose to accept it, is to "fix" the problem(s) in the code, without introducing any new problems, by coding a replacement set of code in any legitimate object-oriented programming of your choice.  Be sure to include a README with instructions on how to compile (if necessary) and run your coded solution.

Very important:  Inside comments at the top of your coded solution, you are to fully describe the problem(s) in the original code as you have determined it, and describe why your solution eliminates the original problem(s).

Step One:

By examining the code, you discover that this C# code implements a shopping cart in an online Etailer.  It allows for a few different types of payment methods:  a customer can pay with paypal or with a credit card (the actual implementations of paypal and cc are stubbed out).  Fine.  That's all there was way back in 2001.  You of course didn't write this code.  A guy who no longer (and for good reason!) works for the company, named Gerald, wrote this slop.  Then, yesterday, your boss came to you and said there's this new payment she wants to accept, called ApplePay, and told you to implement it by updating the CallShoppingCart() method in the ShoppingCart.  (Your boss doesn't actually write code, truth be told, although she has crafted a few compelling Excel macros.  She has her MBA from Harvard Business School.  But she knows she's a genius and how hard can this be?  She's been through Agile training and knows that this can be done quickly and easily if you just welcome change and keep it simple.  So she's kind enough to share her wisdom and experience with you by telling you how to fix this problem.)

You spent the evening last night eating pizza from Pizza Capri and researching ApplePay.  This morning, just a minute ago, she came to you, unbelievably, and says she now wants to accept, in addition to ApplePay, something called SEPA, as well as cryptocurrencies like Bitcoin (OMG!).  I told you she was a genius.  Tomorrow, you fully expect her to come in with some new payment method you've never even heard of.  Such are the vicissitudes of life. 

Now, you can, if you wish, implement SEPA and Cryptocurrency support by adding some new "else if" statements in CallShoppingCart if you want.  That's certainly what Gerald would have done.  It's what The GeniusTM wants you to do.  But you're smarter than that.  You know if you add a few new "else if" statements, that's your future passing before you in a flash.  No.  You want to modify CallShoppingCart so you never have to ever touch that code again.  Ever.  No matter how many new methods of payment show up.

Step Two:

Write a replacement set of code in any legitimate object-oriented programming of your choice, rewriting CallShoppingCart() (and associated stub functions) so that it is more generic and will support any new form of payment method that comes along in the future.  Then stub in the ApplePay and SEPA and Bitcoin payments.  And no, you do not have to actually code the implementation of bitcoin payments, just stub them all out in your replacement version of the code.

References:

You may find the following references helpful:

https://en.wikipedia.org/wiki/Strategy_pattern
https://en.wikipedia.org/wiki/Open–closed_principle

Submitting:

Use the folder named "hw2" in your Subversion repository. See the syllabus for more info about submission using Subversion. Upload your HW2 solution file(s) and any supporting materials to the repo.