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8/11/13 Blackboard Learn 1/4

Ap Application 2

Slightly Taxing

You are a programmer for the software development group of a large retailer. Your company has grown

dissatisfied with its current point-of-sale software, because it has historically struggled with the correct

application and calculation of sales tax on an order. Your business deals primarily with storefront business

and its customers are primarily on foot in the store when they buy, so it is of the utmost importance that

sales tax be calculated quickly and accurately. This may sound like a simple task, but remember that sales

tax varies by state and maybe even by item within that state.

Your supervisor has asked you to pull apart the initial version of a possible new sales system, which he

has colorfully but aptly named SaleBad for the sake of illustration (and perhaps his love of Tarzan

movies). After a quick analysis, you sketch out the following UML diagram to describe it.

It is clear that the program does, indeed, need some work. To demonstrate the problems within the

project, you prepare a brief demonstration of its weaknesses.

Task 1: Highlight the Problems. Unzip and open the project SaleBad. Create some Item objects,

a Saleobject, and some SaleLines objects (via the addItem method in SaleBad). Determine how

the total(from the SaleBad object) is calculated and explain why the class needs to be redesigned. In

one or two paragraphs, explain why the current design will not suffice. For the moment, do not worry

about how, exactly, that redesign will happen.

It occurs to you that the code needs to be more loosely coupled. Changes in tax policy on various items,

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or on the tax rate itself, should affect as few classes and methods as possible. As you begin to

conceptualize the new design, your supervisor pops his head around the corner again. Evidently, the

store has been errantly charging sales tax on food items that are specifically supposed to be tax-free in

some states.

You decide that the Item class is the best place to implement sales tax, because the tax on each item

could vary. The state in which the Item is purchased may also affect the tax, so with these thoughts in

mind, you sketch out a new-and-improved UML class diagram, illustrated below.

This diagram is implemented in the project SaleBetter. Despite the improvements, however,

something is bothering you.

Task 2: What’s Bothering You? You had good reasons to implement tax calculations within the Item class

—it makes much more sense than placing it in the other current classes. However, it simply makes more

sense to introduce an entirely new Tax class. In 1 or 2 paragraphs, explain why, citing principles of good

program design.

Now that you have decided that the taxation policies on items should be handled in a completely

separate class, you proudly craft the UML diagram below. Feeling more and more confident, you

implement it in a project called SaleEvenBetter.

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Task 3: Taxation With Class Representation. Open the SaleEvenBetter project and explore how sales

are calculated. In one or two paragraphs, explain how this design improves upon its predecessor.

One final hurdle needs to be cleared before you can mark this project complete: You still need to account

for Items that are not taxed. You sketch the UML diagram below, splitting the Tax class into two

subclasses, PercentageTax and NoTax. You have a plan in place, and your supervisor approves, so it is

time to implement it.

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Task 4: Let Them Eat (Untaxed) Cake. Using the project SaleEvenBetter as a starting point,

implement the new design, as described in the UML diagram above. Verify that the tax method

withinPercentageTax returns the same value as the getPriceWithTax method in the current version

ofSaleEvenBetter for taxed items, whereas the tax method in NoTax returns a value of zero. To

complete Task 4, submit the new version of SaleEvenBetter.