Connie's Convenience Store - About Peter Coad

64 Connie’s Convenience Store Working Out Problem-Domain Dynamics with Scenarios
What’s being expressed is this (Figure 1–42):
Sale SaleLineItem
Figure 1–42: Picturing messages to some number of objects in a class.
So far, so good.
What’s next? What other sale service needs your attention? It’s time to calculate tax.
(Peter) How do you calculate the tax for a sale?
(Connie) Actually, tax collection varies from country to country. Here’s what we do
in the States. We calculate the subtotal for each tax category. Then we apply the tax rate
to each tax category. Finally, we add up the results, to get the tax for a specific sale.
(Peter) Yuck!
Tax matters always complicate things. Welcome to the real world.
Sketch it out:
A sale asks a store for its tax categories.
A sale tells a sale line item to calculate its tax for a specified tax category.
A sale line item tells its item to calculate its amount in a tax category.
An item gets its price for that date.
A sale asks a tax category for its rate.
You’ve got some new players: tax category and store (store acts as a collection of
tax categories).
Expand the scenario view accordingly (Figure 1–43):