Connie's Convenience Store - About Peter Coad

38 Connie’s Convenience Store Establishing Responsibilities for Problem-Domain Objects
It also needs to know its transactions—so it can answer questions and perform calculations
across its collection of transactions:
– sales made in the store (zero or more).
It also needs to know some descriptive objects:
– tax categories that apply to the store (zero or more).
(From the other direction, each register, item, cashier, and sale knows a corresponding
store. A sale knows its corresponding store. A tax category knows its corresponding
stores.)
Establish responsibilities: “what I do.”
#95. “Establish Place Services” Strategy establishing responsibilities /
what I do (pattern players)
• For a place, include: calculate for me, rate me, is .
• For a place as a collection, include: how many, how much, rank transactions, rank
contents, rank container line items, calculate over transactions, calculate over contents,
calculate over container line items (plus services to enforce business rules across that
collection).
What useful things can a store object do, to support a point-of-sale system?
A couple of things come to mind. How about:
– get item for UPC
– get cashier for number.
What useful things can a store object do, with calculations across its collection of
objects? It can do plenty. As you investigate the content for each window and report,
you can add services that span “across the collection of things in a store” to store itself.
Add store responsibilities to the model (Figure 1–24):
n
Store
name
getItemForUPC
getCashierForNumber
Register Item Cashier
n
1 1
1
n n
n
n
Sale
Figure 1–24: Store: what I know, who I know, what I do.
1
TaxCategory