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Establishing Responsibilities for Problem-Domain Objects Connie’s Convenience Store 47
Establish responsibilities: “who I know.”
A tax category object knows its:
–items.
(And yes, an item knows its tax categories.)
Establish responsibilities: “what I do.”
What does a tax category do? Just the basic services: get, set, add (a connection), remove
(a connection), and delete.
Add tax category responsibilities to your model (Figure 1–31):
TaxCategory
category
rate
effectiveDate
Figure 1–31: Tax category: “what I know; who I know; what I do.”
Responsibilities for problem-domain transactions
Transactions are moments in time in which objects interact, moments in time that a system
is responsible to remember for some future need.
Investigate your transactions:
– sale (and its part, sale line item)
– payment
– session.
Sale
Establish responsibilities: “what I know.”
n n
#54. “Establish Transaction Attributes” Strategy establishing responsibilities /
what I know (pattern players)
• For transactions, include: number, date, time, status.
• For line items, include: quantity (if it knows an item, rather than a specific item), status.
What do you need here, for a sale? In this case, add these two attributes:
– date and time.
Item