Connie's Convenience Store - About Peter Coad

Establishing Responsibilities for Problem-Domain Objects Connie’s Convenience Store 49
Establish responsibilities: “what I do.”
#96. “Establish Transaction Services” Strategy establishing responsibilities /
what I do (pattern players)
• For a transaction, include: calculate for me, rate me, is .
• For a transaction as a collection, include: how many, how much, rank transaction line
items, rank subsequent transactions, calculate over transaction line items, calculate over
subsequent transactions (plus services to enforce business rules across that collection).
• For line items, include: calculate for me, rate me.
So what about for this point-of-sale system? What useful things could a sale do?
How about:
– calculate subtotal for the sale (without tax)
– calculate the discount for the sale
– calculate tax for the sale
– calculate total for the sale (total amount due with tax)
– commit.
Add sale responsibilities to your object model (Figure 1–32):
Sale
date
time
calcSubTotal
calcDiscount
calcTax
calcTotal
commit
1-n
1
SaleLineItem
Session
Figure 1–32: Sale: “what I know; who I know; what I do.”
Sale line item
Establish responsibilities: “what I know.”
In general, what does a transaction line item know? Usually, it knows:
– a date and time, if needed for each line item
– quantity
– tax status (regular, resale, tax-exempt, and the like).
1
n
n
1
Payment