Connie's Convenience Store - About Peter Coad

Establishing Responsibilities for Problem-Domain Objects Connie’s Convenience Store 53
Payment and kinds of payments
What about payments and the three kinds of payments: cash, check, and charge?
Consider what’s the same, what’s different.
Establish responsibilities: “what I know.”
What’s the same? Each payment knows its:
– amount paid.
What’s different?
Each cash object knows its:
– cash tendered.
Each check object knows its:
– bank routing number
– account number
– amount tendered
– authorization code.
Each credit object knows its:
– card type
– card number
– expiration date
– authorization code.
Note that two specializations, check and credit, have a common attribute: authorization
code. If these attributes are the same in name and in meaning, and if you can
add a domain-based generalization, add a new generalization to the gen-spec structure.
Here it is:
payment
cash
authorized payment
check
credit.
By doing this, you explicitly express commonality (Figure 1–35). Good show!
Establish responsibilities: “who I know.”
What’s the same? Each payment knows its corresponding:
– sale.
(And yes, a sale knows about its corresponding payments. Remember, at Connie’s,
a customer may make several payments, with some combination of cash, check, and
charge.)
Okay, what’s different for the kinds of payments? Nothing comes to mind here. Onward!