Connie's Convenience Store - About Peter Coad

46 Connie’s Convenience Store Establishing Responsibilities for Problem-Domain Objects
Cash drawer
Establish responsibilities: “what I know.”
What does a cash drawer object know? It knows its:
– balance
– position (open, closed)
– operational state (okay, jammed, missing).
An important note about operational state
Whenever an object represents an interacting system or device, it needs to know its “operational
state.” You’ll need to add service descriptions that portray state-dependent behavior
(keywords: precondition, trigger, terminate, postcondition).
Establish responsibilities: “who I know.”
A cash drawer object knows its:
– register.
Establish responsibilities: “what I do.”
What does a cash drawer do?
– open.
When you tell a cash drawer object, an abstraction, to open itself, what happens?
The cash drawer object sends an electrical signal to the actual cash drawer device, activating
it, so it will open.
(Peter) Hey, Connie, how about a cash drawer that closes automatically?
(Connie) Nice idea, especially if someone leaves their drawers open in public.
(Sorry about that.) Anyway, automatically closing cash drawers are not available.
Go ahead and add cash-drawer responsibilities to your model (Figure 1–30):
Register
1 1
CashDrawer
operationalState
position
balance
Figure 1–30: Cash drawer: “what I know; who I know; what I do.”
Tax category
Establish responsibilities: “what I know.”
A tax category object knows its category, its rate, and its effective date.
Note: a combination of category and rate values might occur again and again, on
various dates. This makes it a good candidate for an item-specific item pattern (although
this is not pursued further in this chapter).
open