Connie's Convenience Store - About Peter Coad

48 Connie’s Convenience Store Establishing Responsibilities for Problem-Domain Objects
What about total? Here’s the strategy:
#66. “Calculable Result” Strategy establishing responsibilities /
what I know (consider and challenge)
• An attribute for holding a calculable result? At first, don’t show it in the object model.
• At some point, you may want to add the attribute. Here’s how:
Add an explicit “get ” service for it.
Specify that service so it:
checks for an attribute value—and returns it, if it has one
otherwise, invokes a calculate service, saves the result, and returns the result
to the sender.
Add an explicit “calculate ” service, too—so you can tell the object to do the
calculation you want it to do.
You could add “total” as an attribute and “get total” as a service. Here, though, keep
it simple and just plan on adding a “calculate total” service to your model.
Establish responsibilities: “who I know.”
In general, who does a transaction object know?
#76. “Establish Transaction Object establishing responsibilities /
Connections” Strategy who I know (pattern players)
• For a transaction, include object connections to: participants, transaction line items, a
companion transaction, one that occurs at the same moment in time.
• For a transaction line item, include object connections to: the transaction, a companion
“item description” object, a subsequent transaction line item.
How about this time, for a sale? Well, previously, using patterns, you discovered
who a sale knows:
– session
– payment
– sale line items.
That’s that.