Connie's Convenience Store - About Peter Coad

16 Connie’s Convenience Store Selecting Problem-Domain Objects
Sale
One of the transactions is sale. Check it out.
(Peter) Connie, what are the main things you do with a customer?
(Connie) We sell items.
(Peter) Do you have a special name for that?
(Connie) A special name? “Sale.” (Now isn’t that special.)
(Peter) Does it include only items you sell to a customer?
(Connie) Now that you mention it, a sale may include both sales and returns.
Hold it! The domain vocabulary is “sale.” Someone on our team might argue that
you should use a more accurate term, “sales transaction.” Advice? Don’t do it. Key
point: don’t ever change domain vocabulary unless your customer sees the need for and
wants to make the change (otherwise you’ll be forever mapping her words to your
words; that’s both counterproductive and a real nuisance).
Most transactions have parts, called transaction line items. A sale is no exception.
A sale is a collection of some number of sale line items. Add the classes: sale and sale
line item (Fig. 1-8).
Kinds of sales
#34. “Select Kinds of Objects” Strategy selecting objects (kinds of objects)
• Use generalization-specialization (gen-spec) to find additional classes.
Look at each class as a generalization. Name its specializations, ones that fit with
system objectives.
Look at each specialization. Name its generalizations, ones that fit within system
objectives.
• Use gen-spec for domain-based reuse within an application; use configuration
management for reuse across multiple applications, even multiple problem domains.
• Use gen-spec to show domain-based “what’s the same, what’s different,” within an
application.
• Examples: equipment, kinds of equipment; participants, kinds of participants;
transactions, kinds of transactions.
Consider the kinds of sales:
– sales, returns.
Do you need to distinguish between a sale and a return?
(Peter) What’s the difference between a sale and a return?
(Connie) The only difference is whether the amount is positive or negative.
You don’t need to add classes for both “sale” and “return.” At this point, the system’s
responsibilities for them appear to be identical. The only difference is whether the
amount is positive or negative.