Connie's Convenience Store - About Peter Coad

14 Connie’s Convenience Store Selecting Problem-Domain Objects
(Note: a register consists of a number of data entry devices: a mag stripe reader, an
item scanner, a keypad, and a receipt printer. You don’t need these in your object
model; all of these are just data entry device “nuts and bolts.”)
Kinds of things, too
You might be interested in categories of items, such as:
– perishability: perishable, nonperishable
– taxability: taxable, nontaxable
– type: dairy, produce, canned goods, frozen foods.
Should you add some additional classes, representing these different kinds of
items?
Here’s how. For each category, consider the following:
– If you don’t care about the distinction, forget about it.
– If all you care about is the category value, add an attribute.
– If a category implies added attributes, then add a specialized class; put
the added attributes in it.
– If a category implies different services, then add a specialized class; put
the added services in it.
In this case, based upon the system features, you don’t really care about the distinctions.
Just add item, for now.
Descriptive things
#19. “Select Items and Specific Items” Strategy selecting objects (pattern players)
• Look for items, descriptive objects with values that apply to some number of specific
items and actions that apply across those specific items.
• Examples: aircraft–specific aircraft, loan description–specific loan, job
description–specific job, video description–videotape, price category item–specific item,
tax category item–specific item.
What kinds of catalog descriptions or descriptive tables of values do you need to do
business?
In this system, it’s tax tables. Yuck!
Anyway, you really do have to keep track of tax categories, and the taxable rate that
applies for each one. Add a tax category object, so you can keep track of each category,
its rate, and its effective date. Add your “tangible things” and “descriptive things” to
your object model (Figure 1–6):