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Establishing Responsibilities for Problem-Domain Objects Connie’s Convenience Store 43
“Taxable” is a “yes/no” attribute, that is, it has the value “yes” or “no.”
Check it out. “Taxable” is the name of a value: an item is “taxable” or “nontaxable.”
Using a value as an attribute name is boring; it’s a “yes/no” kind of thing. Moreover,
using a value as an attribute is unwise; as soon as you’ve got additional values, you’re in
trouble; at that point, your object model will become more complex than it needs to be.
Think about what happens at a store you visited recently. Are all items taxed at the
same rate? Or do special taxes apply to certain categories of items?
That’s it! Some categories of items may be taxable, while others are not. Instead of
taxable, what an item object really needs to know is its tax category. Model that “need
to know” with an object connection to tax category.
Carefully examining each attribute does pay off. And writing attribute descriptions
certainly gets one to consider that kind of detail. Based upon the added insight, here’s
what item looks like now (Figure 1–27):
Item
number
description
shortDescription
Figure 1–27: Item: “what I know.”
Establish responsibilities: “who I know.”
Who does an item object know?
It knows:
– UPC (zero or more).
– price (zero or more).
– tax category (zero or more).
– sale line item (zero or more).
Yes, each UPC, price, and tax category object knows its items. A sale line item
knows its item, too (Figure 1-28).
Establish responsibilities: “what I do.”
Why do you need this abstraction, an item object?
Certainly one reason is to remember the values of the attributes of an item.
Yet there is a far more important reason why you might add an item object. And
that reason is to capture some system dynamics. What does the system really need to do
regarding its items?
Hopefully, an item is something more than just a data holder. Look for something
more than that.
What can this abstraction do for you?
Apply the “do it myself” strategy. Here it is again (it’s important):