Connie's Convenience Store - About Peter Coad

40 Connie’s Convenience Store Establishing Responsibilities for Problem-Domain Objects
#64. “Attribute with Repeating establishing responsibilities /
Values” Strategy what I know (consider and challenge)
• An attribute that may have a number of values?
• Add a new class to your model. Add an object connection. Look for additional attributes
that may apply.
Add a new class, even if it has only one attribute for now.
Why: make the most of problem-domain partitioning; pave the way for
accommodating future change gracefully.
• Example: Item with the attribute UPCs. Add a UPC class. Add an object connection
between an item and its UPCs.
If you potentially could have attributes for each repeating value, then go ahead and
split what’s being described as another class in your model. It’s domain-based partitioning.
Even if the new class has only one attribute, go ahead and add it anyway. As you
find a need for additional attributes and services, you’ll already have this domain-based
partitioning in your model. And that’s a good deal.
Here, UPC is the repeating value. An item in Connie’s Convenience Store may correspond
to many manufacturer-issued UPCs. Could you have potentially many attributes
for each UPC? Absolutely. A UPC could be described by its code, its bit-map
image, its originator, and its date of first usage.
Right now, within this system’s responsibilities, suppose that all you care about is
the UPC itself. No matter. Go ahead and add a UPC class, with the attribute “upc.”
How about an object connection, too? Yes, you need one. An item knows its UPC
objects. And a UPC object knows its items. Connect item and UPC with an object connection
(Figure 1-26).
Consider price
An item object knows many prices. Hey, you don’t believe it? Check it out with Connie!
(Peter) Tell me about the prices of an item.
(Connie) An item has a regular price, beginning on an effective date; we keep track
of past, present, and future regular prices. In addition, an item often has a promotional
price, good from an effective date through an expiration date; we keep track of past,
present, and future promotional prices. We pride ourselves in giving our customers the
best price that applies at the time of purchase.
(Peter) Do you plan prices in advance? When would you add them to the system?
(Connie) We plan ahead. And we definitely need to enter the new prices in advance
of their effective dates.
So what does all of this mean to your object model?
An item knows its price objects.
An item knows its regular prices (with price and effective date).
An item knows its promotional prices (with price, effective date, and expiration
date).