Connie's Convenience Store - About Peter Coad

44 Connie’s Convenience Store Establishing Responsibilities for Problem-Domain Objects
#86. “Do It Myself” Strategy establishing responsibilities /
what I do (fundamentals)
• This is an aspect of a software object coming to life: “I do those things that are normally
done to the actual object that I’m an abstraction of.”
• Here’s why. “Doing it myself” encapsulates attributes with the services that work on those
attributes. That results in lower coupling and stronger cohesion.
• If you are building a simulation system, then a software object will mimic what an actual
object does. In most systems, this is not so. Software objects do those things that the
system is responsible to do with regard to that object.
Consider the actions done to a real item. Then turn them around, letting your abstraction,
the one that comes to life in software, actually do those actions to (or for) itself.
List some of the things done to an actual object (in this case, an item):
– figure out the price for a given date- calculate a total for a given number
of items.
So let the item abstraction do something useful for you:
– get price for date
– how much for quantity.
You could just ask for the price. Yet “how much for quantity” is a good way to encapsulate
quantity-sensitive pricing for an item.
Add item responsibilities to your object model (Figure 1-28):
SaleLineItem
Item
1 n
number
description
shortDescription
n 1
getPriceForDate
howMuchForQty
n n
1 n
UPC TaxCategory
Figure 1–28: Item: “what I know; who I know; what I do.”
Price