Connie's Convenience Store - About Peter Coad

Working Out Problem-Domain Dynamics with Scenarios Connie’s Convenience Store 63
The “calcSubtotal” sends a “calcSubtotal” message to a sale line item
object (it repeats this for all of its sale line items).
A sale line item is told to calculate its subtotal.
The “calcSubtotal” service sends a “howMuchForQty” message to an
item object, giving it a quantity and a date. 4
An item object is told to calculate its amount for quantity.
The “howMuchForQty” service invokes the “getPriceForDate”
service, giving it a date.
The “getPriceForDate” service returns a price.
The “howMuchForQty” returns an amount.
The “calcSubtotal” service returns a subtotal.
The “calcSubtotal” service returns “subtotal.”
The “calcTotal” service invokes the “calcTax” service.
The “calcTax” service returns “totalTax.”
The “calcTotal” service returns “total.”
Here’s another scenario view convention:
– The “n” on a message arrow indicates that a message may be sent to
some number of objects (Figure 1–40).
So when you see (Figure 1–41):
Sale SaleLineItem
n
Figure 1–41: Sending a message to some number of objects.
4Note that “how much for qty” requires that the quantity be passed to it, as an argument. One
might cry out, “just ask for the value and reduce the argument coupling.” But wait; argument
coupling is not the full story here. Consider the following. Would it be generally useful for an
item to be able to calculate “how much for qty?” Yes. And as the business rules become more
complicated, what object will eventually know enough so it can calculate quantity discounts for
a particular item? An item object (line items never need to know about detail like that).
Very cool.