Connie's Convenience Store - About Peter Coad

32 Connie’s Convenience Store Establishing Responsibilities for Problem-Domain Objects
Participant: cashier
Establish responsibilities: “what I know.”
#50. “Select Attributes from Actual Ones” Strategy establishing responsibilities /
what I know (fundamentals)
• This is an aspect of a software object coming to life: “I know selective things that describe
the actual object that I’m an abstraction of.”
• Select the attributes for your software objects, taken from the abundance of the attributes
that could be used to describe a corresponding actual (real-world) object.
• Consider attributes from a broad-sweeping perspective; then work your way into the
specific ones you need. Look at attributes in general, in this domain, and ultimately just
within this system’s responsibilities.
Okay. What do you need to know about each cashier?
– number
– password
– authorization level
and, as discussed earlier in the chapter, an attribute to distinguish that one session has
special meaning, namely:
– current session.
Model it (Figure 1–21):
Cashier
number
password
authorizationLevel
currentSession
Figure 1–21: Cashier: “what I know.”
Establish responsibilities: “who I know.”
Who does a cashier object know?
It knows its sessions, the ones it participates in. (And each session knows its
cashier.)
It knows its corresponding person object, too (Figure 1–22):