Connie's Convenience Store - About Peter Coad

56 Connie’s Convenience Store Applying Patterns: Establish Problem-Domain Responsibilities
Collection-worker
Consider the fundamental pattern of object modeling: collection-worker.
#1. “Collection-Worker” Pattern the fundamental pattern
• Collection-worker is the fundamental object-model pattern.
• All other object-model patterns are variations on this theme.
• Typical object interactions
howMany —> calcForMe calcOverWorkers —>calcForMe
howMuch —> calcForMe rankWorkers —>rateMe
• Other notes
“aboutMe” helps one think about what other attributes might be needed
“calcForMe” helps one think about what specific calculations might be needed
“rankMe” helps one think about what ordering or comparrison services might be
“rateMe” helps one think about what self-assessment services might be needed
A collection object knows about some number of workers.
A collection object only does those things which apply across its collection of
workers, e.g., it calculates, rates, and selects.
A worker does all that it can do with what it knows, or with what others may tell it
in combination with what it knows.
How do you find places to apply a pattern? Just look for potential pattern players!
In this case, look for objects that know some number of objects. Such a “knowing”
is indicated with an object connection (or a special kind of object connection, a wholepart
connection) that has an upper-bound constraint of “many.”
Many patterns are collection-worker patterns. Apply some here.
Participant-transaction
number
name
date
time
status
aboutMe
Collection
howMany
howMuch
rankWorkers
calcOverWorkers
calcForMe
n 1
number
name
aboutMe
Worker
calcForMe
rateMe
Okay, you worked with this one a bit earlier. Consider cashier and session. A cashier
object knows all of the sessions it has participated in. So a cashier object is the logical
place to put any service that needs to do something across such a collection of sessions.
For example, you could ask a cashier object to calculate the quantity sold over a specified
interval. You could also let a cashier object calculate its total amount of sales made
over a specified interval. Hence: