Connie's Convenience Store - About Peter Coad

20 Connie’s Convenience Store Applying Patterns: Select and Organize Problem-Domain Objects
Apply the participant-transaction pattern (Figure 1–9):
Cashier
Register
Figure 1–9: Cashier, register-session.
Note: the connecting lines are object connections. An object connection is a graphical
attribute, showing that one object knows about some number of other objects.
Note: the markings at the end of each object connection are called object connection
constraints; each constraint is placed next to each object, expressing what it knows
about other objects. Here, a session knows one cashier and one register; a cashier object
knows some number of session objects; a register object knows some number of session
objects, too (Figures 1–10 and 11):
Cashier Session
Figure 1–10: An object connection with a constrant.
Cashier
n
n
n
Session
Session
Figure 1–11: What an object connection and its constraint represents.
Another note: this is just the opposite of the entity-relationship convention for what
that approach calls “multiplicities.” Why? There is a good reason for this. (Actually, the
credit for this goes to our clients and seminar participants. They asked us again and
again: if you’re constraining a session, why are you marking it on the other end of the
line? That got us thinking. And it helped us to change.) You see, entity-relationship
markings reflect “table think”: one row here keys to some number of rows there. Some
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