Specification of Reactive Hardware/Software Systems - Electronic ...

392 Case Study
The next process class we describe is the Product Info Keeper class (see Figures 12.6 and
12.8). The first part of the specification is as follows:
process class
Product Info Keeper myId ¡
instance variables
myId
productInfo
: PIKeeperId
: PI
Recall that a Product Info Keeper contains zero or one PI instances. The aggregation relation
was shown in the Object Class Diagram of Figure 12.9. This aggregation relation is
implemented by the instance variable productInfo. If the keeper contains no PI instances,
the variable is nil. Otherwise it refers to a data object of class PI.
From Figures 12.6 and 12.8 it is straightforward to deduce the next part of the class
description:
communication channels
pihp prks pisa pipo
message interface
pihp!free(PIKeeperId)
pihp?store(PIKeeperId,PI)
prks?serviceRequired(PIKeeperId,Service)
prks!required
prks!notRequired
pisa?servicePerformed(PIKeeperId,Service)
pipo?retrieve(PIKeeperId)
pipo!take(PI)
A Product Info Keeper can be in one of two behaviour states. The keeper is either free,
or it is occupied by a product info. The two states will be modelled by the two methods
noProductInfoStored respectively productInfoStored. Upon instantiation, a keeper always
starts in state noProductInfoStored. Within this state it tries to send a free message to the
Product Input Handler and waits for the arrival of a new product info (see Figure 12.6).
The acceptance of the free message means that a Product Info Keeper is going to be used
by the receiver of this message. The keeper makes a transition to state productInfoStored
if the new PI is received. The specification is as follows:
initial method call
noProductInfoStored
instance methods
noProductInfoStored id:PIKeeperId
pihp!free(myId);
pihp?store(id,productInfo id=myId);
productInfoStored