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

82 Abstraction of a Problem Domain
4.6.3 Aggregate Object
Object objB
(Part)
Aggregate
object
(Whole)
Object objC
(Part)
Figure 4.19: An Aggregate
An aggregate object (an instance of a whole class), is the ’point of address’ of the aggregate.
Figure 4.19 shows this. The flows to collaborating objects outside the aggregate are
connected to the aggregate object. The parts communicate only with the aggregate
object. The aggregate object only represents the aggregate, it is not the aggregate itself.
The aggregate is the entire composite. Unfortunately this distinction is often neglected in
the world of object-orientation. According to the definition of an aggregate, an aggregate
x
Aggregate
object A
(Whole)
y
Object C
(Part)
a b
c
Object B
(Part)
Figure 4.20: Channel Structure of an Aggregate
object hides part objects. The hiding of parts is achieved by connecting the channels that
connect the aggregate to the objects outside the aggregate solely to the aggregate object,
and not to the part objects. In Figure 4.20 the channels x and y form such connections to
the objects outside the aggregate. The parts C and B do not have a channel to any object
outside the aggregate.