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

100 Abstraction of a Problem Domain
Object C
(Part)
Sensor 1
(terminator)
Aggregate
object
(Whole)
Cluster
system boundary
Virtual cluster
Object B
(Part)
Actor 1
(terminator)
Figure 4.27: Aggregate and System Boundary
We visualise instances of these classes in an Object Instance Model. Besides aggregates
we offer clusters to create a hierarchy of subsystems. In this section we summarise the
relation between the concepts: object, aggregate, composite, cluster and hierarchical
structuring.
4.11.1 Hierarchy in Class Models
Object-oriented methods usually present analysis results graphically in so called object
class models (in contrast to instance models). It is confusing that both whole-part
relations and subtype/supertype relations are represented as trees, see Figure 4.28. The
triangle symbol and the diamond symbol distinguish both relationships, however they
do not directly catch the eye. Notice that the two sorts of trees denote completely
different concepts.
Whole-part relations denote that objects may be composed or aggregated. Subtype/supertype
relations denote common properties (generalisation). They can be used
for an efficient notation of properties. Rumbaugh et al. [R 91] notice the following differences
between aggregation and generalisation. Aggregation relates instances of distinct
objects, one is a part of the other, whereas generalisation relates classes and is a way of structuring
the description of a single object. Aggregation is sometimes called an ’and-relationship’, and
generalisation an ’or-relationship’. An aggregation is this and that object, but an object is this