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

4.6 Aggregates 81
Therefore a complex system can be visualised as a hierarchy, where entities on a particular
level in the hierarchy fall apart into parts on the next lower level. Analysis of
whole-part relations between entities is therefore an important issue for revealing the
structure of a system. This is not only true for the implementation of a system. On the
conceptual level objects appear to have whole-part relations too. During analysis of a
problem domain such relations must be revealed.
4.6.2 Whole-Part Relations
Class B
Attributes
Messages
Class A
Attributes
Messages
Class C
Attributes
Messages
Whole class
Whole-part
symbol
Part class
Figure 4.18: Whole-Part Relation between Classes
Clusters have been introduced in the previous subsection, as a mechanism for hierarchical
composition of finer granules into more coarse granules. Traditional object-oriented
methods do not offer nested clusters. When they need to express that an entity is composed
of objects they define a ’whole- part relationship’ between classes, also called
WPA (Whole-Part Association), or aggregation relation. An aggregation relation is defined
between a whole class and one or more part classes in an Object Class Diagrams. We
disapprove of the name aggregation relation. After all, an aggregate is a special kind of
composite, and a whole-part relation denotes a composite. Figure 4.18 shows a whole
class, called Class A, that has a whole-part relation with both Class B and Class C. Figure
4.19 shows how an aggregate of instances corresponding to the class model of Figure
4.18 can look. The whole object is an instance from the whole class. The part objects objB
and objC are instances of Class B, and Class C respectively. Notice that a whole (aggregate
object) needs its parts. In contrast a part may be instantiated as an independent object,
it does not necessarily need the whole.
An aggregate with many kinds of parts must have many whole-part relationships. By
defining individual whole-part relationships per whole-part class pair we can specify
the multiplicity of each component within the composite. For such details we refer to
[R 91]. A tree of classes with whole-part relations may have more than two levels, so a
part may again be an aggregation of parts, etcetera.