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

4.6 Aggregates 85
An old philosophical question is, whether a whole is more than its parts. Is there a
need for an aggregate object, when parts can be composed into an aggregate. When their
joining is the whole, there is no need for a separate additional object. This intuitive
reasoning may seem to hold, when a system is integrated by composing its parts to a
whole. However a model of an aggregate shows an aggregate object that seems to add
something to the parts. This can be understood by observing that the parts are only
models. They are abstractions. The properties modelled in its class are defined such
that the objects are generic reusable entities. Instances of such a class may be parts of
completely different composites, or may be completely independent entities. Possible
properties of a whole are not distributed to parts. Therefore the joined parts cannot be an
adequate abstraction of an aggregate. An aggregate object is an abstraction of a whole.
Specific properties that characterise solely the whole are modelled in the aggregate object
class. The properties that are encapsulated in the parts do not have to be modelled in
the aggregate object’s class again.
A concrete disagreement on aggregates appears in statements about properties. Rumbaugh
et al. state that ’The most significant property of aggregation is transitivity, that is if
A is part of B and B is part of C, then A is part of C. Aggregation is also antisymmetric, that
is, if A is part of B, then B is not part of A’ [R
91]. Others state that aggregations are not
transitive [Mor94]. Aggregations are not transitive when the semantics of the aggregation
relations differ over the hierarchical levels. Various combinations of properties, that
aggregates can have, show that there are different sorts of whole-part relations.
Coad and Yourdon distinguish three sorts of aggregation [CY91a]:
Assembly-Parts (example: An aircraft is an assembly with four engines).
Container-Contents (example: An aircraft contains a pilot, the pilot is not a part of the
aircraft).
Collection-Members (and their different varieties). (Example: A flight plan is an ordered
collection of flight segments.)
Moreira [MC93] distinguishes two main sorts of aggregation:
Non-shared aggregation (object components are not shared by aggregate objects).
Shared aggregation (object components are shared by aggregate objects)
Besides that she allows aggregates to have a static or dynamic number of components.
Rumbaugh et al. [R
of view:
91] mention sorts of aggregation from a completely different point
Fixed aggregate (a fixed structure; number and type of parts are predefined).
Variable aggregate (finite number of levels, but the number of parts may vary).