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

4.12 Object Variety 101
Part class
Class B
Attributes
Messages
or that subtype [R
Class A
Attributes
Messages
91].
Class C
Attributes
Messages
Whole
class
Whole-part
symbol
4.11.2 Hierarchy of Subsystems
Class B
Attributes
Messages
Class A
Attributes
Messages
Figure 4.28: Different Class Hierarchies
Class C
Attributes
Messages
Superclass
Specialisation
symbol
Subclass
A system composition is visualised as a hierarchical tree of modules (subsystems). A
composite can be modelled as a cluster. Clusters offer visual hiding of composites at
the various hierarchical levels of a model. A module (subsystem) can be a cluster or a
process object. (See Figure 4.16 in Section 4.5). We visualise (nested) clusters in Object
Instance Diagrams, and not in Object Class Diagrams.
Although clusters are described in classes, they are not visualised in Object Class Diagrams.
Whole-part relations between object classes in Object Class Diagrams show
conceptual relations. Aggregates and clusters in Object Instance Models show actual
relations. Actual composites are instances of conceptual composites that are denoted
as whole-part relationships in Object Class Diagrams. Object Instance Models offer the
possibility to integrate design aspects into a model. Clusters can visualise instances
of conceptual relations as well as topological constraints derived from architecture and
implementation considerations.
4.12 Object Variety
Objects can represent a wide variety of concepts and entities. During analysis objects
may come from various sources. Coad and Yourdon mention the following items that
must be looked for to find classes and objects: ’structures, other systems, devices, things
or events remembered, roles played, operational procedures, sites, and organisational units.’
This leads to a natural variety of objects that are originating from the problem domain
analysis. Traditional object-oriented methods apprehend this variety by defining classes
and the relations between them.