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

104 Abstraction of a Problem Domain
we constrained the role of object class modelling by not offering inheritance, for the time
being.
We selected two methods, viz. OMT of Rumbaugh et al. [R 91], and Fusion of Coleman
et al. [C 94]), to provide the concepts and symbols needed for object modelling. We
refer to these books for detailed covering of the subject. This section gives only a brief
introduction to object class modelling concepts, and the related fundamental problems.
Only the information that is necessary in the context of this thesis, and the differences
between our approach and theirs are covered. Especially OMT offers, besides definitions,
a lot of very useful guidelines and heuristics that support the actual modelling
activities. Therefore we do not cover heuristics, and refer to their work to learn about
actual ’object modelling’.
4.13.1 Relationship Concept
Relationships, also called associations or (conceptual) relations, exist when objects have
some form of dependency. Relationship is a rather vague concept, that can have various
meanings. For instance objects of a class have to do with, refer to, are linked to, are involved
with, are concerned with, deal with, and are correlated with objects of another (or the same)
class. A relation in an object model is usually shown as a named connection between two
or more object classes in a graphical diagram (see Figure 4.29). A connection denotes that
there is a relationship between object instances of the classes that have the relationship.
A concrete relationship between a pair of objects is called a link.
Candidate relationships can be found by looking at all kinds of dependencies of classes.
Examples are: comparisons, communications, groupings, similarities, containments,
mutual activities and actions. Notice that Object Class Diagrams, in comparison to
Class A Relationship name
Class B
Figure 4.29: Related Classes
Message Flow Diagrams give a rather weak and inadequate view with respect to possible
communications between objects. Message Flow Diagrams show objects that have
concrete relationships, in the sense that they can communicate via concrete message
flows. In contrast, relations in an object model are not concrete. They describe various
diverging concepts that do not necessarily lead to communication between objects.
Relations are very abstract. Relationships between objects may possibly lead to behaviour
when they are broken or created. Object class models are not flow models.
Usually they show various forms of hierarchy relations in one model. Misinterpretation