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

4.13 Object Class Models 105
of object models is presumably one of the largest problems in learning and understanding
object-oriented analysis methods.
Complex reactive and control-oriented systems tend to have a large artificial part that
must be ’designed’ and cannot simply be ’found’ in the problem domain. Object class
modelling is clearly a modern form of data analysis, directed to classifying data as
objects in the problem domain. Most practical systems are a mix of data-oriented parts
and control-oriented parts. Therefore an inventory of relationships will probably be
necessary to make an analysis complete.
We experienced it very hard to find adequate relationships. It is not an easy way to
completeness. A major problem is the vagueness of the relationship concept. Even if
one finds a relation, one still does not know what it is. A relationship may appear to be
an object. A relationship may also become an attribute or an operation, or may appear
to be superfluous. The traditional object-oriented modelling approach did not give
the feeling to make progress when applied to analysis and design of complex reactive
systems. It gave the feeling of using philosophy instead of craftsmanship for the creation
of a product.
4.13.2 Multiplicity
Cardinality constraints specifies the multiplicity of, i.e. ’how many’ instances of one class
can be associated with a single instance of a related class. OMT defines a notation to
denote multiplicity. We refer to [R 91] for details. Besides ’one’ and ’many’, OMT allows
the specification of numerical values, intervals, and sets of intervals in the diagrams at
the end of a relationship symbol.
4.13.3 Invariants
Besides qualifiers that constrain multiplicity, there may be constraints on the relationship
as a whole, or even across relationships. Textual annotations in the diagram, called
invariants can be used to describe such constraints.
4.13.4 Relationship Attributes
Properties of objects are modelled as attributes in classes. Links between objects can
have properties too. These can be modelled as attributes of relationships. It depends on
the multiplicity of a relationship how these attributes are implemented in objects. They
may become attributes of objects on either side. An alternative is that a relationship
becomes an object class itself. In this case the attributes of the relationship become
attributes of this class. If the name of the relationship is a verb, the name of such a class
becomes a gerund of that verb.