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

106 Abstraction of a Problem Domain
4.13.5 Roles
Objects play roles in a relationship. These roles may be documented at both ends of the
relationship symbol. Roles are particularly needed when relations are specified between
objects of the same class, or for distinguishing two relationships between the same pair
of classes [R 91].
4.13.6 Aggregation
Aggregation is a special sort of relationship. Its semantics and symbols have been discussed
in Section 4.6.
4.13.7 Generalisation and Specialisation
Generalisation and specialisation are a special kind of relationship. The concepts of generalisation
and of specialisation in relation with inheritance have been discussed in
Subsection 4.3.3
4.14 Attributes and Variables
4.14.1 Attributes
Objects are abstractions of things or concepts that we want to model as separate distinguishable
entities. Objects have properties that characterise them. These properties
fall apart into observable behaviour and descriptive features. Observable behaviour is
visible via the class interface which defines all exported services of its objects. The services
perform some form of computation, thereby performing a corresponding dynamic
behaviour, which yields, in general, an observable result. The descriptive features have
a more static character. During analysis they are modelled as attributes specified in their
class. The attributes get specific values in object instances of that class. The domain of
values for each attribute must be specified. This is a premise for the later implementation
of the methods that implement the services of the object. Attribute values are protected
by the object’s interface. Values can only be modified as a result of communication via
the object’s interface with other objects.
There are various views on attributes that are interesting to get a more complete understanding.
Coad and Yourdon refer to the Webster’s dictionary (1977) that defines
attributes as ’Any property, quality, or characteristic that can be ascribed to a person or thing.
They refine this definition (for OOA) to ’an attribute is some data (state information) from
which each Object in a Class has its own value’ [CY91a]. This latter definition emphasises
the ’data character’ of attributes and also refers to the concept of state. Most objectoriented
methods and languages follow this interpretation. As is explained in Section
6.4, we do not define state such that the state space of an object is determined by the
product of the data domain spaces of all its internal data. Some of the data is only
for local use and cannot be retrieved via the object interface. This local data does not