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

66 Abstraction of a Problem Domain
which we call specification inheritance. Parts of the description specified in classes can be
inherited and therefore reused. This appears to be a very limited form of inheritance.
The form of inheritance is based on the subtype/supertype relation. Attributes and
messages are specified in the subtype/supertype hierarchy of classes in Object Class
Diagrams. Superclasses contain attributes and messages that are inherited by all their
descendants. These collections of attributes and messages are reused in this way. The
main issue here is that the concepts of specialisation, generalisation and of inheritance
are used for structuring of the problem domain. Reuse is only a secondary issue.
4.3.4.2 Implementation Inheritance
In the object-oriented tradition, inheritance is explained as a mechanism that allows to
reuse the (behaviour) definition of a class. We call this form implementation inheritance, to
distinguish it from specification inheritance. Implementation inheritance is a form of
dynamic hierarchical resource sharing. It is a mechanism for reuse by means of code
sharing. Code implements the objects operations. Each operation of all objects of a class
is described only once in this class, or in one of the ancestor classes.
In Paragraph 4.3.3.2 we interpreted the hierarchy in the tree shown in Figure 4.4 as
supertype/subtype relations. Implementation inheritance interprets this hierarchy differently.
An important aspect is the role for sharing code. Subclasses can inherit the
code for operations from their ancestor classes. In object-oriented implementations this
can be performed at run-time. When an object must perform an operation the code for
this operation is first searched within the object’s class. If the operation is not found, the
ancestors classes are searched successively.
4.3.4.3 Specification Inheritance and Implementation Inheritance
We choose to interpret inheritance as specification inheritance, which is consistent with
the notion of subtyping. Specification inheritance is used in Object Class Diagrams,
which offer a conceptual view of the system to be designed. This view is formalised in a
unified model described in POOSL. To describe specification inheritance, a type system
for POOSL, supporting subtyping, will have to be developed. Further, to support reuse
of specifications, implementation inheritance would have to be supported too. Currently,
the POOSL language is untyped, and no implementation inheritance is offered. Implementation
inheritance and specification inheritance appear to be conflicting concepts.
Problems with respect to type systems and implementation inheritance are described in
[DT92, Hür94, Coo89, AL90, Ame89, AR89a, CHC90]. The development of an appropriate
type system for POOSL that supports implementation inheritance is subject for
future research.
4.3.4.4 Implementation Inheritance and Encapsulation
Another problem of implementation inheritance is that it violates encapsulation. This
problem is described by Snyder [Sny86]. He states that the introduction of implementation
inheritance severely compromises the benefits of encapsulation among classes.