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

90 Abstraction of a Problem Domain
showed that composites can be clustered or not, that they can be aggregates, and that they
can communicate in a weakly distributed fashion or in a strongly distributed fashion.
More in depth research showed that there are more concepts that are needed to manage
complexity. Civello published a study about composites [Civ93]. He recognises the
distinct roles of composites for modelling relations between problem domain concepts,
and for modelling hierarchical software structures.
Civello describes concepts such as of encapsulation, nesting, visibility, sharing, inseparability,
and of ownership. We redefine and refine these concepts for our purpose. Thus
they enable reasoning about various types of modules. This is useful for model design.
It helps in the communication about models among designers. It will appear that these
concepts give also a deeper understanding of composites in general.
4.8.2 Encapsulation and Nesting
We define encapsulation as the circumstance that data and operations are hidden in an
object. The concept of encapsulation refers to the object’s property of protecting individual
objects against each other. An object has a shell that performs the encapsulation and that
hides the internal representation of data and the implementation of operations. This shell
is a message interface that defines the messages for the co-operation with other objects. For
process objects the message interface also defines the communication channels. Objects
exclusively communicate via messages defined in the message interface. They cannot
access attributes of another object directly. Objects that communicate do not use (know)
any information about the internal structure of each other.
Besides encapsulation we define nesting as a separate concept. Nesting is the embedding
of entities in another entity. For instance clusters and objects form composites that can
be nested into a cluster. The latter cluster encompasses the composite. The purpose of
nesting is the hierarchical structuring of complex systems. Nesting simplifies models,
because a nesting entity can hide its internal entities. We call two or more entities disjoint
when none of them is (indirectly) nesting another.
Civello seems to consider encapsulation and nesting as synonyms. ’An encapsulated (or
nested) object is only visible within the scope of its encapsulating object’ [Civ93]. We consider
encapsulation and nesting as separate concepts. Nesting does not necessarily offer
protection as encapsulation does.
Hartmann et al. [HJS92] describe object embedding as a way to create nested composites,
in case of static aggregation or disjoint complex objects. The description of their approach
is in terms of aggregation, not encapsulation. Wegner calls encapsulation ’the fundamental
unit of object granularity’ [Weg90]. America rightly states that ’the essence of object- oriented
is the encapsulation of data and operations in objects and the protection of individual objects
against each other’ [AL90]. Our separation matches with all these points of view.
Protection seems to be the most important property of encapsulation. On the other hand