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

70 Abstraction of a Problem Domain
feedServer
Object A
cardInjector
Object C
Figure 4.7: References
Object B
objects. In order to ask object A what its ’feedServer’ is, object A must export a service
which means that it can accept a message, e.g. whatIsYourFeedServer. When object A
receives this message it returns object B, which means that object A gets a reference to
object B. So the sender of the message receives object B as a result of the communication.
Notice that there is no real distinction between receiving a reference and receiving a
data object. There is no explicit notion ’pointer’. An object ’has’ other objects, by having
them as ’acquaintances’ which are implicitly known by references.
4.4.2.4 Primitive Data Classes
Analysis of a problem domain may lead to complex entities formed by data objects.
Data objects that are linked by references can form such complex structures. The basic
building blocks of such a structure can be relatively simple data objects. The work of
defining all classes is decreased by having some primitive classes. POOSL offers the
following primitive data classes: Boolean, Integer, Real, and Char. These classes define
the usual operations such as relational and mathematical operations.
The primitive data classes are extended with primitive data objects: bunk, iunk, runk, and
cunk. They represent the ’unknown’ value in their respective classes Boolean, Integer,
Real, and Char. They have been introduced to model non- deterministic behaviour of
a system. Another primitive object is nil. A nil object can prevent the occurrence of
a so-called ’dangling pointer’. Such a void reference is properly represented by a nil
object. Nil solely offers the primitive equality operation == which determines whether
two objects refer to the same object.