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

4.3 Classes 63
A message interface defines the names of messages that can be accepted. Collaborating
objects communicate by mutual sending of messages. Classes can be visualised, in so-
Class
symbol
Class sort
P: Motor
Attributes:
Speed
Messages:
Start
Stop
Class name
Figure 4.2: Object Class Symbol
Class
attributes
field
Class
messages
field
called Object Class Diagrams, by the notation in Figure 4.2. The upper field contains class
sort and class name. Figure 4.2 shows a class Motor with class sort P. The class sort is
either P or D. P denotes that it is a class of Process objects, and D denotes Data objects.
The distinction between these sorts of objects is explained in Section 4.4. Attribute names
and Message names are shown in the corresponding Attributes and Messages fields.
4.3.2 Class and Type
Besides the concept of class the concept of type can be used to find objects that have
identical properties. Objects that are similar are said to be of the same type. Type and
class are not the same concept. An object class contains objects that are of the same
type. However, objects that are the same type may be put in separate classes. Figure
4.3 visualises this. Five objects are of type X. All objects in class A are of the same type,
which is type X. The same holds for class B. However, although the five objects are of
the same type, they are separated into different classes A and B. Recognising types is a
premise for classification. However, an object’s type does not determine its class.
Classes are constructed by grouping entities based on particular selected aspects and
properties. Objects are abstractions of ’real’ entities from the problem domain. These
entities may have many different properties and forms of behaviour. What counts,
however, is what properties are selected to be modelled. Only properties that are
relevant for the behaviour of the system will be incorporated into the model. All other
properties disappear by abstraction. Discovering common properties of objects is called
generalisation. A part of the properties of a collection of entities is general; another part
is specific. Classification requires finding collections of entities with the same properties
and capabilities.