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

4.14 Attributes and Variables 107
characterise the object, but is determined by the implementation of the behaviour of the
object. This data ’lives’ temporarily during specific activities of an object. We will define
(in Subsection 6.6.3) the ’state’ of an object based on observable behaviour, instead of
on data. In contrast to local data, the data that really characterises an object is stored
as variables that can be retrieved and/or transformed via its message interface. In this
sense these variables are indirectly ’available’ or ’exported’. These variables make or
form the object. They are always needed, even when the class would be implemented
afresh, with an alternative behaviour description of its internal operations. In an Object
Class
symbol
Class sort
P: Motor
Attributes:
Speed
Messages:
Start
Stop
Class name
Figure 4.30: Class Symbol
Class
attributes
field
Class
messages
field
Class Model attributes can be denoted in the class symbols in a class attributes field,
see Figure 4.2. All objects are instantiated with the attributes of its class. Rumbaugh
notices that an attribute should be a pure data value, not an object [R 91]. Analysis is a
continuous process of making decisions whether a property of an object must become
an attribute, an association or a new class. The task during analysis is defining what
properties of a class are necessary for the working of the system to be designed. This is
all what abstraction is about. The best model is the most simple one that is appropriate
for the tasks to be performed.
Conceptual modelling, which is performed when creating an Object Class Model, is rather
different from the instance approach we recommend for reactive systems. A clear understanding
of the differences lets us decide what form of modelling is most appropriate
for the system part to be analysed. For Object Class Modelling it is recommended to
avoid defining attributes that are references to other objects. They should be modelled as
relationships instead. An Instance Approach works in a completely different way. Instead
of defining classes, collaborating objects are defined. By determining the message flows,
objects can get ’a network’ of acquaintances. When defining classes from this approach