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

62 Abstraction of a Problem Domain
constructed, and cannot be found by simple observation of a problem domain. They
often come from technical solutions, mathematical models, and conceptual solutions.
At least a part of the objects must be constructed. This defines a design activity during
analysis. The design of an object requires the identification of adequate abstraction
boundaries around collections of data and operations (services). Design must always
deal with finding compromises between conflicting requirements. There must be dealt
with conflicting requirements such as minimum interfaces, logical grouping, and adequate
naming related to the problem domain. Properties of objects may be modelled as
internal data (attributes), but they can also be modelled as separate objects.
4.3 Classes
4.3.1 Class Concept
An object class is a collection of identical objects. Objects that belong to the same class
have the same externally observable behaviour. Reuse is always mentioned as one of
the most important advantages of an object-oriented approach. Classification of objects
is a means to achieve reuse. When objects are identical it is not necessary to describe the
behaviour for each object individually. Once the behaviour has been defined, it can be
reused by all similar objects. Therefore, behaviour of objects of a class is defined only once
in an object class definition. This mechanism can consequently be used if we require that
any object belongs to some class. Even when there is only one object with some specific
type of behaviour its behaviour will be described in a class.
An object class definition serves as a template for all objects of that class. The creation or
derivation of an object from a class is called instantiation. An object is an instance from a
class. Notice that class and class definition are different concepts. From now on we will
use this terms without the sharp distinction.
The objects of a class have the same properties. When we talk about properties we mean
static properties as well as variable properties and capabilities (behaviour). The objects
in a class represent identical things in the problem domain. Their meaning is the same.
However, the individual objects of the same class can do different things. They may be
in different states. They can perform different actions at some point in time.
An object class definition defines the attributes of its objects. By the mechanism of
instantiation, all individual objects of a class have the same set of attributes. However,
an attribute may have different values in different objects from that class. The set of
attributes in a class determines the set of properties of typical objects of that class. The
attributes represent the data encapsulated in an object.
Operations can access or transform attributes. Attributes are specified in the class
definition. An operation in an object is activated after the reception of a message by
that object. A class definition specifies the message interface of its objects (instances).