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

4.3 Classes 65
Class B
Attributes
Messages
Class A
Attributes
Messages
Class C
Attributes
Messages
Superclass
Specialisation
symbol
Subclass
Figure 4.4: Generalisation/Specialisation Relation
Relations are represented as connections between the class symbols. A triangle symbol
represents a generalisation/specialisation relation. (There are various other forms of
relations and corresponding symbols for Object Class Models. These are described in
Section 4.13). Figure 4.4 can be interpreted as showing the superclass-subclass relation
between classes. The superclass contains the messages and attributes that are necessary
for its instances. The subclass adds additional attributes and messages. An instance of a
subclass contains these attributes and messages, but contains also the common attributes
and messages defined in the superclass. After all, a subclass has at least the properties
that its superclass has.
4.3.4 Inheritance
Object Class Diagrams give a static, so-called conceptual view on a problem domain. Figure
4.4 shows a static hierarchical structure of classes. This hierarchy can be interpreted
using a parent-child metaphor. Super and subclass have similarities and differences
like parent and child. The parent- child metaphor can be extended with the notion of
inheritance. A child inherits properties of its parents. Correspondingly, a subclass can
inherit properties, such as attributes, of its superclass. Other terms that are used to refer
to generalisation of classes across multiple levels are: ancestor (parent) and descendant
(child). The hierarchical structure may have several levels, and form an inheritance tree.
The leafs of the tree inherit from all their ancestors.
4.3.4.1 Specification Inheritance
The concept of inheritance is considered as a very important property of object-oriented
methods and languages. This concept can be used to specify common properties only
once, which is the foundation for reuse. Generalisation/specialisation relations visualised
in Object Class Diagrams are a communication tool among people for the structuring
of the problem domain. We offer reuse in the form of inheritance in a specific way,