Page 37 - DCAP308_OBJECT_ORIENTED_ANALYSIS_AND_DESIGN

Unit 3: Class Modelling

Generalization is the relationship between a class, and it defines a hierarchy of abstraction in Notes
which subclasses (one or more) inherit from one or more superclasses. Generalization and
inheritance are transitive across a subjective number of levels in the hierarchy. Generalization
is an “is-a-kind of” relationship. For instance, Saving Account is a kind of Account; PG student is
kind of Student, etc.
The notation for generalization is a triangle connecting a super class to its subclasses. The
superclass is connected by a line to the top of the triangle. The subclasses are connected by lines
to a horizontal bar attached to the base of the triangle.

!
Caution Generalization is a very useful construct for both abstract modeling and
implementation.

Example: A generalization of account class is shown in Figure 3.8.
Figure 3.8: Generalization of Account Class

Account

Saving Account Current Account

Source: http://vedyadhara.ignou.ac.in/wiki/images/a/aa/B1U1mcs-032.pdf

3.3.2 Inheritance

Inheritance is taken in the sense of code reuse within the object oriented development. During
modeling, we look at the resulting classes, and try to group similar classes together so that code
reuse can be enforced. Generalization, specialization, and inheritance have very close association.
Generalization is used to refer to the relationship among classes, and inheritance is used for
sharing attributes and operations using the generalization relationship.

Notes In respect of inheritance, generalization and specialization are two phases of a coin
in the sense that if a subclass is seen from a superclass the subclass is seen as a specialized
version of superclass and in, reverse, a superclass looks like general form of subclass.

During inheritance, a subclass may override a superclass feature by defining that feature with
the same name. The overriding features (the subclass feature with the same names of superclass
features) refine and replace the overridden feature (the superclass feature).

Example: Let us look at the diagram given in Figure 3.9. In this diagram, Circle, Triangle,
and Square classes are inherited from Shape class. This is a case of single inheritance because
here, one class inherits from only one class.

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