Inheritance & Classification Hierarchies

Lecture-8

Inheritance & Classification Hierarchies

Inheritance is one of the most powerful features of O-O programming

By organising classes into Classification Hierarchies we can

add extra dimensions to encapsulation by grouping

ADT's and enables classes to inherit from other classes thus extending the attributes and methods of the class

which inherits The inheriting class may then also add extra functionality

and attributes to produce a new object

Terminology of inheritance The following terms are used to describe the different

inheritance properties of an object Derived Class or Sub Class or Child Class

A Class which inherits some of its attributes and methods from another class

Base Class or superclass or Parent Class A Class from which another class inherits

Ancestor A Classes ancestors are those from which its own

superclass inherit Descendant

A classes descendants are those which inherit from its superclasses

Why do we need Inheritance ?

What is the purpose of inheritance and why should we wish to inherit the attributes and methods of other Objects ?

There are two main reasons for using inheritance in O-O which are

Specialisation Extending the functionality of an existing class

Generalisation Sharing commonality between two or more classes

Classification Hierarchy

The product of inheritance is a classification Hierarchy

This is a relationship between classes where one class is said to be 'a kind of' other class

As the class hierarchy is traversed from top to bottom we move from generalisation to specialisation of classes

This is done by adding functionality to extend what exists at each level of the class hierarchy from the base class

An Example

Geometric Object

2D 3D

Triangle Square Sphere Cube

Class Hierarchy Diagram The base of this diagram is the Geometric Object class

From this the 2D and 3D objects inherit the base features ( what could these be ?)

After this there are specialised versions of the classes using the 'kind of' relationship

where 2D objects such as triangle and Square inherit the base classes of a Geometric Object and then add special features specific to the type of object

The same then applies for the other classes

This tree runs from a generalisation of the base class to a specialisation of the actual object

'a kind of' or 'a part of' Each level of a classification hierarchy contains more specific

types of class each one of which must be 'a kind of' the class from which it

inherits This distinction is important as the difference between 'a kind

of' and 'a part of' is very different The distinction between different Objects and the same Objects

with different states This mean analysis to see if difference of Objects are

dependent upon the Object type or a state of that Object For Example Equilateral Triangle and Isosceles Triangle are

inappropriate Object classifications as they are both really triangles (with just the angle between vertices being different) which we could store in a general triangle class

What Do Objects Inherit ?

A class does not contain any state values, only a 'blue print' for what value are to be contained

Therefore a 'derived class' inherits all of the attributes from the 'base class'

A derived class is by definition identical to the base class, but it can be built on to extend and modify the base class

Objects of the derived class do not inherit anything from the objects of the base class

As far as objects are concerned there is no hierarchy

Example

Line+startPosition:+endPosition:+Draw()

Line Object

start = (x,y)end = (x,y)

draw(star,end)

ColoredLine

+color

+set color()

Line Object

start = (x,y)end = (x,y) color=?

draw(star,end)set color(col)