03: abstraction, encapsulation, and inheritance

Learning Objectives

After completing this session, you will be able to:

‰  Describe abstraction, encapsulation, and inheritance

Abstraction

One of the chief advantages of object oriented programming is the idea that programmers can

essentially focus on the “big picture” and ignore specific details regarding the inner-workings of an

object. This concept is called abstraction.

Encapsulation

‰  Abstraction in OOP is closely related to a concept called encapsulation.

‰  The Object Orientation has two major promises/benefits. They are: flexibility and

maintainability.

‰  You have to write your classes and code in a way that supports flexibility and

maintainability.

‰  The ability to make changes in your implementation code without breaking the code of

others who use your code is a key benefit of encapsulation.

‰  If you want maintainability, flexibility and extensibility your design must include

encapsulation.

‰  The following are some of the ways to include encapsulation:

oKeep instance variables protected (with an access modifier, often private).

oMake public accessor methods, and force calling code to use those methods

rather than directly accessing the instance variable.

oFor the methods, use the JavaBeans naming convention of

set<someProperty> and get<someProperty>

‰  Encapsulation is the mechanism that binds together the code and the data it

manipulates, and keeps both safe from outside interference and misuse.

Encapsulation: Example

Bank machine

Hidden data

Account balance

Personal information

Interface

Deposit, Withdraw, Transfer

Display account information

Class Hierarchy

Classes are arranged in a tree hierarchy:

‰  A class’ “superclass” is the class preceding it in the hierarchy

‰  Classes following it are “subclasses”

Classes have all the properties of their superclasses:

General:Towards the root (top)

More specific:Towards the leaves (down)

NB:In Computer Science trees grow upside down!

Inheritance

One of the main tenets of OOP is inheritance.

The process by which a class inherits the properties of its superclasses is called inheritance:

‰  Methods

‰  Instance variables

A child class inherits its properties and attributes from its parents.

Inheritance is the process by which one object acquires the properties of another object. By use of

inheritance, an object needs only to define all of its characteristics that make it unique within its

class; it can inherit its general attributes from its parent.

The inheriting class contains all the attributes and behaviors of its parent class. Moreover the

inheriting class can define its own attributes and behaviors.

The inheriting class can override the definition of existing methods by providing its own

implementation.

The code of the inheriting class consists only the changes and additions to the base class.

Need of inheritance

Inheritance is required for the following reasons:

Modular coding, which means less code and easier to get an idea about the code

‰  Code reuse:

‰  Do not break what is already working

‰  Easier updates

Tips and Tricks

1.The typical problems include real-time problems and challenges:

2.Are there any practical limits on the level of subclassing using Java?

3.Can you extend any class in Java?

Solution:With regard to Java API, there is no hard limit for the limits on the level of subclassing.

Using Java, you cannot extend any class as such. The factors like access control and access

modifier determine whether a class can be subclassed.

Summary

A class can inherit instance variables and methods from a more abstract superclass.