Fundamental Principles of Object Oriented

Programming Language

Lecture Code: CIT 301

Bowen University, Iwo Nigeria

Learning Objectives: To discuss the principles of OOP with respect to JAVA

Programming Language

Targeted Students: Year Three (3)

Course Title: Java Programming Language

Reminder: Introduction to Object Oriented Programming

Literally – an Object as STATE and BEHAVIOUR

STATE: are the Descriptive characteristics of the Object

BEHAVIOUR: What the Object can do or what can be done to an Object

Object-oriented programming is a programming

paradigm where everything is represented as an object.

Objects pass messages to each other. Each object decides

what to do with a received message. OOP focuses on each

object’s states and behaviors.

4 Basic Principles of OOP

Encapsulation

Encapsulation is a process of wrapping code and data

together into a single unit.

Encapsulation is the mechanism of hiding of data

implementation by restricting access to public methods.

Instance variables are kept private and accessor methods are

made public to achieve this.

We can create a fully encapsulated class in Java

by making all the data members of the class

private. Now we can use setter and getter

methods to set and get the data in it.

To achieve encapsulation in Java −

•Declare the variables of a class as private.

•Provide public setter and getter methods to modify and

view the variables values.

It provides you the control over the data.

It is a way to achieve data hiding

The encapsulate class is easy to test.

It improves maintainability and flexibility and re-usability:

User would not be knowing what is going on behind the scene.

public class EncapTest {private String name;public void setName(String newName) {

name = newName;}

public void setIdNum( String newId) {idNum = newId;

}}

Advantages of Encapsulation

Inheritance

Inheritance is a mechanism in which one object

acquires all the states and behaviors of a parent object.

Inheritance uses a parent-child relationship (IS-A relationship).

The following are inherited:

• public instance methods.

• private instance variables (private instance variables can be

accessed only through public getter and setter methods).

Inheritance allows us to reuse of code, it improves reusability

in your java application.

class Subclass-name extends Superclass-name {

//methods and fields }

class Employee{

float salary=40000;

}

class Programmer extends Employee{

int bonus=10000;

public static void main(String args[]){

Programmer p=new Programmer();

System.out.println("Programmer salary is:"+p.salary);

System.out.println("Bonus of Programmer is:"+p.bonus);

}

}

How does Inheritance Work?

A class can extend only one class however it can implement any

number of interfaces. An interface can extend more than one

interfaces.

The extends keyword is used to indicate that the class which is

being defined is derived from the base class using inheritance. So

basically, extends keyword is used to extend the functionality of

the parent class to the subclass.

The implements keyword is used to implement an interface. An interface is a special type

of class which implements a complete abstraction and only contains abstract methods.

Extend VS Implement Keyword

We discuss Abstraction and Polymorphism

Questions and Contributions are Welcome