This document discusses key concepts of object-oriented programming including classes, objects, instance variables, methods, constructors, access specifiers, and more. A class defines the data and behavior of objects through instance variables and methods. An object is an instance of a class that allocates memory and can access class members. Constructors initialize objects. Access specifiers determine object member accessibility. Examples demonstrate how to declare classes, objects, and access members to understand OOP concepts.

1. B Y :
H U M A S A M I N
Object Oriented Programming
Concepts

2. Class and Object
 Class is a user defined datatype.
 The class definition provides a template or blueprint,
which describes
 the data (instance variables) contained within, and
 the behavior (methods) common to all objects of a class.
 Object is a variable of a class.

3. Syntax of class
class ClassName
{
//data or instance variables
//behavior or methods
}

4. Data or Instance Variables
 The data is contained in variables defined within the
class
 Often called instance variables, data members,
properties or attributes.
 The instance variables are variables of the primitive
types or they can be reference variables of objects of
other classes.

5. Example
class Student
{
//data members or instance variables
int rollno;
String name;
int semester;
int[] marks;
//behavior or methods
}

6. Behavior or Methods
 The behavior is controlled by methods defined
within the class.
 Often called member methods or member functions.
 Syntax:
returntype methodName(parameterlist)
{
//valid java statements
}

7. Example
class Student
{
//data members or instance variables
int rollno;
String name;
int semester;
//behavior or methods
void displayValues( )
{
System.out.println(rollno);
System.out.println(name);
System.out.println(semester);
}
}

8. Object
 Object is a variable of a class.
 Object is the implementation of class.
 It is a software bundle of variables and methods.
 It is also known as instance of a class.
 The members of the class both data members and
methods are accessed with the help of the object.

9. Declaration and Definition of Object
 Syntax:
 Declaration of object:
ClassName objectName;
 Definition:
ObjectName=new ClassName( );
 Shortcut:
 ClassName objectName=new ClassName( );

10. Example
 Objects are created in the main method or any other
class.
public class StudentDriver
{
public static void main(String args[])
{
Student s;
s=new Student( );
}
}

11. Accessing Members of the class
 The members of the class(both data members and
methods) are accessed with the help of the object of
the class.
 Syntax:
objectName.instanceVariable=value;
OR
objectName.methodName();

12. Access Modifiers or Access Specifiers
Access
Specifier
Class Package SubClass World
private Y N N N
package Y Y N N
protected Y Y Y N
public Y Y Y Y

13. Example
public class Student
{
//data members or instance variables
private int rollNo;
public int semester;
//behavior or methods are kept public
public void displayValues( )
{
System.out.println(rollno);
System.out.println(name);
System.out.println(semester);
}
}
Note: Data Members are kept private and methods are kept public

14. Example Continued..
public class StudentDriver
{
public static void main(String args[])
{
Student s;
s=new Student( );
s.semester=2;
s.rollNo=123; //Not Allowed as its private
s.displayValues( );
}
}

15. Accessing private members
 To access the private members of the class, we have
to provide getter and setter methods in the class.
 The getters and setters have public access specifier.
 If x & y are the instance variables then for setters,
word “set” is used before the instance variable name
like setX, setY.
 For getters, word “get” is used before the instance
variable name getX, getY.

16. Example
public class Student
{
//data members or instance variables
private int rollNo;
public int semester;
//behavior or methods are kept public
public void setrRollNo(int r)
{
rollNo=r;
}
public int getRollNo( )
{
return rollNo;
}
public void displayValues( )
{
System.out.println(rollNo);
System.out.println(semester);
}
}

17. Example cont..
public class StudentDriver
{
public static void main(String args[])
{
Student s;
s=new Student( );
s.semester=2;
System.out.println(s.semester);
s.setrollNo(123);
System.out.println(s.getrollNo( ));
int r;
r=s.getrollNo();
System.out.println(r);
s.displayValues( );
}
}

18. Constructors
 Constructor is a special kind of method of the class
having the same name as that of the class and has a
no return type.
 It is called when an object of the class is going to be
created.
 The main purpose of writing constructor is
initialization of instance variables.

19. Constructor Example
public class Student
{
//data members or instance variables
private int rollNo;
public int semester;
//behavior or methods are kept public
//Constructor
public Student( )
{
rollNo=10;
semester=2;
}
public setrollNo(int r)
{
rollNo=r;
}
public int getrollNo( )
{
return rollNo;
}
public void displayValues( )
{
System.out.println(rollNo);
System.out.println(semester);
}
}

20. Constructor
 If you don’t provide constructor for class, the JVM
will provide a default (zero argument) constructor
and initialize the instance variables to default values.

21. Lab Work
 Write a program to create a class named Circle
having radius as a data member. The class should
contain two methods to calculate the area and
circumference of the circle.
 You have to create two objects of the Circle class and
display their area and circumference.