oops concept chapter

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.