Embed Size (px)
DESCRIPTION
oops concept chapter
Citation preview
B Y :
H U M A S A M I N
Object Oriented Programming Concepts
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.
Syntax of class
class ClassName
{//data or instance variables
//behavior or methods
}
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.
Example
class Student
{
//data members or instance variables
int rollno;
String name;
int semester;
int[] marks;
//behavior or methods
}
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
}
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);
}
}
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.
Declaration and Definition of Object
Syntax:
Declaration of object:
ClassName objectName;
Definition:
ObjectName=new ClassName( );
Shortcut:
ClassName objectName=new ClassName( );
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( );
}
}
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();
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
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
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( );
}
}
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.
Example
public class Student{
//data members or instance variablesprivate int rollNo;public int semester;
//behavior or methods are kept publicpublic void setrRollNo(int r) {
rollNo=r;}
public int getRollNo( ) {
return rollNo;}
public void displayValues( ){
System.out.println(rollNo);System.out.println(semester);
}}
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( );
}
}
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.
Constructor Example
public class Student{
//data members or instance variablesprivate int rollNo;public int semester;
//behavior or methods are kept public//Constructorpublic 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);
}}
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.
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.
