Session - 6 / Defining Classes in Java

Introduction (10 minutes)

Definition of Class (15 minutes)

Creating Class & Objects (20 minutes)

Assessment (5 minutes)

Defining Classes in Java

Each class is a collection of data and the functions that manipulate the data. The data components of the class are called data fields and the function components of the class are called member functions or methods.

• Thus the state of an object is represented using data fields (data members) and behaviour of an object is represented by set of methods (member functions). The class template can be represented by following Fig. 1.14.1.

• The Java Class can written as follows

class Customer {

int ID;

int Age;

String Name;

Customer() {

}

Customer(int ID) {

}

double withdraw_money() {

…..

…..

}

• Encapsulation means binding of data and method together in a single entity called class. Thus a class is used to achieve an encapsulation property.

• This class is not having the main function. The class that contains main function is called main class.

• Following is a simple Java program that illustrates the use of class

//Program for demonstrating the concept of class

import java.io.*;

import java.lang.*;

import java.math.*;

public class MyClass

{

String name;

int roll;

double marks;

public void display(String n,int r,double m)

{

System.out.println();

System.out.println("Name: "+n);

System.out.println("Roll number: "+r);

System.out.println("Marks: "+m);

}

public static void main(String args[])

{

int a,b;

MyClass obj1= new MyClass();

MyClass obj2=new MyClass();

MyClass obj3=new MyClass();

obj1.display("Amar", 10,76.65);

obj2.display("Akbar", 20,87.33);

obj3.display("Anthony",30,96.07);

}

}

Output

Name: Amar

Roll number: 10

Marks: 76.65

Name: Akbar

Roll number: 20

Marks: 87.33

Name: Anthony

Roll number: 30

Marks: 96.07

Program Explanation

• In above program we have declared one simple class. This class displays small database of the students. The data members of this class are name,roll and marks. There is one member function named display which is for displaying the data members. In the main function we have created three objects obj1,obj2 and obj3. These objects represent the real world entities. These entities are actually the instances of the class. The class representation of the above program can be as follows-

Assessment Questions:

1. Which of the following is the correct syntax to declare a class in Java?

• A) class MyClass { }

• B) class MyClass[] { }

• C) MyClass class { }

• D) class = MyClass { }

2. What is the default value of an instance variable in Java if it is not explicitly initialized?

• A) 0

• B) null

• C) false

• D) Depends on the type of the variable

3. Which keyword is used to refer to the current object in Java?

• A) this

• B) super

• C) self

• D) object

4. What is the correct way to create an object of the class Car in Java?

• A) Car obj = new Car();

• B) Car obj;

• C) new Car obj();

• D) Car obj = Car();

5. Which of the following constructors is used when no constructor is defined in the class in Java?

• A) Default Constructor

• B) Parameterized Constructor

• C) Copy Constructor

• D) Private Constructor

Answers:

1. A) class MyClass { }

2. D) Depends on the type of the variable

3. A) this

4. A) Car obj = new Car();

5. A) Default Constructor