Class and Object Practical

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Where to Create a Class

In Java, you can write multiple classes inside the same file.
However, only one class can be declared as public, and the file name must match that public class name.

For example:

File name → Demo.java
Public class inside → public class Demo { ... }

This rule helps Java identify the main entry point and organize code in a structured way.

Example – Creating and Using a Class

class Calculator {
    public int add(int num1, int num2) {
        System.out.println("In add");
        return num1 + num2;
    }
}

public class Demo {
    public static void main(String[] args) {
        Calculator obj = new Calculator(); // object creation
        int result = obj.add(5, 4);        // calling method using object
        System.out.println(result);
    }
}

This program defines a Calculator class and uses it inside the Demo class to perform an addition.

Step-by-Step Explanation

1. Defining a Class

class Calculator {
    public int add(int num1, int num2) {
        System.out.println("In add");
        return num1 + num2;
    }
}

A class:

• Contains properties (variables) and behaviours (methods)
• Acts as a blueprint for creating objects

Here:

• The class name is Calculator
• It contains one method add(), which returns the sum of two integers
• public before the method means it can be called from other classes

2. Creating the Main Class

public class Demo {
    public static void main(String[] args) {
        Calculator obj = new Calculator();
        int result = obj.add(5, 4);
        System.out.println(result);
    }
}

Demo contains the main() method, which is the starting point of every Java program.

Inside main():

1. An object of Calculator is created
2. The add() method is called using that object
3. The result is printed

Important Concepts

Object Creation

Calculator obj = new Calculator();

This line performs three roles:

1. Calculator → the type of the object (class name)
2. obj → reference variable (stores address of the object)
3. new Calculator() → creates the actual object in memory

Think of it like this:

• The object (new Calculator()) is the actual device
• The reference (obj) is the remote control used to operate the device

Without the reference, you cannot call methods of the object.

Method Call Using Object

int result = obj.add(5, 4);

• The object obj calls the method add()
• The values 5 and 4 are passed as arguments
• The method returns the sum, which is stored in result

Output

In add
9

The line "In add" prints from inside the method, and 9 is the final result of the addition.

Summary

• A class defines what an object will contain (properties) and what it can do (methods).
• An object is created using the new keyword.
• The reference variable (like obj) is used to access methods and variables of the object.
• Only one public class is allowed per .java file, and the filename must match that class name.
• Methods inside the class are invoked using the object created from that class.

Written By: Shiva Srivastava