Java Program to Calculate Factorial Using Scanner

Introduction

The factorial of a non-negative integer ( n ) is the product of all positive integers less than or equal to ( n ). Factorials are commonly used in permutations, combinations, and other mathematical calculations. This guide will show you how to create a Java program to calculate the factorial of a number using the Scanner class to accept user input.

Problem Statement

Create a Java program that:

• Takes an integer input from the user.
• Calculate the factorial of that number.
• Displays the factorial to the user.

Example 1:

• Input: 5
• Output: 120 (Factorial of 5 is ( 5 \times 4 \times 3 \times 2 \times 1 = 120 ))

Example 2:

• Input: 7
• Output: 5040 (Factorial of 7 is ( 7 \times 6 \times 5 \times 4 \times 3 \times 2 \times 1 = 5040 ))

Solution Steps

1. Prompt the User for Input: Use the Scanner class to read an integer input from the user.
2. Calculate the Factorial: Use a loop to multiply the integers from 1 to the input number.
3. Display the Result: Output the calculated factorial to the user.

Java Program

import java.util.Scanner;

/**
 * Java Program to Calculate Factorial using Scanner
 * Author: https://www.javaguides.net/
 */
public class FactorialCalculator {

    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);

        // Step 1: Prompt the user for input
        System.out.print("Enter a number to calculate its factorial: ");
        int number = scanner.nextInt();

        // Step 2: Calculate the factorial
        long factorial = 1;  // Factorial of 0 is 1
        for (int i = 1; i <= number; i++) {
            factorial *= i;
        }

        // Step 3: Display the result
        System.out.println("Factorial of " + number + " is " + factorial);
    }
}

Explanation

Step 1: Prompt the User for Input

• The program uses the Scanner class to read an integer input from the user. This input represents the number for which the factorial will be calculated.

Step 2: Calculate the Factorial

• A for loop is used to calculate the factorial:
  • The loop starts from 1 and multiplies the current value of factorial by the loop counter i until i reaches the input number.
  • The variable factorial is initialized to 1 because the factorial of 0 is defined as 1.

Step 3: Display the Result

• The calculated factorial is printed to the console using System.out.println.

Output Examples

Example 1:

Enter a number to calculate its factorial: 5
Factorial of 5 is 120

Example 2:

Enter a number to calculate its factorial: 7
Factorial of 7 is 5040

Conclusion

This Java program calculates the factorial of a number entered by the user using the Scanner class. It employs a simple loop to perform the multiplication required to compute the factorial. This method is efficient and easy to understand, making it a useful tool for learning and applying basic Java concepts.