Java: Let's Practice - How to calculate factorial through recursion

I have already explained about recursion in earlier post and shown the example Reverse the string. In this post, we will use recursion method to calculate the factorial of a number.

What is factorial of a number?

Factorial of a number is the product of all numbers counting backwards from the given number till number 1. Suppose number is 5, so factorial of number 5 is equal to 5 * 4 * 3 * 2 * 1

f(5) = 5 * 4 * 3 * 2 * 1

f(4) = 4 * 3 * 2 * 1

f(3) = 3 * 2 * 1

We can notice the pattern to calculate the factorial.

f(n) = n * f(n-1)

We will use this pattern for recursion. The program will call the same method recursively till the number is 1 and then it will calculate the backwards and return the result.

Here is the program -

import java.util.Scanner;

public class FactorialDemo {
    public static void main(String args[]) {
        Scanner sc = new Scanner(System.in);
        System.out.print("Enter the number: ");
        int n = sc.nextInt();
        int fact = factorial(n);
        System.out.println("Factorial of the number " + n + " is: " + fact);
    }

    public static int factorial(int n) {
        int result = n;
        if(n != 1)
            result = result * factorial(n-1);
        return result;
    }
}

Here is the output - 

Enter the number: 5

Factorial of the number 5 is: 120

The program asks user to enter any number. It passes the number to a method factorial() which calculates the factorial of the number. 

• The method factorial() method calls itself recursively by decreasing the number by 1 till the number is 1. It returns 1 when the number is 1. 
• Then it multiplies by 2 and then returns result back to the calling method. 
• Multiplies by 3 and then returns result back to the calling method.
• And so on till it reaches the original number
• When it reaches the original number, multiplies and it returns the final result back to main method.

Code can be found here on Github.