Example: Factorial with Recursion in Java

// Factorial with Recursion
import java.util.Scanner;

// Main class containing the entry point of the program
public class Main {

    // Recursive method to calculate factorial
    public static long calculateFactorial(int n) {
        // Step 1: Define the base case
        // The factorial of 0 or 1 is 1
        if (n == 0 || n == 1) {
            return 1;
        }
        // Step 2: Define the recursive step
        // n! = n * (n-1)!
        else {
            return n * calculateFactorial(n - 1);
        }
    }

    public static void main(String[] args) {
        // Step 1: Create a Scanner object to read user input
        Scanner scanner = new Scanner(System.in);

        // Step 2: Prompt the user to enter a non-negative integer
        System.out.print("Enter a non-negative integer: ");
        int number = scanner.nextInt();

        // Step 3: Validate the input to ensure it's non-negative
        if (number < 0) {
            System.out.println("Factorial is not defined for negative numbers.");
        } else {
            // Step 4: Calculate the factorial using the recursive method
            long factorialResult = calculateFactorial(number);

            // Step 5: Display the result
            System.out.println("The factorial of " + number + " is: " + factorialResult);
        }

        // Step 6: Close the scanner
        scanner.close();
    }
}