A basic program that gets input number (Integer/Long Integer) from the user and prints the factorial(n). By this program, you can understand the usage of commandline arguments, use of arithmetic operators, printing variables, user-defined method, Logic behind finding Factorial of a number and recursive method to generate the factorial of a number.

Factorial of a non-negative integer, is multiplication of all integers smaller than or equal to n. For example factorial of 6 or (6!) is 6*5*4*3*2*1 which is 720.

It must be also noted that:

• 0! = 1
• 1! = 1
• (-1!) = undefined

Task: Print the Factorial of a number using recursion.

  Eg: number = 5
  Final Output: 
         Factorial of 5 is 120.

The filename must have the same name as the public class name, This is to ensure that the Java Virtual Machine (JVM) understands the entry point of the program

To run the program, open your commandline and type the following:

• Without Commandline Arguments

 $ ..\JavaRecursion> javac .\Factorial\Factorial.java
 $ ..\JavaRecursion> java Factorial.Factorial

• With Commandline Arguments

 $ ..\JavaRecursion> javac .\Factorial\Factorial.java
 $ ..\JavaRecursion> java Factorial.Factorial 10

package Factorial;

import java.util.Scanner; // Scanner class is required to get input from user

public class Factorial {
    static long number = 0; // the required number to find the factorial of that number.
    static long factorialValue = 0; // the factorial value of the given number.

    public static void main(String[] args) {
        // if user provided commandline-arguments, then
        if (args.length != 0) {
            if (args.length == 1)
                number = Long.parseLong(args[0]);
            else
                System.out.println("[!] Available arguments: [Number]");
        } // else get input from user
        else {
            Scanner scanner = new Scanner(System.in);
            System.out.print("[<-] Enter a number to find the factorial: ");
            number = scanner.nextLong();
        }

        factorialValue = generateFactorial(number);
        if (factorialValue == -1)
            System.out.println("[->] Factorial of " + number + " is undefined.");
        else
            System.out.println("[->] Factorial of " + number + " is " + factorialValue + ".");
    }

    // a recursive method to generate the Factorial of a number n
    public static long generateFactorial(long number) {
        if (number < 0) // if the number is negative then return -1
            return -1;
        if (number == 1 || number == 0) // if the number is 1 or 0 then return 1 or 0 respectively
            return 1;
        else
            return number * generateFactorial(number - 1);
    }
}

import java.util.Scanner; : imports Scanner class from java > util package. Scanner is a class that lets you get input form the user. Scanner can take any primitive data type as input from the user.

public static void main(String[] args) : main method/function is the first method that will be executed in any java programs.

String[] args : args is an array of String type. It is used to get commandline arguments for your program.

args.length > 0 : checks whether args is empty or not

System.out.println(); : Prints the String and changes output cursor to next line

Scanner scan = new Scanner ( System.in ); : scan is an object(a special variable that stores the address in RAM/main memory) of the class Scanner. Here scan acts as the variable that has the access to System.in stream (An input stream that connects keyboard input to the program.

scan.nextInt() : gets Integer input from user.

scan.close(); : closes the input stream link/pipe.

public static long generateFactorial(long number) : user defined recursive method that returns the factorial of a number n.