A basic program that gets input n (Integer) from the user which is considered as the limit number and prints the sum of the first n natural numbers. By this program, you can understand the usage of commandline arguments, use of arithmetic operators, printing variables, user-defined method and recursive method to find the sum of first n-natural numbers.

A natural number is an integer which is positive and is greater than 0.

The natural number series goes on like this: 1, 2, 3, 4, 5, ... upto infinity.

Task: Print the sum of first n-natural numbers where 'n' is the input from the user.

  Eg: n = 10
  Final Output: 
         Sum of the first 10 integers is 55.
         
  Eg: n = 0
  Final Output: 
         Sum of the first 0 integers is 0.
         
  Eg: n = -1
  Final Output: 
         Seems like you entered a non-natural number, cannot find the sum !

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 .\SumNaturalNum\SumNaturalNum.java
 $ ..\JavaRecursion> java SumNaturalNum.SumNaturalNum

• With Commandline Arguments

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

package SumNaturalNum;

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

public class SumNaturalNum {
    static int n = 0; // n number of integers, initialised to zero for now.
    static int sumValue = 0; // sum of the first n integers, initialised to zero for now.

    public static void main(String[] args) {
        // if user provided commandline-arguments, then
        if (args.length != 0) {
            if (args.length == 1)
                n = Integer.parseInt(args[0]);
            else
                System.out.println("[!] Available arguments: [n, the first 'n' integers upto which sum is to be found]");

        } // else get input from user
        else {
            Scanner scanner = new Scanner(System.in);
            System.out.print("[<-] Enter the number of terms: ");
            n = scanner.nextInt();
        }

        sumValue = findSumNaturalNumbers(n); // getting the result from the recursive method
        if (sumValue == -1)
            System.out.println("[->] Seems like you entered a non-natural number, cannot find the sum !");
        else
            System.out.println("[->] Sum of the first " + n + " integers is " + sumValue + ".");
    }

    // a recursive method to find the sum of first n-natural numbers.
    public static int findSumNaturalNumbers(int number) {
        if (number < 0) // if the number is a negative integer then return -1
            return -1;
        if (number == 0 || number == 1) // if the number is 0 or 1 then return 0 or 1 respectively.
            return number;
        return number + findSumNaturalNumbers(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 int findSumNaturalNumbers(int number) : user defined recursive method that returns the sum of first n natural numbers where n is a parameter.