Java Program to Find Sum of N Natural Numbers

You can find the sum of n natural numbers in Java using the formula sum = n * (n + 1) / 2 or by looping from 1 to n and adding each number.

Examples

Input5

OutputSum of first 5 natural numbers is 15

Input10

OutputSum of first 10 natural numbers is 55

Input1

OutputSum of first 1 natural number is 1

How to Think About It

To find the sum of the first n natural numbers, think of adding all numbers from 1 up to n. You can do this by either adding each number one by one or by using the formula n * (n + 1) / 2 which gives the total directly.

Algorithm

1

Get the input number n from the user.

2

Calculate the sum using the formula sum = n * (n + 1) / 2.

3

Print the sum.

Code

java

import java.util.Scanner;

public class SumNaturalNumbers {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        System.out.print("Enter a positive integer: ");
        int n = scanner.nextInt();
        int sum = n * (n + 1) / 2;
        System.out.println("Sum of first " + n + " natural numbers is " + sum);
        scanner.close();
    }
}

Output

Enter a positive integer: 5
Sum of first 5 natural numbers is 15

Dry Run

Let's trace the input 5 through the code

1

Input

User enters n = 5

2

Calculate sum

sum = 5 * (5 + 1) / 2 = 5 * 6 / 2 = 30 / 2 = 15

3

Output

Print "Sum of first 5 natural numbers is 15"

n	sum
5	15

Why This Works

Step 1: Input reading

The program reads the number n from the user using a scanner.

Step 2: Sum calculation

It uses the formula n * (n + 1) / 2 which adds all numbers from 1 to n efficiently.

Step 3: Output display

The program prints the calculated sum with a clear message.

Alternative Approaches

Using loop

java

import java.util.Scanner;

public class SumNaturalNumbersLoop {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        System.out.print("Enter a positive integer: ");
        int n = scanner.nextInt();
        int sum = 0;
        for (int i = 1; i <= n; i++) {
            sum += i;
        }
        System.out.println("Sum of first " + n + " natural numbers is " + sum);
        scanner.close();
    }
}

This method uses a loop to add each number one by one, which is easy to understand but less efficient for very large n.

Using recursion

java

import java.util.Scanner;

public class SumNaturalNumbersRecursion {
    public static int sumNatural(int n) {
        if (n == 1) return 1;
        return n + sumNatural(n - 1);
    }

    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        System.out.print("Enter a positive integer: ");
        int n = scanner.nextInt();
        int sum = sumNatural(n);
        System.out.println("Sum of first " + n + " natural numbers is " + sum);
        scanner.close();
    }
}

This method uses recursion to add numbers, which is elegant but can cause stack overflow for very large n.

Complexity: O(1) time, O(1) space

Time Complexity

Using the formula, the calculation is done in constant time without loops.

Space Complexity

Only a few variables are used, so space is constant.

Which Approach is Fastest?

The formula method is fastest; looping or recursion take O(n) time and more memory.

Approach	Time	Space	Best For
Formula	O(1)	O(1)	Fastest and simplest for any n
Loop	O(n)	O(1)	Easy to understand, good for small n
Recursion	O(n)	O(n)	Elegant but risky for large n due to stack