Java Program to Find the Sum of First N Odd & Even Numbers

When any number which ends with 0,2,4,6,8 is divided by 2 that is an even number. And when any number ends with 1,3,5,7,9 is not divided by two is an odd number.

Example:

Input : 8
Output: Sum of First 8 Even numbers = 72
        Sum of First 8 Odd numbers = 64

Approach #1: Iterative

1. Create two variables evenSum and oddSum and initialize them by 0.
2. Start For loop from 1 to 2*n.
3. If i is even Add i with evenSum.
4. Else add i with oddSum.
5. Print evenSum and oddSum at the end of loop.

Below is the implementation of the Java program:

Java

// Calculate the Sum of First N Odd & Even Numbers in Java
import java.io.*;

public class GFG {

    // Driver function
    public static void main(String[] args)
    {
        int n = 8;
        int evenSum = 0;
        int oddSum = 0;

        for (int i = 1; i <= 2 * n; i++) {
            // check even & odd using Bitwise AND operator
            if ((i & 1) == 0)
                evenSum += i;
            else
                oddSum += i;
        }
        // Sum of even numbers less than 17
        System.out.println("Sum of First " + n
                           + " Even numbers = " + evenSum);

        // sum of odd numbers less than 17
        System.out.println("Sum of First " + n
                           + " Odd numbers = " + oddSum);
    }
}

Output

Sum of First 8 Even numbers = 72
Sum of First 8 Odd numbers = 64

Time Complexity: O(N), where N is the number of First N even/odd numbers.
Auxiliary Space: O(1)

Method 2: Using AP Formulas.

• Sum of First N Even Numbers = n * (n+1)
• Sum of First N Odd Numbers = n * n

Below is the implementation of the above approach:

Java

// Calculate the Sum of First N Odd & Even Numbers in Java
import java.io.*;

public class GFG {

    // Function to find the sum of even numbers
    static int sumOfEvenNums(int n) { return n * (n + 1); }

    // Function to find the sum of odd numbers.
    static int sumOfOddNums(int n) { return n * n; }

    // Driver function
    public static void main(String[] args)
    {
        int n = 10;
        int evenSum = sumOfEvenNums(n);
        int oddSum = sumOfOddNums(n);

        // Sum of even numbers
        System.out.println("Sum of First " + n
                           + " Even numbers = " + evenSum);

        // sum of odd numbers
        System.out.println("Sum of First " + n
                           + " Odd numbers = " + oddSum);
    }
}

Output

Sum of First 10 Even numbers = 110
Sum of First 10 Odd numbers = 100

Time Complexity: O(1)
Auxiliary Space: O(1)