Odd summation not working

Question

I have to write a program that adds all the odd numbers between two bounds. I got it to add the odd numbers, however I can't get it to work if one of the bounds is a negative. This is the code I have already.

import java.util.Scanner;

/**
   Computes a sum of odd integers between two bounds. 
   Input: a, the lower bound (may be odd or even).
   Input: b, the upper bound (may be odd or even).
   Output: sum of odd integers between a and b (inclusive).
*/
public class OddSum
{
   public static void main(String[] args)
   {
      // Read values for a and b
      Scanner in = new Scanner(System.in);
      int a = in.nextInt();
      int b = in.nextInt();
      int sum = 0;
      int swap;
      if(a > b) {
          swap = a;
          a = b;
          b = swap;
      }
      for (int i = a; i <=b; i++){
          if (i % 2 ==1)
              sum +=i;
      }
      System.out.println(sum);
   }
}

Answer 1

Annoyingly, the condition for an odd number is

n % 2 != 0

n % 2 == 1 does not work for negative odd numbers because n % 2 gives -1 .

Answer 2

Rather then testing i for oddness every loop iteration, I would suggest you start with the first odd number following your lowest value in range and then increment by 2 in the loop. Something like,

int a = in.nextInt();
int b = in.nextInt();
int lo = Math.min(a, b);
int hi = Math.max(a, b);
if (lo % 2 == 0) { // <-- ensure that lo is odd.
    lo++;
}
int sum = 0;
for (int i = lo; i <= hi; i += 2) {
    sum += i;
}
System.out.println(sum);

Answer 3

You need to use i % 2 != 0 as your condition to check whether a number is odd or not because the condition you're currently using unfortunately won't work for negative numbers.
After you've sorted out whether a or b should be first, you could get the sum in one line using IntStream :

int sum = IntStream.rangeClosed(a, b).filter(i -> i % 2 != 0).sum();

This will take the sum of all odd numbers. Don't forget to import

import java.util.stream.IntStream;

Cheers!