Find mean of array recursively

The mean is the average of the numbers. Add up all the numbers, then divide by how many numbers there are.

import java.util.Objects;

public class MeanCalc {

 public static double getMean(int arr[]) {
  Objects.nonNull(arr);
  if (arr.length == 0)
   throw new IllegalArgumentException("Array is empty");

  return sum(arr, 0) / arr.length;
 }

 private static double sum(int[] arr, int index) {
  if (index == arr.length - 1)
   return arr[index];
  return arr[index] + sum(arr, ++index);
 }
}

Following is the junit test case for above program.

import static org.junit.Assert.assertTrue;

import org.junit.Test;

public class MeanCalcTest {

 @Test
 public void test1() {
  int arr1[] = { 1, 2, 3, 4 };
  int arr2[] = { 5 };

  assertTrue(MeanCalc.getMean(arr1) == 2.5);
  assertTrue(MeanCalc.getMean(arr2) == 5);
 }
}