Recursive program to find maximum in array

Write a recursive program to find greatest elements in array.

import java.util.Objects;

public class MaximumFinder {

 public static int getMaximum(int arr[]) {
  Objects.nonNull(arr);
  if (arr.length < 0)
   return Integer.MIN_VALUE;

  return maximum(arr, 0);
 }

 private static int maximum(int arr[], int index) {
  if (index == arr.length - 1)
   return arr[index];

  int val = maximum(arr, index + 1);
  if (val < arr[index])
   return arr[index];
  return val;
 }
}

Following is the junit test case for above program.

import static org.junit.Assert.assertEquals;

import org.junit.Test;

public class MaximumFinderTest {
 @Test
 public void test1() {
  int arr1[] = { 1, 2, 3, 4, 5 };
  int arr2[] = { 5, 2, -2, 10, 5 };
  int arr3[] = { 1 };

  assertEquals(MaximumFinder.getMaximum(arr1), 5);
  assertEquals(MaximumFinder.getMaximum(arr2), 10);
  assertEquals(MaximumFinder.getMaximum(arr3), 1);
 }
}