Find the first element that appears an even number of times.

Given an array, find the first element that appears an even number of times.

import java.util.LinkedHashMap;
import java.util.Objects;
import java.util.Optional;
import java.util.Set;

public class ArrayUtil {
 public static Optional<Integer> getFirstEvenNumber(int[] arr) {
  if (Objects.isNull(arr))
   throw new NullPointerException("Array Shouldn't be null");

  LinkedHashMap<Integer, Integer> map = new LinkedHashMap<>();

  for (int data : arr) {
   if (!map.containsKey(data)) {
    map.put(data, 1);
    continue;
   }

   map.put(data, map.get(data) + 1);
  }
  
  Set<Integer> keys = map.keySet();

  for (int key : keys) {
   if (map.get(key) % 2 == 0) {
    return Optional.of(key);
   }
  }

  return Optional.empty();

 }
}

import static org.junit.Assert.assertEquals;
import static org.junit.Assert.assertTrue;

import org.junit.Test;

public class ArrayUtilTest {

 @Test
 public void test() {
  int arr1[] = { 1, 2, 3, 4, 1, 2, 3, 4, 1, 3, 2 };
  int arr2[] = {};
  int arr3[] = { 1, 2, 3, 4, 1, 2, 3, 4, 1, 3, 2, 4 };
  int arr4[] = { 1, 2, 3, 4, 4, 3, 2, 1 };
  int arr5[] = {5, 1, 2, 3, 4,  4, 3, 2, 1, 5};

  int result1 = ArrayUtil.getFirstEvenNumber(arr1).get();
  int result4 = ArrayUtil.getFirstEvenNumber(arr4).get();
  int result5 = ArrayUtil.getFirstEvenNumber(arr5).get();

  assertEquals(result1, 4);

  if (!ArrayUtil.getFirstEvenNumber(arr2).isPresent()) {
   assertTrue(true);
  } else {
   assertTrue(false);
  }

  if (!ArrayUtil.getFirstEvenNumber(arr3).isPresent()) {
   assertTrue(true);
  } else {
   assertTrue(false);
  }

  assertEquals(result4, 1);
  assertEquals(result5, 5);
 }
}