private void validateInput(int[] numbers) {
    if (numbers == null) {
      throw new IllegalArgumentException("You can't pass a null array of numbers.");
    }
  }
Another iterative solution with a much better performance. This algorithm has a complexity
order in space terms of O(N) where N is the number of elements in the array and O(N) in time
terms because we are going to iterate over the numbers array just one time instead of two. The
base idea in this algorithm is to store in a map the number you need to complete the sum just
in cause you can find it in the future.
src/main/java/com/github/pedrovgs/problem28/FindSums.java