Write a Java program modifying the standard Merge Sort algorithm to perform the merging of the two subarrays in-place, without using any additional memory for creating temporary arrays. The goal is to reduce the overall space complexity of the algorithm.

public class MergeSort {
  public static void main(String[] args) {
    int[] arr = { 5, 1, 6, 2, 3, 4 };
    mergeSort(arr);
    for(int i = 0; i < arr.length; i++) {
      System.out.print(arr[i]);
    }
  }
  public static void mergeSort(int[] arr) {
    int size = arr.length;
    if(size < 2) {
      return;
    }
    int mid = size / 2;
    int leftSize = mid;
    int rightSize = size - mid;
    int[] left = new int[leftSize];
    int[] right = new int[rightSize];
    for(int i = 0; i < mid; i++) {
      left[i] = arr[i];
    }
    for(int i = mid; i < size; i++) {
      right[i - mid] = arr[i];
    }
    mergeSort(left);
    mergeSort(right);
    merge(left