Generate a Java program to implement Merge Sort algorithm to efficiently sort an array of elements in ascending order.

int[] arr = {10, 2, 20, 1, 3, 100, 300, 4};

for(int i = 0; i < arr.length; i++)
{
  for(int j = i; j < arr.length; j++)
  {
    if(arr[i] > arr[j])
    {
      int temp = arr[i];
      arr[i] = arr[j];
      arr[j] = temp;
    }
  }
}

for(int i = 0; i < arr.length; i++)
  System.out.println(arr[i]);