Efficient System.arraycopy on multidimensional arrays

Question

I'm aware that a common performance refactoring is to replace simple for's by System.arraycopy.

I want to ask about:

1. When exactly does system.arraycopy begin to make sense (considering it's a native method call). Does copying small things say, < 32 have any advantage?

2. Is it my impression, or is it not simply possible to copy (efficiently) a cycle like this with arraycopy:

      for (int j = 0; j < 2; ++j) {
          vpr[m][s + j][i] = vr[j];
      }
   

Answer 1

It's not difficult to use System.arraycopy to do a quick deep copy. Here's an example for a 2D array:

for (int i = 0; i < src.length; i++) {
    System.arraycopy(src[i], 0, dest[i], 0, src[0].length);
}

From a quick timing test, using this to copy a 1000x1000 2D array 100 times takes 40 milliseconds, versus 1740 milliseconds using the more obvious two for loops and assignment.

Answer 2

AFAIK, System.arrayCopy is the most efficient and best way to copy your arrays. I am not aware of any situations where the alternate way of implementing your own loops would be much more efficient for direct copies.