java - Performance comparison of array of arrays vs multidimensional arrays

Question

When I was using C++ in college, I was told to use multidimensional arrays (hereby MDA) whenever possible, since it exhibits better memory locality since it's allocated in one big chunk. Array of arrays (AoA), on the other hand, are allocated in multiple smaller chunks, possibly scattered all over the place in the physical memory wherever vacancies are found.

So I guess the first question is: is this a myth, or is this an advice worth following?

Assuming that it's the latter, then the next question would be what to do in a language like Java that doesn't have true MDA. It's not that hard to emulate MDA with a 1DA, of course. Essentially, what is syntactic sugar for languages with MDA can be implemented as library support for languages without MDA.

Is this worth the effort? Is this too low level of an optimization issue for a language like Java? Should we just abandon arrays and use Lists even for primitives?

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Another question: in Java, does allocating AoA at once (new int[M][N]) possibly yield a different memory allocation than doing it hierarchically (new int[M][]; for (... new int[N])?

Answer 1

Java and C# allocate memory in much different fashion that C++ does. In fact, in .NET for sure all the arrays of AoA will be close together if they are allocated one after another because memory there is just one continuous chunk without any fragmentation whatsoever.

But it is still true for C++ and still makes sense if you want maximum speed. Although you shouldn't follow that advise every time you want multidimensional array, you should write maintainable code first and then profile it if it is slow, premature optimization is root for all evil in this world.