Java Performance - ArrayLists versus Arrays for lots of fast reads

Question

I have a program where I need to make 100,000 to 1,000,000 random-access reads to a List-like object in as little time as possible (as in milliseconds) for a cellular automa

Answer 1

There will be an overhead from using an ArrayList instead of an array, but it is very likely to be small. In fact, the useful bit of data in the ArrayList can be stored in registers, although you will probably use more (List size for instance).

You mention in your edit that you are using wrapper objects. These do make a huge difference. If you are typically using the same value repeatedly, then a sensible cache policy may be useful (Integer.valueOf gives the same results for -128 to 128). For primitives, primitive arrays usually win comfortably.

As a refinement, you might want to make sure the adjacent cells tend to be adjacent in the array (you can do better than rows of columns with a space filling curve).

Answer 2

If you're not going to be doing a lot more than reads from this structure, then go ahead and use an array as that would be faster when read by index.

However, consider how you're going to get the data in there, and if sorting, inserting, deleting, etc, are a concern at all. If so, you may want to consider other collection based structures.

Answer 3

Primitives are much (much much) faster. Always. Even with JIT escape analysis, etc. Skip wrapping things in java.lang.Integer. Also, skip the array bounds check most ArrayList implementations do on get(int). Most JIT's can recognize simple loop patterns and remove the loop, but there isn't much reason to much with it if you're worried about performance.

You don't have to code primitive access yourself - I'd bet you could cut over to using IntArrayList from the COLT library - see http://acs.lbl.gov/~hoschek/colt/ - "Colt provides a set of Open Source Libraries for High Performance Scientific and Technical Computing in Java") - in a few minutes of refactoring.

Answer 4

An Array will be faster simply because at a minimum it skips a function call (i.e. get(i)).

If you have a static size, then Arrays are your friend.

Answer 5

If you're creating the list once, and doing thousands of reads from it, the overhead from ArrayList may well be slight enough to ignore. If you're creating thousands of lists, go with the standard array. Object creation in a loop quickly goes quadratic, simply because of all the overhead of instantiating the member variables, calling the constructors up the inheritance chain, etc.

Because of this -- and to answer your second question -- stick with standard ints rather than the Integer class. Profile both and you'll quickly (or, rather, slowly) see why.

Answer 6

The options are:
1. To use an array
2. To use the ArrayList which internally uses an array

It is obvious the ArrayList introduces some overhead (look into ArrayList source code). For the 99% of the use cases this overhead can be easily ignored. However if you implement time sensitive algorithms and do tens of millions of reads from a list by index then using bare arrays instead of lists should bring noticeable time savings. USE COMMON SENSE.

Please take a look here: http://robaustin.wikidot.com/how-does-the-performance-of-arraylist-compare-to-array I would personally tweak the test to avoid compiler optimizations, e.g. I would change "j = " into "j += " with the subsequent use of "j" after the loop.