Java: fixed size array of 2D int array

Question

Is it possible to create an array of 2D int arrays like:

int n = 100;
int[][] [] a = new int[][] [n];

Array has a fixed length n and matrices (2D arrays) have different non-zero sizes (at least 1 x 1).

For performance I would like to store that in the stack, not like:

ArrayList<int[][]> a = new ArrayList<int[][]>(n);

which will be stored on the heap as far as I know.

Arrays are *always* stored in the heap - at least notionally. They're reference types. (It's possible that very smart JVMs will perform escape analysis, but you should *assume* that it will be on the heap.) — Jon Skeet, Aug 15 '12 at 18:31
Your arrays wouldn't be stored on the stack if they go into a collection. Java *may* try to optimize this ([link](http://stackoverflow.com/a/2099726/335858); see Jon's note at the bottom of the answer) but it does not have to. — Sergey Kalinichenko, Aug 15 '12 at 18:31
Suggestion, 2D array are overrated: Use a pseudo-2D-array with size of length * width, and access with `[x+(length*y)]` — TheZ, Aug 15 '12 at 18:32
Even some simple array like `int[] a = new int[5]` will be on the heap.. Alright, thanks for information, so `ArrayList` is a solution. — Sophie Sperner, Aug 15 '12 at 18:33
@ TheZ: very interesting, would be nice to see the reasons why [x*y] linear array is better than 2D array.. — Sophie Sperner, Aug 15 '12 at 18:34
@SophieSperner a 1D array is often chosen when an array is non-jagged and rectangular as it can in cases be more efficient, but in this case your array __is__ jagged, so ignore him. — obataku, Aug 15 '12 at 18:36
Whoops, forgot the length and typed it wrong! Fixed. Sleepy brain is not good... for clarity: capacity of `xsize * ysize`, and access with `[x+(xsize*y)]`. Note that the technique can be expanded to any size multidimensional array as needed (it just complicates the access calculation a little more) — TheZ, Aug 15 '12 at 18:36
@SophieSperner hopefully I've answered your question! Note that `List` and `ArrayList` are __not__ meant as replacements for arrays. — obataku, Aug 15 '12 at 18:39
@veer Often ArrayList is a better choice than using array and the performance difference is often not important. — Peter Lawrey, Aug 15 '12 at 18:52
@PeterLawrey maybe if what you want is a `List`, then sure; my point is that they're not congruent, not that there is a performance difference. — obataku, Aug 15 '12 at 18:56
@veer its not a drop in replacement that's for sure. There is a performance difference between `int[]` and `List` but even then it might be as much you might expect esp if the values are small. ;) — Peter Lawrey, Aug 15 '12 at 18:58

score 4 · Answer 1 · answered Aug 15 '12 at 18:36

4

To create a 3D array

int n = 100;
int[][][] a = new int[n][][];

This creates 100 array of array of any dimension.

This is almost as (in)efficient as

List<int[][]> a = new ArrayList<int[][]>(n);

answered Aug 15 '12 at 18:36

Peter Lawrey

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@downvoter Can you comment as to why? – Peter Lawrey Aug 15 '12 at 18:37
+1 as consolation for the downvote (5:1 payback--win!). – Ted Hopp Aug 15 '12 at 18:58
Just for humor: nothing can be "almost as (in)efficient as X" since it would have to approach the efficiency of X from both sides at once :) – Marko Topolnik Aug 15 '12 at 19:28
@MarkoTopolnik true, the difference doesn't matter either way. ;) – Peter Lawrey Aug 15 '12 at 19:36

score 3 · Answer 2 · edited Aug 22 '14 at 23:44

3

Is it possible to create an array of 2D int arrays like:
int n = 100;
int[][] [] a = new int[][] [n];

--> this is invalid with syntax, you will get compiler error. Use :

int n = 100;
int[][] [] a = new int[n][] [];

but a is an object as arrays in java are object so a will be stored on heap not on stack.

edited Aug 22 '14 at 23:44

Sam R.

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answered Aug 15 '12 at 18:40

Nandkumar Tekale

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Java: fixed size array of 2D int array

2 Answers2