Java: fixed size array of 2D int array [duplicate]

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
– Jon Skeet, Commented Aug 15, 2012 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; see Jon's note at the bottom of the answer) but it does not have to. — Sergey Kalinichenko
– Sergey Kalinichenko, Commented Aug 15, 2012 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
– TheZ, Commented Aug 15, 2012 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
– Sophie Sperner, Commented Aug 15, 2012 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
– Sophie Sperner, Commented Aug 15, 2012 at 18:34

Peter Lawrey · Accepted Answer · 2012-08-15 18:36:24Z

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, 2012 at 18:36

Peter Lawrey

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4 Comments

@downvoter Can you comment as to why?

+1 as consolation for the downvote (5:1 payback--win!).

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 :)

@MarkoTopolnik true, the difference doesn't matter either way. ;)

Sam R. · Accepted Answer · 2014-08-22 23:44:00Z

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.

Sam R.

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answered Aug 15, 2012 at 18:40

Nandkumar Tekale

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