This question will probably require some compiler knowledge to answer. I am currently working on a project where I will be creating an array that may be either
int[2][veryLargeNumber]
or
int [veryLargeNumber][2]
It makes no difference logically but I was thinking that the form (and therefore size) in memory may differ (perhaps the question should be, are the compilers clever enough to rearrange arrays to suit them)?
The size of the integer is 4 bytes.
Thus, if you add a short to an int, short is the narrower type, so a temporary int with the closest value to the short is created. This temporary int value is added to the other int operand, and results is an int.
An array is a container object that holds a fixed number of values of a single type. The length of an array is established when the array is created. After creation, its length is fixed. You have seen an example of arrays already, in the main method of the "Hello World!" application.
Interesting question, I ran a simple program
int N = 100000000; long start = System.currentTimeMillis(); int[][] a = new int[2][N]; System.out.println(System.currentTimeMillis() - start + " ms");
Which resulted in 160 ms
. Then I ran the other variant
int N = 100000000; long start = System.currentTimeMillis(); int[][] a = new int[N][2]; System.out.println(System.currentTimeMillis() - start + " ms");
Which resulted in 30897 ms
. So indeed the first option seems much better.
Java only actually implements single dimensional arrays. It has multi-dimensional types, however two dimensional arrays are actually implemented as an array of arrays. Each array has an overhead of about 16 bytes. You are better off with int[2][x]
to minimise overhead.
You can avoid this issue entirely by using helper methods.
final int[] array = new int[2 * veryLargeNumber]; public int get(int x, int y) { return array[idx(x, y)]; } public void set(int x, int y, int val) { array[idx(x, y)] = val; } private int idx(int x, int y) { return x * 2 + y; // or x * veryLargeNumber + y; }
To provide this to yourself, each object hash a unique, generate hashCode which is stored in its Object header.
You can see from http://ideone.com/oGbDJ0 that each nested array is an object in itself.
int[][] array = new int[20][2]; for (int[] arr : array) { System.out.println(arr); }
prints the internal representation of an int[]
which is [I
followed by @
followed by the hashCode() stored in the header. This is not as some believe, the address of the object. The address can't be used as the hashCode, as the object can be moved at any time by the GC (unless you have a JVM which never moves objects)
[I@106d69c [I@52e922 [I@25154f [I@10dea4e [I@647e05 [I@1909752 [I@1f96302 [I@14eac69 [I@a57993 [I@1b84c92 [I@1c7c054 [I@12204a1 [I@a298b7 [I@14991ad [I@d93b30 [I@16d3586 [I@154617c [I@a14482 [I@140e19d [I@17327b6
You can see how much memory is used if you turn off the TLAB with -XX:-UseTLAB
https://github.com/peter-lawrey/Performance-Examples/blob/master/src/main/java/vanilla/java/memory/ArrayAllocationMain.java
public static void main(String[] args) { long used1 = memoryUsed(); int[][] array = new int[200][2]; long used2 = memoryUsed(); int[][] array2 = new int[2][200]; long used3 = memoryUsed(); if (used1 == used2) { System.err.println("You need to turn off the TLAB with -XX:-UseTLAB"); } else { System.out.printf("Space used by int[200][2] is " + (used2 - used1) + " bytes%n"); System.out.printf("Space used by int[2][200] is " + (used3 - used2) + " bytes%n"); } } public static long memoryUsed() { Runtime rt = Runtime.getRuntime(); return rt.totalMemory() - rt.freeMemory(); }
prints
Space used by int[200][2] is 5720 bytes Space used by int[2][200] is 1656 bytes
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