Question

An int array of 100 million ints should be 400 MB, right?
One int = 4 bytes

So if I set the VM to have a max heap of 1024m with -Xmx1024m why does this code fail:

public static void main(String[] args) {

    int[] b = new int[100000000]; //100 mil = 400mb
    System.out.println("ok");
    int[] c = new int[100000000];

}

This is the output:

ok
Exception in thread "main" java.lang.OutOfMemoryError: Java heap space
at Main.main(Main.java:9)

Solution – break it up!

public static void main(String[] args) {

    int[] a = new int[50000000]; //50 mil = 200mb
    System.out.println("ok 1");
    int[] b = new int[50000000];
    System.out.println("ok 2");
    int[] c = new int[50000000];
    System.out.println("ok 3");
    int[] d = new int[50000000];
    System.out.println("ok 4");


}

Answer 1

The problem lies in the fact that when you’re creating this int[], it requires contiguous memory of 400mb. Basically this means, you need a solid block of 400mb of memory for use. Due to fragmentation of the heap, it can’t find a solid 400mb block of memory to use. That’s why you’re getting the out of memory error in this situation.