Do Java arrays have a maximum size?

Haven't seen the right answer, even though it's very easy to test.

In a recent HotSpot VM, the correct answer is Integer.MAX_VALUE - 5. Once you go beyond that:

public class Foo {
  public static void main(String[] args) {
    Object[] array = new Object[Integer.MAX_VALUE - 4];
  }
}

You get:

Exception in thread "main" java.lang.OutOfMemoryError:
  Requested array size exceeds VM limit

This is (of course) totally VM-dependent.

Browsing through the source code of OpenJDK 7 and 8 java.util.ArrayList, .Hashtable, .AbstractCollection, .PriorityQueue, and .Vector, you can see this claim being repeated:

/**
 * Some VMs reserve some header words in an array.
 * Attempts to allocate larger arrays may result in
 * OutOfMemoryError: Requested array size exceeds VM limit
 */
private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;

which is added by Martin Buchholz (Google) on 2010-05-09; reviewed by Chris Hegarty (Oracle).

So, probably we can say that the maximum "safe" number would be 2 147 483 639 (Integer.MAX_VALUE - 8) and "attempts to allocate larger arrays may result in OutOfMemoryError".

(Yes, Buchholz's standalone claim does not include backing evidence, so this is a calculated appeal to authority. Even within OpenJDK itself, we can see code like return (minCapacity > MAX_ARRAY_SIZE) ? Integer.MAX_VALUE : MAX_ARRAY_SIZE; which shows that MAX_ARRAY_SIZE does not yet have a real use.)