Why I can't create an array with large size?

Question

Why it is impossible to create an array with max int size?

int i = 2147483647;
int[] array = new int[i];

I found this explanation:

Java arrays are accessed via 32-bit ints, resulting in a maximum theoretical array size of 2147483647 elements.

But as you can see my code doesn't work. It is also impossible to create an array with size

new int[Integer.MAX_VALUE - 5];

Technical details

• 64-Bit HotSpot JVM
• OSX 10.10.4

PS

And why -5 actually?

Answer 1

Some VMs reserve some header words in an array.

The maximum "safe" number would be 2 147 483 639 (Integer.MAX_VALUE - 8)

Source-http://www.docjar.com/html/api/java/util/ArrayList.java.html

**
  191        * The maximum size of array to allocate.
  192        * Some VMs reserve some header words in an array.
  193        * Attempts to allocate larger arrays may result in
  194        * OutOfMemoryError: Requested array size exceeds VM limit
  195        */
  196       private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;

So It depends on the maximum memory available to your JVM on your SYSTEM NOW

Edit : Why It's Showing OOM.

Number of Elements = 2 147 483 639

number of bytes required for one element = 4

Total Memory for just Element 8589934556 KB == 8.589934555999999 GB

Now If the total memory usage of the array is not a multiple of 8 bytes, then the size is rounded up to the next mutlitple of 8 .

So You need more than what you are allocating due to Overheads too and that should be continuous memory

Answer 2

Theory

There are two possible exceptions:

• OutOfMemoryError: Java heap space means your array does not fit into java heap space. In order to solve you can increase the maximum heap size by using JVM option -Xmx. Also take into account that the maximum size of object cannot be larger than the largest heap generation.
• OutOfMemoryError: Requested array size exceeds VM limit means platform-specific size was exceeded:
  • the upper bound limit is set by the restrictions of the size type used to describe an index in the array, so theoretical array size is limited by 2^31-1=2147483647 elements.
  • the other limit is JVM/platform specific. According to chapter 10: Arrays of The Java Language Specification, Java SE 7 Edition there is no strict limit on array length, thus array size may be reduced without violating JLS.

Practice

In HotSpot JVM array size is limited by internal representation. In the GC code JVM passes around the size of an array in heap words as an int then converts back from heap words to jint this may cause an overflow. So in order to avoid crashes and unexpected behavior the maximum array length is limited by (max size - header size). Where header size depends on C/C++ compiler which was used to build the JVM you are running(gcc for linux, clang for macos), and runtime settings(like UseCompressedClassPointers). For example on my linux:

• Java HotSpot(TM) 64-Bit Server VM 1.6.0_45 limit Integer.MAX_VALUE
• Java HotSpot(TM) 64-Bit Server VM 1.7.0_72 limit Integer.MAX_VALUE-1
• Java HotSpot(TM) 64-Bit Server VM 1.8.0_40 limit Integer.MAX_VALUE-2

Useful Links

• https://bugs.openjdk.java.net/browse/JDK-8059914
• https://bugs.openjdk.java.net/browse/JDK-8029587

Answer 3

It's not enough to just have enough heap for that allocation; you need to have a single heap region of sufficient size. As you know, heap is divided into generations.

For a single allocation of 8 GB you must ensure that much for a single heap region (plus some overhead). With 12 GB of -Xmx you may still be short. Use additional options to control the size of the Old Generation.

Answer 4

find your max heap size by going to cmd and enter this line

javaw -XX:+PrintFlagsFinal | find "MaxHeapSize"

and then divide it by 1.5, you will get the approximate maximum size of the array for your computer