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Consider the below case 

int[] anArray = new int[10];

Allocates memory for 10 elements of each 32-bit on heap.Right ?

So what is the size of the element if the element type is Object. ??

like 

Object[] objArray = new Object[10];

How much memory allocated now on heap ? I just got the doubt by ssing the source code of ArrayList.

  private transient Object[] elementData;

Just tried this line in my machine 

List<String> s = new ArrayList<String>(Integer.MAX_VALUE);

results 

Exception in thread "main" java.lang.OutOfMemoryError: Requested array size exceeds VM limit
    at java.util.ArrayList.<init>(Unknown Source)
    at com.nextenders.server.guice.actions.servlets.Test.main(Test.java:13)

So I'm trying to know how much memory allocated.

Suresh Atta
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    Possible duplicate of [Arrays in Java and how they are stored in memory](http://stackoverflow.com/questions/5564423/arrays-in-java-and-how-they-are-stored-in-memory). See also [How big is an object reference in Java and precisely what information does it contain?](http://stackoverflow.com/questions/981073/how-big-is-an-object-reference-in-java-and-precisely-what-information-does-it-co) – Duncan Jones Apr 08 '14 at 09:03
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    Allow me to extract (and slightly adjust) the relevant snippets from @Duncan linked questions: `New Object[10] creates space for 10 Object references only. It does not create 10 Object objects (or even free space for 10 Object objects)` and `Each reference will be an address: 32 bit on 32 bit CPU, 64 at 64` – Ceiling Gecko Apr 08 '14 at 09:10
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    Yes , see the line `or even free space for 10 Integer objects`. So why `OutOfMemoryError` occurred just for initialization ?? – Suresh Atta Apr 08 '14 at 09:15
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    Because `ArrayList(Integer.MAX_VALUE);` requested 2^(31-1) number of addresses. – Ceiling Gecko Apr 08 '14 at 09:23
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    strange case I am curious of the solution – RMachnik Apr 08 '14 at 09:23
  • Also, I hope you do realize that arrays are not in fact infinite. – Ceiling Gecko Apr 08 '14 at 09:24
  • Google can answer your question easily. – Germann Arlington Apr 08 '14 at 09:24

3 Answers3

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So what is the size of the element if the element type is Object. ??

The size of an Object[] array element is the size of a reference.

  • On a 32 bit JVM, a reference is 32 bits (4 bytes)

  • On a 64 bit JVM, a reference is 64 bits (8 bytes), or 32 bits if the "compressed oop" optimization is enabled and the heap size is less than 32Gb.

So your array allocation will allocate a heap object containing roughly 4 x 10 or 8 x 10 bytes ... plus about 12 bytes of object header overhead.

Just tried this line in my machine

List<String> s = new ArrayList<String>(Integer.MAX_VALUE);

Under the covers, you are attempting to allocate attempting to allocate an array containing between 233 and 234 bytes. With a 32bit JVM, that is guaranteed to not work. With a 64 bit JVM, you'd need a heap of at least 8Gb (compressed oop) or 16Gb for that to work.

Stephen C
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  1. 10 x 4bytes = 40 bytes
  2. The maximum size for the object store is 256 MB of compressible, nonvolatile, RAM storage. The size of the object store can be changed to customize the amount of persistent storage available for applications and their related data. default Object = 4bytes in x32bit and 8bytes in x64bit
  3. It is depend on what items of object are holding. Since you are not assign any items in object so the Heap will be calculate by default Object = 4bytes x 10 = 40bytes.
Minh
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It will create an array of 10 32 bit references to objects (the memory address where the object is stored) as all objects in java are just pointers to the memory where the object is stored. Or if you are using a 64 bit machine the addresses will be 64 bits

06needhamt
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