java integer array size confusion

Question

I have this code

int[] i = new int[127*1024];

is working

int[] i = new int[128*1024];

is throwing

Exception in thread "main" java.lang.OutOfMemoryError: Java heap space at memory.main(memory.java:123)

running this using the following command in a 64 bit windows machine, jdk 1.8 java -Xms1M -Xmx1M memory

As per my understanding, java int is of 4 bytes(32 bit) and int[] array is an object. In the above case its failing if the size is 131.072 bytes(128 * 1024) with 1mb of heap space Can anyone help in explaining why its going out of memory?

• Louis Wasserman: I'm sorry, did you assume nothing else in the heap was taking up any space? Say, all the other infrastructure for a Java VM?
• biziclop: And the size is 128*1024*4 bytes (+a few bytes of overhead), so 4 times as much as you said.
• user207421: You're using at least 128*1024*4=524288 bytes for this array, leaving less than 1/2Mb for the rest of Java. Never seen it run that small in 20 years.
• Vinod Krishnan: this counts to 4 * 128 * 1024 ~512 bytes... u mean additional bytes overhead also accounts to 512 bytes to fill 1 mb?

Answer 1

Space for every element of the array is allocated on array creation.
All elements are initialized to 0.

int values are 4 bytes, so this array causes 128 * 1024 * 4 bytes to be allocated, which is 0.5 Mb.

Your array is not the only object on the heap at that line of code, which is why your JVM explodes, even though you gave it 1 Mb of memory.

You need to give more memory to your JVM to avoid this error.

• Bohemian: @VinodKrishnan there's not much more to explain. Try this: All the space ever needed by the array is allocated when it's created. Assigning values to array elements uses zero memory - the place to hold the value has already been allocated.