Why maximum size of an java array is Integer.MAX_VALUE/7?

Question

I am little surprised to see why on my machine, the maximum size of an array is Integer.MAX_VALUE/7
I know that the arrays are indexed by integers, and so the array size cannot be greater than Integer.MAX_VALUE. I also read some stackoverflow discussions where I found that it varies on the JVM, and some(5-8 bites) are used by JVM.
In that case also, the maximum values should be Integer.MAX_VALUE-8.

Any value in between Integer.MAX_VALUE-2 and Integer.MAX_VALUE/7 gives me the error: Exception in thread "main" java.lang.OutOfMemoryError: Java heap space

int[] arr = new int[Integer.MAX_VALUE/7];

This is the largest value I can assign to array on my machine. Are specific reasons for this?

Update: I am running the code from eclipse in which the default heap size is 1024Mb. Below are the more details from my environment:

System.out.println(Runtime.getRuntime().totalMemory()/(1024*3));
System.out.println(Runtime.getRuntime().freeMemory()/(1024*3));
System.out.println(Runtime.getRuntime().maxMemory()/(1024*3));

give the output:

40618
40195
594773

@cloudwalker I have already mentioned that link in my question. And I came to create a new question only when I didn't find that link helpful. — The Coder
– The Coder, Commented Jan 20, 2018 at 20:19
Dear downvoters, it would be very soothing to my mind if anyone could explain me for your downvotes!!!! — The Coder
– The Coder, Commented Jan 20, 2018 at 20:20

libik · Accepted Answer · 2018-01-20 23:09:29Z

2

As mentioned already by cloudworker, the real limits for array are explained here: Do Java arrays have a maximum size?

In your case 1GB is just not enough heap space for array that huge.

I do not know what exact processes are run in JVM, but from what I am able to count:

Integer.MAX_VALUE= ~2 billions
int = 4bytes
2billions*4bytes=8billions bytes = 8GB memory

With 1GB heapspace you should be able to have ~ /8 of MAX_VALUE. (I think that reason that you can actually get more than /8 is some optimization in JVM)

answered Jan 20, 2018 at 23:09

libik

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The Coder Over a year ago

I ran into a situation where I need to store around 300mb xml file. Since String is backed by a char[] only, and the maximum value a char[] can consume would be ~2billion * 2byte = 4GB. Will it be a bad practice to store such large file into String instance?

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