does array indexes affects performance/memory usage if they has no value

Question

I'm new to Java and I've been wondering lately about memory consumption of huge, but partly empty array like in this case:

I need array of unknown size- it can be 300k of values, it can be 300m, but to keep in mind it might as well keep only like 50 values,

i'm initialising array with size of

int values[] = new int[Integer.MAX_VALUE];

user generates some number of values that has to be stored in array every time given generating method is used

Is there any contraindication of using excessively huge arrays? How the size of array matters in case of performance when, say only 0,1% of that array is used vs 100% array usage?

In other words- when im calling empty array of X int values, will the JVM on initialisation reserve memory for X*(memory used to store 1 int value) even if there isn't any value stored yet?

Thanks, Daniel.

@BoristheSpider are you seriously suggesting using an ArrayList of size Integer.MAX_VALUE for storing int values? Just no. — davmac
– davmac, Commented Jun 26, 2016 at 13:30
The OP says up to 300m values; this should be fine for an ArrayList. — Boris the Spider
– Boris the Spider, Commented Jun 26, 2016 at 13:32
@BoristheSpider I wouldn't make sense to me to use an ArrayList here, since there is only one reason why OP might create such a large array, but doesn't fill it completely (as far as I see it): he prefers to access the items by their index. So an ArrayList is no help here. A Map should be more suited, with Integer as the key. — Tom
– Tom, Commented Jun 26, 2016 at 14:03
@BoristheSpider my assumption from the question was that the indexes aren't known. So "300m" values doesn't mean 300 million values ranging from 0 through 299999999, it means 300 million arbitrary values. An ArrayList is not a good choice in this case, especially considering the overhead of storing an int (as an object) as compared to storing into an int[]. — davmac
– davmac, Commented Jun 26, 2016 at 14:16

Jon Skeet · Accepted Answer · 2016-06-26 13:43:21Z

6

How the size of array matters in case of performance when, say only 0,1% of that array is used vs 100% array usage?

There's no such thing as an "empty array of X int values". Every element will have a value - it'll just be 0 by default. Every element will take 4 bytes, regardless of its value. Your new int[Integer.MAX_VALUE] array will take 8GB of memory.

If you want to create a sparse array, you might want to consider using a Map instead. However, note that while you can have an int array, a map would need an Integer type for the value - if you get outside the range of cached Integer values (and if you have a lot of different values) you could end up wasting a lot of space on Integer objects (and references to them).

edited Jun 26, 2016 at 13:43

answered Jun 26, 2016 at 13:28

Jon Skeet

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Brandon Over a year ago

Or ArrayList, or any kind of List really.

Jon Skeet Over a year ago

@Brandon: Well, yes to ArrayList - but it would be perfectly feasible to create a List implementation designed for sparse population.

Daniel Stradowski · Accepted Answer · 2016-06-26 13:30:26Z

0

When you initialize an array, you reserve a place in memory for it. This memory size will not change depends on the values in the array. The default value in int array is 0, so your array is not partially empty.

answered Jun 26, 2016 at 13:30

Daniel Stradowski

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