java.lang.OutOfMemoryError : Java heap space

Question

I was playing with some examples of Collections from Oracle website

public class Timing {
 public static void method(){
 List numbers = new ArrayList();
 for (double i = 1; i <= Double.MAX_VALUE; i++)
 numbers.add(new Double(i));
 Collections.shuffle(numbers);
 List winningcombination = numbers.subList(0, 10);
 Collections.sort(winningcombination);
 }
 public static void main(String[] args)
 {
 long start = System.currentTimeMillis();
 method();
 long end = System.currentTimeMillis();
 System.out.println("time elapsed : " + (end-start));
 }
}

I tried to see how long it will take to do it for Double.MAX_VALUE. And I got this :

Exception in thread "main" java.lang.OutOfMemoryError: Java heap space
 at java.util.Arrays.copyOf(Unknown Source)
 at java.util.Arrays.copyOf(Unknown Source)
 at java.util.ArrayList.ensureCapacity(Unknown Source)
 at java.util.ArrayList.add(Unknown Source)

I there a way to fix this ?

• Thorbjørn Ravn Andersen: The actual exception happens when the ArrayList chooses to grow (and copy its old contents to the new backing array)

Answer 1

Is there a way to allow you to create and store Double.MAX_VALUE objects in a Collection? No. There's not that much RAM on Earth. Double.MAX_VALUE is about 2 times ten to the 308th power: that's 2 followed by over 300 zeros. Give Best Buy a call, see how much they'd charge to put that in your computer.

• Michael Borgwardt: @OpenMind: The solution is not to attempt the impossible.
• Sebastian Hoffmann: There is no solution. You cant store that much!
• Ernest Friedman-Hill: The solution is to use a much smaller upper limit for your loop -- no more than, say, a million or so.
• Dilum Ranatunga: @OpenMind, exactly what are you trying to do? Generate 10 random numbers that are guaranteed to not have duplicates? If so, look into pseudo-random number generators. Also, I don't understand why you are generating winning combinations of doubles rather than longs or alphanumerals etc.
• KARASZI István: Write a better algorithm, just store the already selected numbers and do a random again if you've got the same and you don't have enough.

Answer 2

Even if you had enough memory, ArrayList can have at most Integer.MAX_VALUE elements. Double.MAX_VALUE far exceeds said limit.

In this case, you ran out of memory during an add that caused the array list to grow.

• user166390: Exceeds by "How much?" (I just want to see a big number ^^)

Answer 3

Yet another reason why your code cannot work: double can only represent integers exactly up to about 2^52 - after that, i++ will have no effect and the for loop will never terminate.

You should never use floating-point variables as loop counters. Use int or long instead.

• user166390: Although I think there might already be problems if code tries to loop 2^52 times... ;-)
• Clockwork-Muse: Well, it'll give him slightly unexpected results, at least. That, and because he actually tests i <= Double.MAX_VALUE, he's going to overflow i regardless...
• Clockwork-Muse: #winces# I didn't realize the imprecision got that bad, even in the upper range... Oh, grr, it would have to increment in a (positive non-zero) power of ten after that, wouldn't it...
• Michael Borgwardt: @X-Zero: power of two, actually. And it's not that the imprecision gets worse - it's always the same, relative to the magnitude of the number being represented. Once you're up to 2^52, an increase of 1 is (justly) assumed not to matter much. OTOH, the format can distinguish between 0.00000000000001 and 0.00000000000002 - Any attempt to represent an uncountably infinite set in will require some tradeoffs, and it's really quite marvelous how it can be done in only 64 bits and work quite well most of the time.
• Clockwork-Muse: ... I'm really showing my lack of a CS degree here, aren't I... That, and being too used to an AS/400 shop, where most stuff is being coded as BCDs

Answer 4

Instead of doing what you are currently doing, you should just obtain 10 random doubles, add them to an ArrayList and sort it. That is basically what your method is doing.

To obtain a random double, look at Random.nextDouble().

Answer 5

You are trying to allocate of the order of 10^308 values. That's a lot of values.

• user166390: Put into a real perspective? 10^308 just looks small ;p
• Paul Cager: @pst 100000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000. Yup, still looks small...

Answer 6

Increasing the size of the heap will do. Just run the program with this argument:

-Xmx512m

It will increase your heap size to 512 MB. You can specify as much as you want: 1g, 2g and so on.

Answer 7

for (double i = 1; i <= Integer.MAX_VALUE; i++)
 numbers.add(new Double(i));

• user166390: But how will this address the OutOfMemory exception? (A PC still won't come close.)
• Ernest Friedman-Hill: That's better, although you probably still don't have that much memory. That's about 2 billion objects, at least 12 bytes each, plus the array in the array list; so 24GB for the objects and another 8B for the array.
• Michael Borgwardt: @Ernest: well, it's not completely impossible for a consumer-grade PC to have 24GB of RAM; in fact, in 5 years' time it will probably be normal.
• Ernest Friedman-Hill: Indeed, that's why I said "you probably still don't have that much". I have a server that does, but none of my desktops do!

Answer 8

In you loop:

for (double i = 1; i <= Double.MAX_VALUE; i++)
 numbers.add(new Double(i));

An ArrayList will just add the value to the ArrayList if there is room. If not it will increase the size of the ArrayList and then continue adding.

So what you are basically doing is using all the memory allocated in your heap when you are creating this ArrayList. If you make your ArrayList smaller you should be able to hold it in memory.