java.lang.OutOfMemoryError: Java heap space for java 8

Question

I use Java 8 (In Eclipse) when I got this error:

java.lang.OutOfMemoryError: Java heap space

I've tried the -Xmx command but no use even changing the MetaspaceSize doesn't solve it. This problem occurs when I tried to swap elements of list (it size<= 395).

public void permutation(double[] arr, int pos, ArrayList<double[]> list) {
    if (arr.length - pos == 1)
        list.add(arr.clone());
    else
        for (int i = pos; i < arr.length; i++) {
            swap(arr, pos, i);
            permutation(arr, pos + 1, list);
            swap(arr, pos, i);
        }
}

Side note: I strongly recommend you use braces in your branching structures, even if there is just one statement. — Joffrey
– Joffrey, Commented Apr 28, 2015 at 17:56
@JaredBurrows I believe the list is just here to keep a trace of the steps, so cloning is necessary to keep the state of the array at each step unchanged. I might be wrong, though I haven't read properly yet. — Joffrey
– Joffrey, Commented Apr 28, 2015 at 17:57
@StuPointerException because it is a pure imperative method, not mutating or using any state of the enclosing object, and I really believe there is no reason this would be overriden at all (given the nature of the function). That being said, static lets the next programmer know at a glance that this method does not mutate/use any state, and can also improve static analysis of the code. — Joffrey
– Joffrey, Commented Apr 28, 2015 at 18:14

StuPointerException · Accepted Answer · 2015-04-28 18:08:44Z

2

Changing the amount of heap space you are allocating won't solve the problem.

Your heap is being used up because you are creating an incredible number of objects and looking at what you are trying to achieve, this is likely a bug in your code.

You need to debug your application to determine where all of the objects are being created and why the garbage collector can't clear them up.

answered Apr 28, 2015 at 18:08

StuPointerException

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Peter Lawrey · Accepted Answer · 2015-04-29 11:12:10Z

Instead of building a List of every possible combination will use a lot of memory and when exploring combinations the number of results can grow exponentially with the number of elements you input.

A simple way around this it to provide a call back to process each combination as it happens. This will leave the amount of work much the same, but the memory consumption drops from O(x^n) to O(1)

public void permutation(double[] arr, int pos, Consumer<double[]> consumer) {
    if (arr.length - pos == 1)
        consumer.accept(arr);
    else
        for (int i = pos; i < arr.length; i++) {
            swap(arr, pos, i);
            permutation(arr, pos + 1, list);
            swap(arr, pos, i);
        }
}

Note: you can create a consumer which adds clones to a List, but you can also print or process the results as you go.

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