Heap memory error in Java

Question

I'm tinkering a bit with Java but have a lot of experience in some other languages. I have a test problem that I know solution to (and can easily produce in Python and C++). But running the following Java code gives an

Exception in thread "main" java.lang.OutOfMemoryError: Java heap space

I'm wondering if I'm making a simple mistake, I would not expect the memory footprint of this program to be very large at all:

public static void main(String[] args) { ArrayList<Integer> longest_sequence = new ArrayList<>(); ArrayList<Integer> this_sequence; int n = 0; for (int i = 1; i < 1000000; i++) { this_sequence = new ArrayList<Integer>(); n = i; this_sequence.add(n); while (n != 1) { if (n % 2 == 0) { n = n / 2; } else { n = 3*n + 1; } this_sequence.add(n); } if (this_sequence.size() > longest_sequence.size()) { longest_sequence = this_sequence; } } System.out.println(longest_sequence.get(0)); System.out.println(longest_sequence.size()); }

To clarify a bit more:

A new list is created in each iteration of the program. It is either kept by assigning the longest_sequence to it, or it is discarded and overwritten by a new list instance.

I'm guessing my assumptions about that are incorrect, and instances are being preserved? The size of the lists should not be a problem (about 500 elements for the largest one).

You could try increasing the heap space for the running program. It probably defaults to less than 1G — OneCricketeer
– OneCricketeer, Commented Apr 29, 2017 at 17:07
Try using this_sequence.clear(), though, and use only one reference to the list rather than recreate in the loop — OneCricketeer
– OneCricketeer, Commented Apr 29, 2017 at 17:08
probably, the memory footprint will be very big due to your while loop, how can you know how big the largest one is ? — Luatic
– Luatic, Commented Apr 29, 2017 at 17:09
The problem here is that the integer is overflowing and your Collatz conjecture code is not terminating at 1 as you expect. Replace int with long and everything is fine. This is, of course, still a wildly inefficient way to measure the length of a sequence which doesn't require keeping anything in memory at all. — pvg
– pvg, Commented Apr 29, 2017 at 17:17

Matthieu Saleta · Accepted Answer · 2017-04-29 17:29:10Z

2

It will fail even if you increase your heap space.

For n = 113383, an operation make you go through Integer limit and n becomes negative and this is ending in a infite loop.

It works if you change Integer by Long.

answered Apr 29, 2017 at 17:29

Matthieu Saleta

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thearn Over a year ago

Yep, and a few minutes after I posted this question I looked at my old c++ code and immediately realized that I was hitting an integer overflow. I've been spending too much time in dynamically typed languages I think! Thank you.

Kirill · Accepted Answer · 2017-04-29 17:09:11Z

I think the easiest to avoid this is to add memory using -Xmx flag

https://docs.oracle.com/cd/E13150_01/jrockit_jvm/jrockit/jrdocs/refman/optionX.html here's official docs to read more about it

Anshul Sharma · Accepted Answer · 2017-04-29 17:09:26Z

to change the VM for Eclipse you can change the amount of the MV from Windows> Preferences> Java> Installed JREs from there select the JRE and click edit, then write in the Default VM Arguments: to -Xmx1024M or any other amount of memory ...

Well, it's fairly self-explanatory: you've run out of memory.

You may want to try starting it with more memory, using the -Xmx flag, e.g.

java -Xmx2048m [whatever you'd have written before] This will use up to 2 gigs of memory.

See the non-standard options list for more details.

Thank you, the process of assigning more memory to the VM makes sense, but is sort of orthogonal to my question: Why is this program running out of memory?
The answer to the question 'why is this running out of memory' can't really be 'just add more memory'.

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