Question

I have a problem with my Java progam suddenly exiting, without any exception thrown or the program finishing normally.

I’m writing a program to solve Project Euler‘s 14th problem. This is what I got:

private static final int INITIAL_CACHE_SIZE = 30000;
private static Map<Long, Integer> cache = new HashMap<Long, Integer>(INITIAL_CACHE_SIZE);

public void main(String... args) {
    long number = 0;
    int maxSize = 0;

    for (long i = 1; i <= TARGET; i++) {
        int size = size(i);
        if (size > maxSize) {
            maxSize = size;
            number = i;
        }
    }
}
private static int size(long i) {
    if (i == 1L) {
        return 1;
    }
    final int size = size(process(i)) + 1;
    return size;
}

private static long process(long n) {
    return n % 2 == 0 ? n/2 : 3*n + 1;
}

This runs fine, and finishes correctly in about 5 seconds when using a TARGET of 1 000 000.

I wanted to optimize by adding a cache, so I changed the size method to this:

private static int size(long i) {
    if (i == 1L) {
        return 1;
    }
    if (cache.containsKey(i)) {
        return cache.get(i);
    }
    final int size = size(process(i)) + 1;
    cache.put(i, size);
    return size;
}

Now when I run it, it simply stops (process exits) when I get to 555144. Same number every time. No exception, error, Java VM crash or anything is thrown.

Changing the cache size doesn’t seem to have any effect either, so how could the cache
introduction cause this error?

If I enforce the cache size to be not just initial, but permanent like so:

    if (i < CACHE_SIZE) {
        cache.put(i, size);
    }

the bug no longer occurs.
Edit: When I set the cache size to like 2M, the bug starts showing again.

Can anyone reproduce this, and maybe even provide a suggestion as to why it happens?

Answer 1

This is simply an OutOfMemoryError that is not being printed. The program runs fine if I set a high heap size, otherwise it exits with an unlogged OutOfMemoryError (easy to see in a Debugger, though).

You can verify this and get a heap dump (as well as printout that an OutOfMemoryError occurred) by passing this JVM arg and re-running your program:

-XX:+HeapDumpOnOutOfMemoryError

With this it will then print out something to this effect:

java.lang.OutOfMemoryError: Java heap space
Dumping heap to java_pid4192.hprof …
Heap dump file created [91901809 bytes in 4.464 secs]

Bump up your heap size with, say, -Xmx200m and you won’t have an issue – At least for TARGET=1000000.