HashMap bug for large sizes

Doug Lea dl at cs.oswego.edu
Fri Jun 1 13:36:36 UTC 2012

On 06/01/12 05:29, Kasper Nielsen wrote:
> Hi,
> I don't know if this has been discussed before. But I was looking at
> the HashMap implementation today and noticed that there are some
> issues with very large sized hashmaps with more then Integer.MAX_VALUE
> elements.

I think this arose on this list (or possibly some side-exchanges)
three years ago when discussing possible HashMap improvements. It
seems impossible to fix this without breaking serialization
compatibility, so people seemed resigned to let the limitations
remain until there was some change forcing incompatibility anyway.
At the least though, an implementation note could be added to
the javadocs. I think several other java.util classes also have
this problem, but none of the java.util.concurrent ones do.
So as a workaround, people can use ConcurrentHashMap even if
they aren't using it concurrently.


> 1.
> The Map contract says that "If the map contains more than
> Integer.MAX_VALUE elements, returns Integer.MAX_VALUE." The current
> implementation will just wrap around and return negative
> numbers when you add elements (size++).
> 2.
> If the size of a HashMap has wrapped around and returns negative size
> you cannot deserialize it. Because of this loop in readObject
> for (int i=0; i<size; i++) {
>     K key = (K) s.readObject();
>     V value = (V) s.readObject();
>     putForCreate(key, value);
> }
> If someone wants to play around with the size limits of HashMap I
> suggest taking the source code of HashMap and change the type of the
> size field from an int to a short, in which case you can test this
> with less the xx GB of heap.
> There are probably other map implementations in the JDK with the same issues.
> Cheers
>    Kasper

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