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.
-Doug
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
