HashMap bug for large sizes

Kasper Nielsen kasperni at gmail.com
Fri Jun 1 09:29:16 UTC 2012


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

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++).

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.


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