I did exactly that last night. I built a test that allocated HashEntry<K,V>[] just like it does in the Segment constructor. I don't have the numbers in front of me right now, but at the maximum size (2^30) and without specifying concurrency it was trying to allocate 32 Segment objects each having a HashEntry<K,V>[] with 2^25 elements. Each one of these HashEntry<K,V>[] takes up around 128MB of memory.