A reference to an Object on a 32 bit JVM (at least on Hotspot) takes up 4 bytes.
Does the 64 bit Hotspot JVM need 8 bytes? Or is some clever compression going on?
If not, every Object[] would require twice as much heap memory, which I somehow think (hope, expect) is not the case.
Update/extra question: Does this really matter, or is this a negligible increase, because most references point to objects that are much larger than a few bytes (whereas one might argue that those objects are in turn mostly comprised of references to other objects)?
In a 64-bit system, object references are typically 8-byte long. But in recent JVMs from Sun/Oracle you can enable Compressed Oops, which reduce reference size to 4 bytes at the cost of a smaller limit on heap size.