reference type size in java

Question

Why does a reference type in java take 8 bytes? Why not less or more than 8 bytes?

Answer 1

Actually, it is nowhere specified how much bytes a reference variable shall have, and in fact, it is not everywhere the same.

Common virtual machines for 32-bit systems (i.e. systems with 32 bit adresses in the memory bus) usually use 32-bit (= 4 bytes, same as int and float) as the size for object references, while virtual machines for 64-bit systems often use the native address size 64 bits (= 8 bytes) for these. (Note that most 64 bit systems can run 32-bit programs, too, so often even there you'll be using a 32 bit VM.)

It is simply a matter of simplifying the implementation, if you can use an actual memory address for your references instead of something else.

As this increases the size of memory used (and often we don't actually need to access that much memory), from Java 7 on the 64-bit HotSpot VM will be able to use 32-bit references under certain conditions, i.e. when the heap is smaller than 32 GB (8·2³² bytes). To convert them to a actual memory address, they are multiplied by 8 (as objects will be aligned on 8-byte-boundaries) and then added to the base address (if this is not zero). (For heaps smaller than 4 GB, we don't need the multiplication step.)

Other VMs might use similar tricks.