So... Why exactly does it need 24 bytes if a long is only 8 bytes (64 bits)?

kaushiks · on July 11, 2016

For the 16 byte (on x86_64) header (markOop, klass *).

jcdavis · on July 12, 2016

The header is only 12 bytes with compressedOops (which most people use), but the long is aligned on an 8 byte boundary so the advantage is wasted here.

alblue · on July 12, 2016

And even if the long wasn't on an aligned boundary, the object itself is. So you either have padding at the start or end of the object.