What JVM checks in Java object equality (==)?

The == operator only checks reference equality. It doesn't call any methods on the object... it just checks whether the two references involved are equal, i.e. they refer to the same object.

In simple cases I believe this is just a matter of comparing the references in a bitwise fashion - checking whether or not they consist of the same bytes. For more complex environments (e.g. "compressed oops") there could be slightly more work involved, in order to compare different representations. But the internally reference is effectively a pointer of some kind, and it's just a matter of comparing the two pointers.