> On (1), all objects in Java are references, being a pointer to a pointer effectively. This indirection has a cost but has been incredibly useful historically because it enables moving allocation around the address space because none of the references change. They're effectively just pointers to a lookup table and it's the lookup table that changes.
There's only one level of indirection. References are simply pointers. You can't see the raw bits of the pointer without Unsafe, but that's how the Sun JVM implements references. (The implementation calls them "ordinary object pointers".) GC rewrites these pointers when it moves objects; there is no separate lookup table of all objects.
You might have been mislead by the existence of Object.identityHashCode(). The identity hash code is not a memory address and not guaranteed unique. It's an arbitrary value stored in some bits of the object's mark word and copied around when the object moves. That's how it remains stable across GCs.
Or you might be thinking of "compressed oops". Those are still pointers, but encoded as (object_address - start_of_heap) >> 3 to save space.
I think there is a global lookup table for interned strings and symbols loaded from class files, but slots in that table are not themselves pointed to by references.
There's only one level of indirection. References are simply pointers. You can't see the raw bits of the pointer without Unsafe, but that's how the Sun JVM implements references. (The implementation calls them "ordinary object pointers".) GC rewrites these pointers when it moves objects; there is no separate lookup table of all objects.
You might have been mislead by the existence of Object.identityHashCode(). The identity hash code is not a memory address and not guaranteed unique. It's an arbitrary value stored in some bits of the object's mark word and copied around when the object moves. That's how it remains stable across GCs.
Or you might be thinking of "compressed oops". Those are still pointers, but encoded as (object_address - start_of_heap) >> 3 to save space.
I think there is a global lookup table for interned strings and symbols loaded from class files, but slots in that table are not themselves pointed to by references.