Does AtomicReference.compareAndSet(old,new) guarantees old.field wasn't change?

Question

Does AtomicReference.compareAndSet(old,new) guarantee

1. old.field wasn't change?
2. Or it just guarantees old wasn't reassign to a new object?

if 2 is true, does it mean AtomicReference is useful only with immutable objects like String? (Because in case of mutability of old the old.field changes are lost with a successfully compareAndSet)

Answer 1

AtomicReference only checks reference equality, and does not check for changes to fields.

As a result, it is primarily only useful with immutable types.