Question

Suppose I have a weak reference to a car which has an ordinary (strong) reference to an engine. No other references to the car or the engine exist. Can the engine be garbage collected?

Answer 1

Yes it can, that is exactly how weak references are designed to work. The weak reference is the root that your object has to the application, even though the object may have other strong references it is the root reference that matters and since the root reference is a weak reference the object will be a candidate for garbage collection.

For more information please see WeakReference class documentation:

Weak reference objects, which do not
prevent their referents from being
made finalizable, finalized, and then
reclaimed. Weak references are most
often used to implement canonicalizing
mappings.

Suppose that the garbage collector determines at a certain point in time
that an object is weakly reachable. At
that time it will atomically clear all
weak references to that object and all
weak references to any other
weakly-reachable objects from which
that object is reachable through a
chain of strong and soft
references. At the same time it
will declare all of the formerly
weakly-reachable objects to be
finalizable. At the same time or at
some later time it will enqueue those
newly-cleared weak references that are
registered with reference queues.

FYI, along with WeakReference, Java offers two other subclasses of Reference: SoftReference and PhantomReference.