or phantomly reachable so your program can take some action based on that knowledge. For example, a program might perform some post-finalization cleanup processing that requires an object to be unreachable (such as the deallocation of resources outside the Java heap) upon learning that an object has become phantomly reachable. To be placed on a reference queue, a reference object must be created with a reference queue. Soft and weak reference objects can be created with a reference queue or not, but phantom reference objects must be created with a reference queue: ReferenceQueue queue = new ReferenceQueue(); PhantomReference pr = new PhantomReference(object, queue); Another approach to the soft reference example from the diagram on the previous page could be to create the SoftReference objects with a reference queue, and poll the queue to find out when an image has been reclaimed (its reference field cleared). At that point, the program can remove the corresponding entry from the hash map, and thereby, allow the associated string to be garbage collected. ReferenceQueue q = new ReferenceQueue(); Reference r; while((r = q.poll()) != null) { //Remove r's entry from hash map }