Class Reference<T>

This is the base class of all references. A reference allows refering to an object without preventing the garbage collector to collect it. The only way to get the referred object is via the get()-method. This method will return null if the object was collected.
A reference may be registered with a queue. When a referred element gets collected the reference will be put on the queue, so that you will be notified.
There are currently three types of references: soft reference, weak reference and phantom reference.
Soft references will be cleared if the garbage collector is told to free some memory and there are no unreferenced or weakly referenced objects. It is useful for caches.
Weak references will be cleared as soon as the garbage collector determines that the refered object is only weakly reachable. They are useful as keys in hashtables (see WeakHashtable) as you get notified when nobody has the key anymore. Phantom references don't prevent finalization. If an object is only phantom reachable, it will be finalized, and the reference will be enqueued, but not cleared. Since you mustn't access an finalized object, the get method of a phantom reference will never work. It is useful to keep track, when an object is finalized.

See Also: WeakHashMap