| PhantomReference<T> | Implements a phantom reference, which is the weakest of the three types of references. |
| Reference<T> | Provides an abstract class which describes behavior common to all reference objects. |
| ReferenceQueue<T> | The ReferenceQueue is the container on which reference objects are
enqueued when the garbage collector detects the reachability type specified
for the referent. |
| SoftReference<T> | A reference that is cleared when its referent is not strongly reachable and there is memory pressure. |
| WeakReference<T> | Implements a weak reference, which is the middle of the three types of references. |
