| PhantomReference | A phantom reference is useful, to get notified, when an object got finalized. |
| Reference | This is the base class of all references. |
| ReferenceQueue | This is the queue, where references can enqueue themselve on. |
| SoftReference | A soft reference will be cleared, if the object is only softly reachable and the garbage collection needs more memory. |
| WeakReference | A weak reference will be cleared, if the object is only weakly reachable. |