Android APIs
Added in API level 1



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.