@Documented @Target(value={FIELD,METHOD,PARAMETER,LOCAL_VARIABLE}) @Retention(value=CLASS) public @interface NullUnknown
null under certain
defined circumstances. The necessity of the non-null check depends
on situations described in the javadoc (so the non-null check is
not required by default).
Consider Map.get(java.lang.Object) as an example of
such a method - depending on usage of the Map
null may be legal or forbidden.