class Dictionary<A,B> extends Object {...}
class Hashtable<A,B> extends Dictionary<A,B> {...}

Dictionary<String,Integer> d;
Object o;

(Hashtable<String,Integer>) d;  // legal
(Hashtable) o;                  // legal
(Hashtable<Float,Double>)   d;  // illegal, kein Subtyp
(Hashtable<String,Integer>) o;  // illegal, kein eindeutiger Subtyp