--- On Thu, 10/16/08, Jonathan Cast
But I can't say new HashSet()?
No... but you can say 'new HashSet<T>()' where T is a type variable, and then put a value of type T into your set, which is probably generally what you want. HashSet<?> is a set of unknown (at compile time) element type. It is not safe to put any element into such a set. Consider: void wrong(List<?> foo, List<?> bar) { foo.add(bar.get(0)); // illegal... but if it weren't... } ... List<Integer> x = ...; List<String> y = ...; wrong(x, y); // then this would put an String into a list of ints... --- Perhaps there was confusion over what you meant by 'conjure up a value of an unknown type'... you can't explicitly instantiate a parameterized class with a wildcard type variable (e.g. new HashSet<?>). However, you can conjure up an instance of any class for which you have a Class object handy, provided it is non-abstract and has public constructors, and then assign it to a variable with a wildcard in its type.