Re: limitations of generic reflection
On Apr 16, 12:58 pm, Roger Levy <sinos...@gmail.com> wrote:
I think I have hit up against an interesting limitation of generics in
Java, and I want to confirm that I'm understanding the limitations
properly. I would like to write a method that takes a parameterized
Collection of type C<A>, and apply to each member of the Collection a
method that takes an A and returns a B. The result should be a
Collection of type C<B>. The code would ideally look something like:
public <A, B, C extends Collection> C<B> applyAll(C<A> as,
Function<A,B> f) {
C<B> result = (C<B>) as.getClass().newInstance();
for(A a : as)
result.add(f.apply(a));
return result;
}
with the appropriate exception handling. But it seems like this is
impossible because type parameters themselves cannot be
parameterized. Is there a way around this limitation that I haven't
thought of?
You can parameterize type bounds:
public static
<A, B,
CA extends Collection<A>,
CB extends Collection<B>>
CB apply(
CA as, Function<A, B> f) {
}
however, once you're past that hurdle, you're going to discover that
you can't, eg., do 'new CB', which you'd need for a truly generic
implementation of the map meta-function, nor can you specialize
generics on some arguments the way you could with C++ templates.
-o