Re: Reflection and access to type parameter?

From:
Owen Jacobson <angrybaldguy@gmail.com>
Newsgroups:
comp.lang.java.programmer
Date:
Thu, 22 May 2008 16:46:40 -0700 (PDT)
Message-ID:
<d4949986-36a0-4873-968e-b90bd25720dd@m73g2000hsh.googlegroups.com>
On May 22, 4:28 pm, Jim Garrison <j...@acm.org> wrote:

Given the SSCCE below, the need to pass A.class and B.class in
lines 25 and 25 seems redundant. However, I can find nothing in
the language that would let that be done in the constructor,
between lines 18 and 19. The obvious would be

      Class<T> x = T.class;

but that of course does not work. Is there any bridge at all
between reflection and generics? I suspect the answer is no
and the code below is the best that can be done, but I'm
not sure.

  1 import java.lang.reflect.Method;
  2 public class TestEnums
  3 {
  4 public static enum A
  5 {
  6 V1,
  7 V2;
  8 }
  9 public static enum B
10 {
11 X1,
12 X2,
13 X3;
14 }
15 public static class C<T extends Enum<?>>
16 {
17 public C(Class<T> x) throws Exception
18 {
19 T[] eVal = x.getEnumConstants();
20 for (Enum<?> v : eVal) System.out.println(v.toS=

tring());

21 }
22 }
23 public static void main(String[] args) throws Exception
24 {
25 C<A> ca = new C<A>(A.class);
26 C<B> cb = new C<B>(B.class);
27 }
28 }


You can, at least, refactor some of the repetition:

public static <T> C<T> createC (Class<T> clazz) {
  return new C<T>(clazz);
}

...
  C<A> = createC(A.class);
  C<B> = createC(B.class);
...

Java allows type parameters to be inferred in certain contexts.
Unfortunately, constructor invocation isn't one of them...

-o

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