Re: Generics: how to read actual type parameters

From:
Lew <lew@lewscanon.com>
Newsgroups:
comp.lang.java.programmer
Date:
Sun, 30 Sep 2007 16:24:30 -0400
Message-ID:
<osGdnfToNdxjmJ3anZ2dnUVZ_hOdnZ2d@comcast.com>
Joshua Cranmer wrote:

marek.dudek@gmail.com wrote:

For example:

public class Pair<S> {
    public String toString() {
        Class clas = ??? ;
        return "Pair of " + clas.toString();
    }
    public S first;
    public S second;
}


1. Don't use tabs in Usenet posts.
2. What you are probably intending to do is impossible as specified.
There is no possible way at runtime to get the class of S. Java erases
the types of the parameters at runtime.
3. Class is generic. Use Class<?> instead.

The easiest thing you can do is:
Class<?> clas = first.getClass();

A potentially tighter bound is:
Class<?> left = first.getClass();
Class<?> right = second.getClass();
Class<?> clas = left;

while (!clas.isAssignableFrom(right))
   clas = clas.getSuperclass();

(This returns the last common ancestor of the classes of first and second)


Another hack in a class you own is to have a Class<?> instance variable to
provide runtime type information.

And here's a recent article I just googled up that delves into the issue (GIYF):
<http://www.artima.com/weblogs/viewpost.jsp?thread 8860>

--
Lew

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