Re: Incomparable types

From:
Thomas Hawtin <usenet@tackline.plus.com>
Newsgroups:
comp.lang.java.programmer
Date:
Wed, 02 Aug 2006 13:54:38 +0100
Message-ID:
<44d0e71c$0$947$ed2619ec@ptn-nntp-reader01.plus.net>
Jono wrote:

Hi everyone,
I have a reasonably strong .NET background and I have a problem
understanding the Class class in Java. The Type class in .NET (along
with the typeof() operator and System.Object's GetType() method) seems
pretty straightforward to me: an instance of the Type class is
instantiated for every type loaded into the system (even with
generics). But it's not so easy to understand with Java, it seems. If I
try to compile the sample code below it fails on line 3 of the main
method. If I explicitly cast the left-hand and right-hand sides of the
equality expression (as per line 4) to Class, then the compiler is
happy. Please could someone let me know what the subtle difference is
that I am missing out on?

public class NewClass {

    public static void main(String[] args)
    {
        Integer i = new Integer(123);
        String s = "456";
        //System.out.println(i.getClass() == s.getClass()); //
compilation failure
        System.out.println(((Class)i.getClass()) ==
((Class)s.getClass()));
    }

}


An object of type Class<capture of ? extends Integer> cannot be the same
object as an object of type Class<capture of ? extends String> (unless
you write unchecked code, and null is an odd case). The is no type T
such that Class<T> is compatible with both of those types. No T
simultaneously extends both String and Integer.

Similarly the expression "i instanceof String" and "i == s" are nonsense
and will not compile.

Tom Hawtin

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