Re: Confusion with templates, generics, and instanceof

From:
Mark Space <markspace@sbc.global.net>
Newsgroups:
comp.lang.java.programmer
Date:
Sat, 14 Mar 2009 10:35:58 -0700
Message-ID:
<9WRul.12307$hc1.6230@flpi150.ffdc.sbc.com>
Greg Boettcher wrote:

2. What I really wanted to know was whether you can test to see if an
object, o, belongs to the class specified by a type variable, T. The
condition (o instanceof T) doesn't seem to work. This was my main
reason for posting here, and it's too bad I picked a bad example for
it. Anyway, I am starting to think that this is impossible. If I'm
wrong, please let me know.


As far as I know, you can't do an "instance of T" other than as Lew
suggested -- pass a class literal (object.class) as a type token.

You can however test classes and cast at runtime.

         Object o = ...
         JButton b = new JButton();
         JButton b2 = b.getClass().cast( o );
         Class<? extends JButton> c = b.getClass().
                 asSubclass( JButton.class );

I don't think these are applicable to your current situation. That's
why all the idioms you see as examples you instanceof or other similar
patterns. Actually, I'm not wholly clear on when you would use either of
the above methods (cast() or asSubclass()). Any use I can think of
could be duplicated by a cast ((JButton)) or literal (JButton.class).
I'm sure there are situations though, so I just wanted to point out that
these methods exist.

<http://java.sun.com/javase/6/docs/api/java/lang/Class.html>

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