Re: Another generics question: List<Class<?>> ls = ?

From:

Lew <noone@lewscanon.com>

Newsgroups:

comp.lang.java.programmer

Date:

Tue, 21 Apr 2009 08:48:32 -0400

Message-ID:

<gskfb2$cil$1@news.albasani.net>

Mark Space wrote:

I got to thinking about the last generics question, and I don't
understand why the following doesn't work.

Given a list that holds some type of Class:

    List<Class<String>> ls = new ArrayList<Class<String>>();

Why can't I assign this to a List that holds any type of Class?

    List<Class<?>> al1 = ls; // oops
    List<Class<?>> al2 = (List<Class<?>>) ls; // oops

Looking at the raw types, it seems this should work. al1 holds any
type of Class, and ls has a type of class. Yet there appears to be
no way to assign them. I don't know if this is a good idea, I merely
want to understand generics a little better.

Lew a ??crit :

For the same reason you can't do

  List <Number> numbers = new ArrayList <Number> ();
  List <Integer> integers = new ArrayList <Integer> ();
  List <Number> noops = integers; // forbidden
  List <Integer> ieeks = numbers; // forbidden

<http://java.sun.com/docs/books/tutorial/java/generics/subtyping.html>

Given 'Sub' extends 'Parent', it is not true that 'Foo<Sub>' extends
'Foo<Parent>'. If it did, it would allow illegal actions.

Albert wrote:

Well, i [sic] don't if it's a special case, but the following code is valid
for eclipse [sic]:

It's not a special case, just a different one.

And valid for Java is valid for Java, whether from Eclipse or Notepad.

import java.util.Collection;
import java.util.Set;

public class TestsGenerics {

    abstract class A {
        abstract Collection<Integer> foo();
    }

    class Sub extends A {
        Set<Integer> foo() { // implements indicator here
            return null;
        }
    }

}

Note that your case is 'Foo <Bar>', 'SubtypeOfFoo <Bar>', whereas Mark Space
and I were discussing 'Foo <Bar>', 'Foo <SubtypeOfBar>'.

The former shows a subtype relationship, the latter does not.

--
Lew