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