On Tue, Aug 11, 2009 at 7:50 AM, Marshall Schor<m...@schor.com> wrote:
> A small correction: allowing downcasting doesn't generate a warning
> about unchecked casting.
>
> Here's a small test case in case you want to play with this:
>
> package generics;
>
> import java.util.List;
>
> public class TestG {
>
> class Box<T> { // version returning down-castable thing
> <U extends T> U get() {return null;}
> <U extends T> List<U> getL() {return null;}
> }
>
> class StrictBox<T> { // version returning just type T
> T get() {return null;}
> List<T> getL() {return null;}
> }
>
> Box<Number> box = new Box<Number>();
> StrictBox<Number> sbox = new StrictBox<Number>();
>
> void test() {
> // returning elements
> Number n = box.get(); // OK
> Integer n2 = box.get(); // OK
>
> Number sn = sbox.get(); // OK
> Integer sn2 = sbox.get(); // fails
> Integer sn3 = (Integer) sbox.get(); // OK
>
> // returning lists
> List<Number> ln = box.getL(); // OK
> List<Integer> ln2 = box.getL(); // OK
>
> List<Number> sln = sbox.getL(); // OK
> List<Integer> sln2 = sbox.getL(): // fails, no fix available
> }
> }
>
For a complete picture you should also try to implement the methods to
return something other that null. For example how to you implement
<U extends T> U get() such that it returns an instance of U regardless
of what U is? You could only do this with a typecast. And you'd have
to expectation that the typecast would actually succeed.
-Adam
