You can't actually do this: that signature is a promise to return an
instance of *any* class implementing those interfaces, not *some* class
implementing them. If you try to implement your getCloseableFoo method,
you'll find that no implementation compiles. For example:
final class tmp {
static <X extends AutoCloseable & Serializable> X getX() {
class Impl implements AutoCloseable, Serializable {
public void close() {}
}
return new Impl();
}
}
tmp.java:8: error: incompatible types: Impl cannot be converted to X
return new Impl();
^
where X is a type-variable:
X extends AutoCloseable,Serializable declared in method <X>getX()
This is because some caller may define their own MyImpl class implementing
those interfaces, and then write MyImpl i = getX(), instantiating X to
MyImpl, and your method doesn't know how to build such an object.
On Thu, Oct 7, 2021 at 7:37 AM Maurizio Cimadamore <
[email protected]> wrote:
> But I'm puzzled by the fact that programmers can, in a way, do something
> similar to the above with a generic method:
>
> <X extends Foo & AutoCloseable> X getCloseableFoo();
>
> Which kind of works, but it's quite an horrible hack (you introduce a type
> parameter you don't need - which means compiler will try to infer types,
> etc.)
>
>
