Minor improvement to anonymous classes

[Maurizio Cimadamore]

Fully agree... unless you embrace unchecked casts (e.g. if your impl 
cast to X and suppresses the warning, the program compiles).

Which is what Kevin was trying to say: having a generic method that 
doesn't use its type-variable in its parameters can be a bit of a smell, 
in the sense that implementors can forget that they don't have much 
control over what the type parameter might be inferred to.

All this goes back to my original point: we can't express a method that 
returns a conjunction of two types today; the alternative is to declare 
a new type just for that (but some of the arguments originally discussed 
in this thread apply: e.g. I might not have a great name for it, nor a 
desire for actually naming it), or resort to unchecked generic dark arts 
(which, as you and Kevin point out, 95% is subtly broken).

Maurizio

On 07/10/2021 22:25, Alan Malloy wrote:
> 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 
> <maurizio.cimadamore at oracle.com 
> <mailto:maurizio.cimadamore at oracle.com>> 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.)
>
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