Java failed to compile Functional Interface with generic parameter and default method implementation

Question

I'm trying to extend this interface:

@FunctionalInterface
public interface HandlerFunction<T extends ServerResponse> {
    Mono<T> handle(ServerRequest request);
}

Like this:

@FunctionalInterface
public interface HandlerFn<R extends ServerRequest> extends HandlerFunction<ServerResponse> {
    default Mono<ServerResponse> handle(ServerRequest request) {
        return handleFn(request);
    }

    Mono<ServerResponse> handleFn(R request);
}

I'm getting this error (on handleFn(request)):

The method handleFn(R) in the type HandlerFn<R> is not applicable for the arguments (ServerRequest)

In this code snippet, why when calling handleFn(request) the type R is not inferred as ServerRequest ?

Andy Turner · Accepted Answer · 2020-06-14 17:38:54Z

request is a ServerRequest, not an R. R could be ServerRequest, but it could be any other subclass of it.

R is declared on the class, not the method. The type of R is determined at the declaration of the reference to the HandlerFn, not at the call site of the handleFn method.

You could cast inside the default method:

return handleFn((R) request);

which is an unchecked cast (at that point; it would be checked elsewhere).

If you declared the method as a generic method, it would work in handle:

<RR extends ServerRequest> Mono<ServerResponse> handleFn(RR request);

(You wouldn't then need R on the interface. I just called it RR to emphasize that it is a separate type variable).

But, in fact, there is no point in such a type variable: this gives no advantage over declaring the method non-generically:

Mono<ServerResponse> handleFn(ServerRequest request);

which, of course, has no advantage over the handle method.

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