Conditional methods "for free"

[Tomas Mikula]

On Sep 30, 2014, at 8:10 AM, Simon Ochsenreither <simon at
ochsenreither.de> wrote:

>   There was a short mention of "conditional methods", e.g. enabling
>   methods only for a subset of some generic type.

I don't know where conditional methods were mentioned and what their
current state is (any links?), but here is an idea how to get
conditional methods "for free" on top of two other (missing) features,
which would both be useful independently in their own right.

The first feature is lower bounds on type parameters, i.e. <U super T>, as in

    class Optional<T> {
        public <U super T> Optional<U> or(Optional<? extends U> other);
    }

The other one is to allow multiple type paramater bounds, even if one
of the bounds is itself a type variable. That is, allow bounds like <U
extends T & Foo> or <U extends Foo super T>.

I have no idea how technically feasible these features are, but I have
encountered use cases for both of them.

Once we have lower bounds and multiple bounds, we can express
conditional methods like this:

    class Optional<T> {
        default <I extends Integer super T> int getAsInt() {
            I value = get();
            return value.intValue();
        }
    }

Valid substitute for I exists only when T extends Integer, thus the
getAsInt() method can effectively be invoked only on
Optional<Integer>.

Regards,
Tomas