`Future.exceptionNow` and `Future.resultNow`

[benjaminp]

Java 19 added exceptionNow and resultNow methods to the Future interface. There are default implementations on the interface, but Guava's future library could provide more efficient overrides. In particular, exceptionNow() could avoid raising an exception.

There's also Future.state() but that seems harder, since the return value is an enumeration added in Java 19.

[cpovirk]

Thanks. We had speculated about this back before we got Java 21 internally, and then we never revisited. (Google code does tend to operate with async frameworks that essentially eliminate the need for these methods, but they can still be handy for us in some cases and of course for other users. Hmm, depending on what happens with virtual threads and friends, we may see either more or less use of blocking Future methods :))

I do think that adding the two methods you mentioned, exceptionNow and resultNow, to AbstractFuture in particular would be a nice win. As you may have seen, AbstractFuture has a fair amount of cross-platform weirdness around it (including one copy that's not published externally at the moment), so the big question will be how much of a complication that introduces. I will see if this is something that the friendly neighborhood LLM can churn out or whether I'd need to dedicate actual time to it.