1. While language-level extension methods (no JVM involvement) won’t
solve the deprecated-method problem, they do solve the Stream<int>.sum
problem, and quite elegantly.
Sort of -- but only if you buy the partiality.
In order for this to work, you have to (as .NET does) inject methods
into *types*, not into *classes*. This is just a partial method in
disguise! In order for this to work, you have to be comfortable with
the idea that the set of methods on a given receiver is dependent on the
parameterization of the receiver. And you clearly are, since you don't
think extension methods are too complex.
FTR, we considered and rejected use-site extension methods in 8, for a
philosophical reason that is still equally valid here: an API developers
should own their API. What we rejected is the use-site aspect of it;
the part we actually liked (but didn't have enough motivation to embrace
in 8) was the partiality. Just as default methods support the
after-the-fact aspect of API extension that use-site extension would
(without the transparency risks), partial methods (including partial
defaults) support the specialized-receiver aspect of extension methods
that we like but don't yet have.
