What should default interfaces be for?

[Stephen Colebourne]

On 13 March 2012 00:10, David Holmes <david.holmes at oracle.com> wrote:
David said:
> I'm concerned that default methods are being seen as a new way to
> construct type hierarchies, rather than just being a short-term way to
> add a new method to an existing interface. In my opinion developers who
> subclass interfaces to which default methods get added, should implement
> those new methods in the next version of their libraries/apps - so that
> they don't rely on implicit use of defaults (they can make it explicit
> if it suits their needs). Default methods provide an evolution path for
> interfaces, but they can very easily be mis-used.

I think that in all the focus on lambdas, there hasn't been enough
discussion of what the Java community wants from the defaulted
interfaces feature.

David expresses the original lambda-focussed opinion - that default
methods in interfaces solely exist to provide a mechanism for
interface evolution. A necessary evil to provide the ability to
integrate lambdas into existing APIs but not for general use as part
of API design.

I've come to the opposite opinion. That defaulted interfaces are a
major new language feature in their own right, and should be
celebrated as such. I want to use them as a new way of designing
applications, replacing most usages of abstract classes today,
providing much of the feature set asked for by "abstract enums" and
effectively treating them as Java's traits. IMO, the addition of
traits is as important to API design as lambdas themselves.

What we need is a debate on the feature. Do we, as users/developers,
want the minimal feature set necessary to support lambdas, or the
maximal feature set that would occur were we adding traits to Java?

Things that may be options with a fully implemented feature:
- private-scoped methods
- package-scoped methods (would probably require adding a package
keyword for scope)
- protected-scoped methods
- static methods
- what should the default scope be
- whether the name "default interface" is the best choice ("trait interface"?)

So, please chime in on whether you think default interfaces should be
kept simple and minimal (as David suggested), or be thought of as a
major new feature in their own right? (Try to initially focus on the
big question without too much low-level detail)

Stephen