Formal model for defender method resolution

[Brian Goetz]

> I see your point: there is a unique, non-overridden concrete
> implementation here, as in the previous example.
>
> In both this and the previous example, any problem arises only in the
> context of a particular concrete class.  Either the class has a unique
> overrider for each abstract method it inherits, or it does not (and is
> rejected).  Why should we should reject interfaces, which can't
> generally be instantiated in isolation anyway?  I think the primary
> reason (if there is one) is that there are defaults in the hierarchy
> that can't be "used" by a class because of ambiguity, so the defaults
> dont do the clients any good.  I think you're thinking that ought to
> be an error we want to diagnose when the interface is compiled.  Is
> that right?

Right.  If any attempt to inherit from the interface would cause a 
failure, it seems only polite for the compiler to reject that interface.