Request for review: proposal for @FunctionalInterface checking

[Gernot Neppert]

Am 29.12.2012 18:23, schrieb Joe Darcy:
>
>
> Amongst others; we would systematically find and annotate such 
> interfaces in the JDK if the FunctionalInterface type is added. In JDK 
> 7, analagous efforts were undertaken for adding @SafeVarargs and 
> AutoCloseable:
>

Good that you bring it up: @SafeVarargs was a very clever idea because 
it actually conveyed some valualble additional information that could 
not be deduced otherwise from the class' or method's declaration:
namely, that the *implementation* of a method will not access an array 
of a generic type in a way that could compromise type-safety.
IMO, merely stating that an interface has one abstract method is not 
something that cannot be deduced otherwise.

> That is not my assessment of the situation.
>
> If the interface is annotated with @FunctionalInterface that means its 
> future evolution should be constrained to not add, directly or 
> indirectly, new methods without defaults, including in superinterfaces.
>

IIRC, it was this very constraint that spurred the idea of having 
defaulted methods in the first place!

So, if this policy is valid for all interfaces, it doesn't need to be 
explicitly emphasized for SAM interfaces, right?