Re-specification and re-abstraction

[David Holmes]

One of the things that concerns me in the current State of the Lambda is 
the fact that re-specification of method on a sub-interface does not 
re-abstract the method, but continues to allow any default from a super 
interface to be applied to that method. This is inherently wrong in my 
opinion.

Consider this fictitious but plausible example.

We have the present Map interface. We also have a ConcurrentMap 
interface that adds some additional methods that provide atomicity for 
compound actions on maps that support concurrent access.

Suppose that ConcurrentMap actually re-specified each of the Map methods 
to add various thread-safety and atomicity guarantees.

Now suppose that the maintainer of Map decides to take advantage of 
default methods and adds default implementations to some of those Map 
methods. This is not done with ConcurrentMap in mind and those 
implementations offer no specific thread-safety or atomicity guarantees. 
It should be obvious that those implementations are not likely to be 
valid for any class that implements ConcurrentMap.

Given the current semantics however, if I were to write a class that 
implements ConcurrentMap and accidentally forgot to implement those 
methods for which a default exists in Map, then my class will in fact 
"inherit" those implementations. And in doing so my class would be 
completely broken, but I would be unaware of it.

To me this is completely wrong - we should never sacrifice correctness. 
A supertype does not in general know about any subtypes and so a 
supertype's default implementation should not apply if the subtype has 
redefined that method. Any change to the "documentation" of a method is 
a change to its specification. Only the author of that change is in a 
position to say whether a default implementation in a supertype is still 
applicable - so to assume that it is by default (no pun intended) is 
just wrong in my view.

If redefinition of a method always re-abstracted it then, I think, no 
explicit syntax for re-abstraction would be needed.

David Holmes