Re: Default Interfaces: possible Java extension?
On 2011-02-11 21:44:14 -0500, Eric Sosman said:
On 2/11/2011 9:20 PM, Tom McGlynn wrote:
....
Add a new optional element in the definition of an interface which
defines a class that does the default implementation of the
interface.
....
Are there other ways to do this? A factory method requires knowing
the class the factory resides in and I can't really think of other
patterns that address this. E.g., one could add
newSet(), newList() and newMap() methods to Collections but that's not
especially elegant to my eye since there's no special language
relation between the List interface and the Collections class.
I think that's the crux: There's no special relationship between
an interface and its many implementations; the relationship is one-way.
Adding the other-way relation -- the ability of an interface to name a
class and say "This is My beloved Son, in Whom I am well pleased" --
doesn't seem to me to add much utility.
This proposal also "bakes in" a circular dependency between an
interface (List<T>) and its default implementation (ArrayList<T>), such
that there is no way to compile or load either class without the other.
While such circularity is sometimes hard to avoid (enums with
per-constant bodies, for example, are inherently circular), they're not
recommended style and they can lead to hard-to-debug classloading
problems if you're not careful.
I can't think of a use case for this outside of the collection types,
either, and there are already idioms for those. This feels like a
sublimated complaint about the standard library's choice of naming
conventions (List & ArrayList, rather than IList and List as with .Net
or informal protocols and list() like Python).
Interesting proposal, but I think it contains unfixable flaws.
-o