Re: ReversibleCollection proposal

[Anthony Vanelverdinghe]

On Saturday, April 17, 2021 11:48 CEST, Stephen Colebourne <scolebourne at joda.org> wrote: 
 
> On Fri, 16 Apr 2021 at 18:41, Stuart Marks <stuart.marks at oracle.com> wrote:
> > This is a proposal to add a ReversibleCollection interface to the Collections
> > Framework. I'm looking for comments on overall design before I work on detailed
> > specifications and tests. Please send such comments as replies on this email thread.
> 
> I think this could be an interesting addition to the framework.
> 
> > # Ordering and Reversibility
> 
> Reading this section, it seems like ordering is a more significant
> quality than reversibility. Yet the API is named "reversible". That
> seems odd, esepcially given the stream characteristic.

Since `reversible = ordered + sized`, using reversible makes sense imho (even though in the context of the Collections Framework, reversible <-> ordered, since all collections are sized). Also, I think "reversible" is much less likely to clash with existing code (e.g. a quick GitHub search shows 1 result for `ReversibleCollection`, while there's plenty for `OrderedCollection`).

> 
> > SortedSet::addFirst and addLast throw UnsupportedOperationException. This is because
> > SortedSet's ordering is determined by the comparison method and cannot be controlled
> > explicitly.
> 
> This seems undesirable. Maybe SortedSet should not implement
> reversible/ordered? Maybe they should add to the set but validate
> whether they would be in first/last position? Simply allowing users to
> get a new instance with a different (eg. reversed) comparator would
> meet much of the use case.

I'd argue throwing UOE is the right thing to do. Not having `SortedSet` implement `ReversibleSet` doesn't make sense to me. Adding with validation is reasonable in itself, but then you'd have to specify `IllegalArgumentException` in `ReversibleCollection` (where you'd have a hard time specifying the conditions under which it might be thrown without explicitly referencing `SortedSet`), just to accommodate these 2 methods which would be very rarely used.

> 
> Also, SortedSet uses first() and last(), yet the proposed interface
> uses getFirst() and getLast().

Since `Deque` uses `getFirst()` and `getLast()`, it's impossible to match all existing methods in the different interfaces.

> 
> Stephen

Kind regards, Anthony