[External] : Re: ReversibleCollection proposal

[dfranken.jdk at gmail.com]

When I thought about Collection's role in the hierarchy, it seemed to
me that 'Collection' is an interface for describing how elements are
stored in a cardboard box (we can and and remove them) and that we
might need a different, yet related, interface to describe how to
retrieve the items from the box. This way we are not tied to the
Collection hierarchy and we could have one Set implementation which is
ordered and another Set implementation which is not and they would both
still implement Collection, but only one would implement the interface.

Imagine an interface like 'java.lang.OrderedEnumerable` if you will
with methods such as 'first(), last(), etc', then each implementation
would be free to choose to implement the interface or not. I also
thought about 'OrderedIterable', but there would be too much confusion
with 'Iterable', but I think these are related too. Retrieving items is
an iteration problem, not a storage problem.

While I would love to see the Collection hierarchy redesigned to also
allow for ImmutableCollection which for instance would not have an
`add(T e)` method, I don't think we can simply do something like that
without breaking too many things.

So in short, separating retrieval aspects from storage aspects with a
different interface might be the way to go.

Kind regards,

Dave Franken

On Wed, 2021-05-05 at 12:41 +0200, forax at univ-mlv.fr wrote:
> ----- Mail original -----
> > De: "Stuart Marks" <stuart.marks at oracle.com>
> > À: "Remi Forax" <forax at univ-mlv.fr>
> > Cc: "core-libs-dev" <core-libs-dev at openjdk.java.net>
> > Envoyé: Mercredi 5 Mai 2021 02:00:03
> > Objet: Re: [External] : Re: ReversibleCollection proposal
> 
> > On 5/1/21 5:57 AM, forax at univ-mlv.fr wrote:
> > > I suppose the performance issue comes from the fact that
> > > traversing a
> > > LinkedHahSet is slow because it's a linked list ?
> > > 
> > > You can replace a LinkedHashSet by a List + Set, the List keeps
> > > the values in
> > > order, the Set avoids duplicates.
> > > 
> > > Using a Stream, it becomes
> > >    Stream.of(getItemsFromSomeplace(), getItemsFromAnotherPlace(),
> > >    getItemsFromSomeplaceElse())
> > >     .flatMap(List::stream)
> > >     .distinct()              // use a Set internally
> > >     .collect(toList());
> > 
> > The problem with any example is that simplifying assumptions are
> > necessary for
> > showing the example, but those assumptions enable it to be easily
> > picked apart.
> > Of
> > course, the example isn't just a particular example; it is a
> > *template* for a
> > whole
> > space of possible examples. Consider the possibility that the items
> > processing
> > client needs to do some intermediate processing on the first group
> > of items
> > before
> > adding the other groups. This might not be possible to do using
> > streams. Use
> > your
> > imagination.
> > 
> > > I think there are maybe some scenarios where ReversibleCollection
> > > can be useful,
> > > but they are rare, to the point where when there is a good
> > > scenario for it
> > > people will not recognize it because ReversibleCollection will
> > > not be part of
> > > their muscle memory.
> > 
> > I'm certain that uses of RC/RS will be rare in the beginning,
> > because they will
> > be
> > new, and people won't be familar with them. And then there will the
> > people who
> > say
> > "I can't use them because I'm stuck on JDK 11 / 8 / 7 / 6 ...." It
> > was the same
> > thing with lambdas and streams in Java 8, with List.of() etc in
> > Java 9, records
> > in
> > Java 16, etc. This wasn't an argument not to add them, and it isn't
> > an argument
> > not
> > to add RC/RS.
> 
> All the changes you are listing here are "client side" changes, the
> ones that can be adopted quickly because they do not require to
> change the API side of any libraries.
> ReversibleCollection is an API side change, like generics is, those
> changes have a far higher cost because you have to wait your library
> dependencies to be updated.
> On the Valhalla list, we have discussed several times about how to
> alleviate those API side change cost using automatic bridging or
> methods forwarding, even for Valhalla, we are currently moving in a
> state where those mechanisms are not needed. 
> 
> > 
> > > There is a real value to add methods like
> > > descendingSet/descendingList()/getFirst/getLast on existing
> > > collections, but we
> > > don't need a new interface (or two) for that.
> > 
> > It depends on what you mean by "need". Sure, we could get away
> > without this;
> > after
> > all, we've survived the past twenty years without it, so we could
> > probably
> > survive
> > the next twenty years as well.
> > 
> > It would indeed be useful to add various methods to List, Deque,
> > SortedSet, and
> > LinkedHashSet to provide a full set of methods on all of them. And
> > it would also
> > be
> > nice to have those methods be similar to one another. An interface
> > helps with
> > that,
> > but I agree, that's not really the reason to have an interface
> > though.
> > 
> > The reversed-view concept could also be added individually to the
> > different
> > places.
> > A reverse-ordered List is a List, a reverse-ordered Deque is a
> > Deque, a
> > reverse-ordered SortedSet is a SortedSet, and a reverse-ordered
> > LinkedHashSet is
> > a
> > ... ? And what is the type of the keySet of a LinkedHashMap, that
> > enables one to
> > (say) get the last element?
> 
> see below
> 
> > 
> > After working with a system like this for a while, it begins to
> > emerge that
> > there is
> > an abstraction missing from the collections framework, something
> > like an
> > "ordered
> > collection". People have been missing this for quite a long time.
> > The most
> > recent
> > example (which this proposal builds on) is Tagir's proposal from a
> > year ago. And
> > it's been asked about several times before that.
> > ReversibleCollection fills in
> > that
> > missing abstraction.
> 
> The abstraction already exists but it's not defined in term of
> interface because it's an implementation decision and those are
> cleanly separated in the current Collection design.
> 
> Let take a step back, the collection API defines basic data structure
> operations in term of interfaces like List, Deque, Set, etc those
> interfaces are decoupled from implementation capabilities like
> mutable, nullable, ordered and checked.
> 
> Depending on the implementation capabilities, the interfaces method
> implementation may throw an exception, non-mutable implementations
> use UnsupportedOperationException, non-nullable implementations use
> NPE and checked implementations use CCE. 
> 
> So what is missing is methods on Collection interfaces that require
> the collection implementation to be ordered like descendingList(),
> getFirst(), etc.
> Those methods that may throw a specific exception if the
> implementation is not ordered, not UnsupportedOperationException but
> a new one like NotOrderedException.
> 
> So to answer to your question about LinkedHashSet, the reverse-
> ordered LinkedHashSet is a Set with a method descendingSet() that do
> not throw NotOrderedException like any Set with an order.
> 
> To summarize, if we introduce ReversibleCollection, we should also
> introduce ImmutableCollection, NonNullableCollection and
> CheckedCollection.
> I think it's better to consider the fact that being ordered as a
> capability (hint: this is already what the Spliterator API does) and
> not as a specific interface.
> 
> > 
> > s'marks
> 
> Rémi