ReversibleCollection proposal

Peter Levart peter.levart at
Sun Apr 18 07:51:04 UTC 2021

A word for Remi's concern...

While it is good that List (and SortedSet and Deque) extend 
ReversibleCollection in the proposal, it is a pity the same is not true 
for Set and Queue. If Set and Queue could also be 
ReversibleCollection(s), then we would not need ReversibleCollection at 
all and the methods could be added to Collection itself.

So let's see them:

public interface Collection<E> .... {
         default Collection<E> reversed() { return this; }
         default void addFirst(E e) { throw new 
UnsupportedOperationException(); }
         default void addLast(E e) { throw new 
UnsupportedOperationException(); }
         default E getFirst() { return iterator().next(); }
         default E getLast() { return iterator().next(); }
         default E removeFirst() { var i = iterator();; 
i.remove(); }
         default E removeLast() { var i = iterator();; 
i.remove(); }

List, SortedSet and Deque would of course override them with 
implementations as proposed. Would anything be wrong with above 

Collection is an Iterable which allows to iterate over elements. We can 
still iterate a Collection that does not define the order of iteration. 
We can not rely on such undefined order in our code, but we can still 
iterate it. Stream.findFirst() is equivalent to Stream.findAny() for 
unordered streams. So why would Collection.getFirst() and 
Collection.getLast() not be equivalent (to Collection.getAny()) for 
unordered Collections? Why would coll.reversed() be any different than 
coll for unordered collections?


On 4/18/21 3:53 AM, Tagir Valeev wrote:
> Hello, Remi!
> On Sat, Apr 17, 2021 at 11:50 PM Remi Forax <forax at> wrote:
>> Introducing a new interface (two in this case) has a really huge cost and in this case, i've trouble to see why i will want to write a method that takes a ReversibleCollection as parameter, ReversibleCollection as a type does not seem to be useful. About the cost of introducing a new collection interface, it means that we also have to update all emptyXXX, singleXXX, uncheckedXXX with a new interface, and that only for the JDK. Every libraries that proxy collections has also to be updated to take care of this new type, otherwise the integration with an application that uses this new type and the library is not seamless anymore.
> Note that in Stuart's proposal, java.util.List extends
> ReversibleCollection. So, existing emptyList() and singletonList()
> also return ReversibleCollection and no new methods are necessary
> here. Probably unmodifiableReversibleSet could be useful as a
> LinkedHashSet wrapper. On the other hand, we don't have, e.g.
> checkedDeque or synchronizedDeque, and it looks like nobody complains.
> So probably it's not always necessary to create a wrapper for every
> single collection interface.
>> I fully agree that having a way to reverse a collection is a powerful API point, as Brian said, it's better to reverse the collection and then ask for a stream than providing a method reverseStream (and this is true for iterators or views like keySet/entrySet/values or subList too). Several people also ask to have findLast() on Stream, using list.descendingList().findFirst() will be equivalent.
> A dedicated findLast() would be a great addition to reversible
> streams! It could short-circuit for reversible stream
> (reverse().findFirst()) and use reduce((a, b) -> b) for non-reversible
> (so intermediate buffering is not necessary). I've found 8 Stream API
> call chains ending with .reduce((a, b) -> b) in our sources. Clearly,
> some of them could short-circuit, as the source is potentially
> reversible (e.g. allAnchors.subList(0,
> idx).stream().filter(hasNewName).reduce((x1, x2) -> x2)). Having
> dedicated findLast operation would be less error-prone, compared to
> creating the stream as reversed().stream(), as you don't have to think
> whether the stream is reversible or not. If you accidentally add a
> non-reversible operation, you'll miss the optimization but not
> correctness.
> With best regards,
> Tagir Valeev.

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