Hi Stuart, We should be cautious when adding new APIs to existing interfaces. There may be libraries which extend JDK types and already have reversed(), toReversed() or asReversed() APIs and corresponding interfaces.
There are OrderedIterable and ReversibleIterable interfaces and asReversed() and toReversed() methods in the Eclipse Collections API. JavaDoc: https://www.eclipse.org/collections/javadoc/10.4.0/org/eclipse/collections/api/ordered/OrderedIterable.html https://www.eclipse.org/collections/javadoc/10.4.0/org/eclipse/collections/api/ordered/ReversibleIterable.html Code Browser: https://code.yawk.at/org.eclipse.collections/eclipse-collections-api/10.4.0/org/eclipse/collections/api/ordered/OrderedIterable.java https://code.yawk.at/org.eclipse.collections/eclipse-collections-api/10.4.0/org/eclipse/collections/api/ordered/ReversibleIterable.java OrderedIterable and ReversibleIterable appear in the mind map for RichIterable and have corresponding primitive versions in the mind map for PrimitiveIterable. https://medium.com/oracledevs/visualizing-eclipse-collections-646dad9533a9?source=friends_link&sk=3370a5e8bb5a516e6b5d7040f7d0955b There are methods asReversed() and toReversed() on ReversibleIterable(). https://code.yawk.at/org.eclipse.collections/eclipse-collections-api/10.4.0/org/eclipse/collections/api/ordered/ReversibleIterable.java#org.eclipse.collections.api.ordered.ReversibleIterable%23asReversed%28%29 https://code.yawk.at/org.eclipse.collections/eclipse-collections-api/10.4.0/org/eclipse/collections/api/ordered/ReversibleIterable.java#org.eclipse.collections.api.ordered.ReversibleIterable%23toReversed%28%29 The toReversed() method is co-variantly overridden in subtypes of ReversibleIterable. https://code.yawk.at/org.eclipse.collections/eclipse-collections-api/10.4.0/org/eclipse/collections/api/list/ListIterable.java#org.eclipse.collections.api.list.ListIterable%23toReversed%28%29 The asReversed() method returns a ReverseIterable which is lazy. https://code.yawk.at/org.eclipse.collections/eclipse-collections/9.2.0/org/eclipse/collections/impl/lazy/ReverseIterable.java Thanks, Don > On Apr 16, 2021, at 1:40 PM, Stuart Marks <stuart.ma...@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. > > Here's a link to a draft PR that contains the code diffs. It's prototype > quality, but it should be good enough to build and try out: > > https://github.com/openjdk/jdk/pull/3533 > > And here's a link to a class diagram showing the proposed additions: > > https://cr.openjdk.java.net/~smarks/ReversibleCollection/ReversibleCollectionDiagram.pdf > > Thanks, > > s'marks > > > # Ordering and Reversibility > > > A long-standing concept that's been missing from collections is that of the > positioning, sequencing, or arrangement of elements as a structural property > of a collection. (This is sometimes called the "iteration order" of a > collection.) For example, a HashSet is not ordered, but a List is. This > concept is mostly not manifested in the collections API. > > Iterating a collection produces elements one after another in *some* > sequence. The concept of "ordered" determines whether this sequence is > defined or whether it's a coincidence of implementation. What does "having an > order" mean? It implies that there is a first element and that each element > has a successor. Since collections have a finite number of elements, it > further implies that there is a last element that has no successor. However, > it is difficult to discern whether a collection has a defined order. HashSet > generally iterates its elements in the same undefined order, and you can't > actually tell that it's not a defined order. > > Streams do have a notion of ordering ("encounter order") and this is > preserved, where appropriate, through the stream pipeline. It's possible to > detect this by testing whether its spliterator has the ORDERED > characteristic. Any collection with a defined order will have a spliterator > with this characteristic. However, this is quite a roundabout way to > determine whether a collection has a defined order. Furthermore, knowing this > doesn't enable any additional operations. It only provides constraints on the > stream's implementations (keeping the elements in order) and provides > stronger semantics for certain operations. For example, findFirst() on an > unordered stream is the same as findAny(), but actually finds the first > element if the stream is ordered. > > The concept of ordering is thus present in the system but is surfaced only in > a fairly indirect way. We can strengthen abstraction of ordering by making a > few observations and considering their implications. > > Given that there is a first element and a last element, the sequence of > elements has two ends. It's reasonable to consider operations (add, get, > remove) on either end. Indeed, the Deque interface has a full complement of > operations at each end. This is an oft-requested feature on various other > collections. > > Each element except for the last has a successor, implying that each element > except for the first has a predecessor. Thus it's reasonable to consider > iterating the elements from first to last or from last to first (that is, in > forward or reverse order). Indeed, the concept of iterating in reverse order > appears already in bits and pieces in particular places around the > collections: > > - List has indexOf() and lastIndexOf() > - Deque has removeFirstOccurrence() and removeLastOccurrence() > - List has a ListIterator with hasPrevious/previous methods > - Deque and NavigableSet have descendingIterator methods > > Given an ordered collection, though, there's no general way to iterate it in > reverse order. Reversed iteration isn't the most common operation, but there > are some frequent use cases, such as operating on elements in > most-recently-added order. Questions and bug reports about this have come up > repeatedly over the years. > > Unfortunately, iterating in reverse order is much harder than iterating in > forward order. There are a variety of ways to iterate in forward order. For > example, given a List, one can do any of the following: > > for (var e : list) { ... } > list.forEach(...) > list.stream() > list.toArray() > > However, to iterate a list in reverse order, one must use an explicit loop > over ListIterator: > > for (var it = list.listIterator(list.size()); it.hasPrevious(); ) { > var e = it.previous(); > ... > } > > Streaming the elements of a List in reverse order is even worse. One approach > would be to implement a reverse-ordered Iterator that wraps a ListIterator > and delegates hasNext/next calls to the ListIterator's hasPrevious/previous > methods. Then, this Iterator can be turned into a Spliterator, which is then > turned into a Stream. (The code to do this is left as an exercise for the > reader.) Obtaining the elements in reverse via other means -- forEach() or > toArray() -- is similarly difficult. > > Obtaining the elements in reverse order can be accomplished by adopting a > concept from NavigableSet's descendingSet() method, which provides a > reverse-ordered view collection. This view is also a NavigableSet, which > means that any operation that can be performed on the original set can also > be applied to the reverse-ordered view. If it were possible to obtain a > similar reverse-ordered view on any kind of ordered collection, this would > enable the elements to be processed in reverse order in any fashion, not just > special-purposes APIs such as ListIterator or descendingIterator(). > > > # LinkedHashSet > > > The main feature of LinkedHashSet is that it does have a defined ordering, as > opposed to HashSet, which does not. LinkedHashSet clearly has a first and a > last element. However, LinkedHashSet is deficient in a number of ways: > > - access to and removal of the first element is reasonable (using an > iterator) but it is not possible to add at the front > > - it is possible to add an element at the end, but access to and removal of > the last element are expensive > > - it's not possible to iterate in reverse without copying the entire > collection > > Most frustratingly, LinkedHashSet is implemented using a doubly-linked list, > so there is no fundamental implementation reason that prevents these > operations from being supported. The main reason these operations aren't > supported is probably that there hasn't been a good place to push such APIs. > > > # Proposal > > > Introduce a new ReversibleCollection<E> interface. This provides a new method > to obtain a reverse-ordered view. It also contains several methods (copied > from Deque) that allow operating on elements at both ends. > > > interface ReversibleCollection<E> extends Collection<E> { > ReversibleCollection<E> reversed(); > default void addFirst(E e) > default void addLast(E e) > default E getFirst() > default E getLast() > default E removeFirst() > default E removeLast() > } > > > The List, Deque, and SortedSet interfaces, and the LinkedHashSet class are > retrofitted to implement ReversibleCollection. They provide covariant > overriding implementations of the reversed() method. For example, > List.reversed() returns a List. Default implementations for all of these > methods are provided in the appropriate interfaces. > > Covariant overrides are also provided in several other classes. This presents > a difficulty for LinkedHashSet, as there's no suitable return type for > reversed(). To remedy this, we add another interface > > > interface ReversibleSet<E> extends ReversibleCollection<E>, Set<E> { } > > > This adds no new methods but is essentially an intersection of > ReversibleCollection and Set. As such, it's suitable as the return type of > LinkedHashSet.reversed() and the set views of LinkedHashMap. > > SortedSet::addFirst and addLast throw UnsupportedOperationException. This is > because SortedSet's ordering is determined by the comparison method and > cannot be controlled explicitly. > > LinkedHashSet::addFirst and addLast add the element at the appropriate > position or reposition the element if it is already present in the set. > > New methods are added to LinkedHashMap > > V putFirst(K, V) > V putLast(K, V) > > which put the mapping at the designated position or reposition the mapping if > is already present in the map. > > > # Design & Implementation Issues > > > Covariant overrides for reversed() are provided where possible. However, > there is a conflict between List and Deque, as there are examples in the JDK > (such as LinkedList) that implement both List and Deque. Since List is much > more popular than Deque, I've decided against adding a covariant override to > Deque. Instead, a method Deque::reversedDeque is added that returns a Deque, > and Deque::reversed returns ReversibleCollection. > > There is no ReversibleMap interface. Most iteration over maps is over the > entrySet, keySet, or values views. NavigableMap already has descendingMap(), > and LinkedHashMap provides ReversibleX views, which cover most of the cases > already. > > Introducing new methods into an interface hierarchy always raises the > possibility of name clashes. I've done some searching but I haven't found any > major issues, but we should be on the lookout for this. I'll continue to do > more searching for such conflicts. > > Introducing covariant overrides on existing methods (notably LinkedHashMap > entrySet, keySet, and values) could present incompatibility issues with > subclasses that override these methods. I've done some searching and again I > haven't found major problems, but this is nonetheless a point of concern. > I'll do more searching here, too. > > The default implementations for List::reversed, Deque::reversed, and > SortedSet::reversed return reverse-ordered views that are implemented using > only public methods of the original interface. This demonstrates the > feasibility of retrofitting reverse ordering onto any instance of these > interfaces. Similarly, the various add/get/remove methods' default > implementations are all implementable in terms of an iterator or > reverse-ordered iterator. That said, some concurrent collection > implementations will need to override these default implementations in order > to preserve their safety invariants. It would probably also be wise to add > optimized implementations to the more commonly-used collection > implementations. > > Given that a ReversibleCollection has a defined ordering, any spliterator > obtained from such a collection should have the Spliterator.ORDERED > characteristic. > > This proposal is related to a previous discussion on a similar topic, where > Tagir Valeev had proposed OrderedSet and OrderedMap: > > http://mail.openjdk.java.net/pipermail/core-libs-dev/2020-April/066028.html > > I think there were some good ideas in there and in the ensuing discussion, > but it didn't go anywhere. Several of the concepts in this proposal are an > evolution of some ideas discussed in that thread. > > One of the ideas from that thread was to use Deque as a common interface for > many collections to support reversibility and operations at the ends. We've > abandoned that idea in this proposal. The reasons are that it's cluttered > with a bunch of methods that are inherited from Queue. Also, some methods are > null-returning instead of throwing, which introduces confusion for > collections that can contain nulls. > > # END
