ReversibleCollection proposal

[Stuart Marks]

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