> From: "Tagir Valeev" <amae...@gmail.com>
> To: "Stuart Marks" <stuart.ma...@oracle.com>
> Cc: "Remi Forax" <fo...@univ-mlv.fr>, "core-libs-dev"
> <core-libs-dev@openjdk.java.net>
> Sent: Saturday, February 12, 2022 4:24:24 AM
> Subject: Re: [External] : Sequenced Collections
> Wow, I missed that the Sequenced Collections JEP draft was posted!
> Of course, I strongly support this initiative and am happy that my proposal
> got
> some love and is moving forward. In general, I like the JEP in the way it is.
> I
> have only two slight concerns:
> 1. I'm not sure that having addition methods (addFirst, addLast, putFirst,
> putLast) is a good idea, as not every mutable implementation may support them.
> While this adds some API unification, it's quite a rare case when this could
> be
> necessary. I think most real world applications of Sequenced* types would be
> around querying, or maybe draining (so removal operations are ok). Probably it
> would be enough to add addFirst, addLast, putFirst, putLast directly to the
> compatible implementations/subinterfaces like List, LinkedHashSet, and
> LinkedHashMap removing them from the Sequenced* interfaces. In this case,
> SortedSet interface will not be polluted with operations that can never be
> implemented. Well my opinion is not very strong here.
> 2. SequencedCollection name is a little bit too long. I think every extra
> letter
> adds a hesitation for users to use the type, especially in APIs where it could
> be the most useful. I see the Naming section and must admit that I don't have
> better ideas. Well, maybe just Sequenced would work? Or too vague?
> Speaking of Remi's suggestion, I don't think it's a good idea. Maybe it could
> be
> if we designed the Collection API from scratch.
??
Here is the first sentence of the javadoc for java.util.List "An ordered
collection (also known as a sequence )."
And the first paragraph of java.util.RandomAccess "Marker interface used by
List implementations to indicate that they support fast (generally constant
time) random access. The primary purpose of this interface is to allow generic
algorithms to alter their behavior to provide good performance when applied to
either random or sequential access lists"
You can find that the actual design, mixing ordered collection and indexed
collection into one interface named List not great, but you can not say that
this is not the actual design.
> But given the current state of Java collections, it's better to add new
> interfaces than to put the new semantics to the java.util.List and greatly
> increase the amount of non-random-accessed lists in the wild.
> There are tons of code that implicitly assume fast random access of every
> incoming list (using indexed iteration inside). The suggested approach could
> become a performance disaster.
If you take several Java developers, some will stick to the javadoc definition,
a List is either sequential or random access and some will think that a List is
only random access. Because of that, adding more sequential implementations
under the List interface is an issue.
Introducing SequencesCollection (more on the name later), a super interface of
List solves that issue, the new implementations will only implement the
sequential part of interface List.
But it does not solve the other problems, mainly adding 4 interfaces when one
is enough, not being backward compatible because of inference and the weird
semantics of LinkedHashMap.
We still need SortedSet or LinkedHashSet to not directly implement
SequencesCollection but to use delegation and a have a method returning a view.
The same reasoning applied to SortedMap, LinkedHashMap.
By using views, there is no need to the two other proposed interfaces
SequenceSet and SequenceMap.
Another question is ListIterator, a list can be iterated forward and backward,
a SequenceCollection can do almost the same thing, with iterator() and
reversed().iterator(). It's not exactly the same semantics but i don't think it
exist an implementation of SequenceCollection that can be implemented without
the property that given one element, it's predecessor and successor can be
found in O(1).
Do we need a new SequenceCollectionIterator that provides the method
next/hasNext/previous/hasPrevious but not add/set/nextIndex/previousIndex ?
For the name, Java uses single simple name of one syllable for the important
interface List, Set, Map or Deque (the javadoc of Deque insist that Deque
should be pronounced using one syllable).
So the name should be Seq.
The main issue with the name "Seq" is that it is perhaps too close to the name
"Set".
Also, it can not be "Sequence" because of CharSequence.
interface Seq<E> extends Collection<E> {
void addFirst();
void addLast();
E getFirst();
E getLast();
E removeFirst();
E removeLast();
Seq<E> reversed();
}
interface List<E> extends Seq<E> { }
interface SortedSet<E> implements Set<E> { // or NavigableSet
// new methods
Seq<E> asSeq();
}
interface SortedMap<K,V> implements Map<K,V> { // or NavigableMap
// new methods
Seq<K> keySeq(); // do not use covariant return type
Seq<V> valueSeq();
Seq<Map.Entry<K,V>> entrySeq();
}
I'm still not sure that introducing an interface like Seq instead of using List
is the way to go.
If we do that, there will be a lot of blog post/bikeshedding about when to use
List vs Seq and a lot of github issues about taking a Seq instead of a List as
parameter of a method of a library.
> With best regards,
> Tagir Valeev.
Rémi
> On Sat, Feb 12, 2022 at 2:26 AM Stuart Marks < [
> mailto:stuart.ma...@oracle.com
> | stuart.ma...@oracle.com ] > wrote:
>> Hi Rémi,
>> I see that you're trying to reduce the number of interfaces introduced by
>> unifying
>> things around an existing interface, List. Yes, it's true that List is an
>> ordered
>> collection. However, your analysis conveniently omits other facts about List
>> that
>> make it unsuitable as a general "ordered collection" interface. Specifically:
>> 1) List supports element access by int index; and
>> 2) List is externally ordered. That is, its ordering is determined by a
>> succession
>> of API calls, irrespective of element values. This is in contrast to
>> SortedSet
>> et al
>> which are internally ordered, in that the ordering is determined by the
>> element
>> values.
>> The problem with indexed element access is that it creates a bunch of hidden
>> performance pitfalls for any data structure where element access is other
>> than
>> O(1).
>> So get(i) degrades to O(n), binarySearch degrades from O(log n) to O(n).
>> (This
>> is in
>> the sequential implementation; the random access implementation degrades to
>> O(n
>> log
>> n)). Apparently innocuous indexed for-loops degrade to quadratic. This is
>> one of
>> the
>> reasons why LinkedList is a bad List implementation.
>> If we refactor LinkedHashSet to implement List, we basically have created
>> another
>> situation just like LinkedList. That's a step in the wrong direction.
>> Turning to internal ordering (SortedSet): it's fundamentally incompatible
>> with
>> List's external ordering. List has a lot of positional mutation operations
>> such
>> as
>> add(i, obj); after this call, you expect obj to appear at position i. That
>> can't
>> work with a SortedSet.
>> There is implicit positioning semantics in other methods that don't have
>> index
>> arguments. For example, replaceAll replaces each element of a List with the
>> result
>> of calling a function on that element. Crucially, the function result goes
>> into
>> the
>> same location as the original element. That to cannot work with SortedSet.
>> Well, we can try to deal with these issues somehow, like making certain
>> methods
>> throw UnsupportedOperationException, or by relaxing the semantics of the
>> methods
>> so
>> that they no longer have the same element positioning semantics. Either of
>> these
>> approaches contorts the List interface to such an extent that it's no longer
>> a
>> List.
>> So, no, it's not useful or effective to try to make List be the common
>> "ordered
>> collection" interface.
>> s'marks
>> On 2/10/22 3:14 AM, Remi Forax wrote:
>>> I've read the draft of the JEP on sequenced collection, and i think the
>>> proposed
>> > design can be improved.
>>> [ https://bugs.openjdk.java.net/browse/JDK-8280836 |
>> > https://bugs.openjdk.java.net/browse/JDK-8280836 ]
>>> I agree with the motivation, there is a need for an API to consider the
>>> element
>>> of a list, a sorted set and a linked hash set as an ordered sequence of
>>> elements with a simple way to access/add/remove the first/last element and
>>> also
>> > reverse the elements as view.
>>> I disagree about the conclusion that we need to introduce 4 new interfaces
>>> for
>> > that matter.
>> > Here are the reasons
>>> 1/ Usually an ordered collection is called a list. Introducing an interface
>>> SequencedCollection for something which is usually called a list will cause
>> > more harm than good. Or maybe we should rename LISP to SEQP :)
>>> 2/ There is already an interface List in Java, that represents an ordered
>>> sequence of elements, with LinkedList being the name of the the double
>>> linked
>>> list implementation. You can argue that there is a slight difference between
>>> the semantics of java.util.List and the proposed syntax of
>>> java.util.SequencedCollection, but given that people already have
>>> difficulties
>>> to understand basic data structure concepts, as a teacher i dread to have a
>> > discussion on those slight differences that are only true in Java.
>>> If the collection API was not already existing, we may discuss about having
>>> the
>>> same interface java.util.List to both indexed collection and ordered
>> > collection, but that boat has sailed a long time ago.
>>> So in first approach, we should refactor sorted set and linked hash set to
>>> directly implement java.util.List and all the proposed methods into
>>> java.util.List. But as you hint in the Risks and Assumptions section, this
>>> will
>>> cause regression due to inference and also we will have trouble with
>> > LinkedHashMap (see below).
>>> 3/ LinkedHashMap mixes 3 implementations in one class, some of these
>> > implementations does not conform to the semantics of SequencedMap.
>>> - You can opt-out having the key sequentially ordered as defined by
>>> SequencedMap
>>> by using the constructor LinkedHashMap(int initialCapacity, float
>>> loadFactor,
>> > boolean accessOrder) and passing true as last parameter.
>>> - You can opt-out having the key sequentially ordered as defined by
>>> SequencedMap
>>> by overriding removeEldestEntry(), removing the first entry at the same time
>> > you add a new one.
>>> Because all these reasons, i think we should move to another design, using
>>> delegation instead of inheritance, which for the collection framework means
>>> exposing new way to access/modify sorted set and linked hash set through
>> > java.util.List views.
>>> The concept of views is not a new concept, it's used in Arrays.asList(),
>>> List.subList() or Map.keySet()/values()/entrySet() (and more). The idea is
>>> not
>>> that a sorted set is a list but that it provides a method to see it as a
>>> list.
>>> It solves our problem of compatibility by not adding super types to existing
>>> type and also the problem of the semantics of LinkedHashMap because a view
>> > keeps the semantics of the data structure it originated.
>> > Here is the proposed new methods in List, SortedSet and SortedMap.
>> > interface List<E> extends Collection<E> {
>> > // new methods
>> > void addFirst();
>> > void addLast();
>> > E getFirst();
>> > E getLast();
>> > E removeFirst();
>> > E removeLast();
>> > List<E> reversedList(); // or descendingList() ??
>> > }
>> > interface SortedSet<E> implements Set<E> {
>> > // new methods
>> > List<E> asList();
>> > }
>> > interface SortedMap<K,V> implements Map<K,V> {
>> > // new methods
>> > List<K> keyList(); // do not use covariant return type
>> > List<Map.Entry<K,V>> entryList(); // same
>> > }
>>> I believe this design is objectively better than the one proposed because
>>> as a
>>> user being able to use new interfaces is a slow process, the
>>> libraries/dependencies must be updated to take the new interfaces as
>>> parameter
>>> before the new types can be used. By contrast, the proposed design only
>>> enhance
>>> existing interfaces so people will enjoy the new methods directly when
>> > introduced.
>> > Rémi
