[External] : Sequenced Collections

[Stuart Marks]

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
> 
> 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
>