Hi Tagir,
A few quick thoughts on this.
There does seem to be a conceptual hole here. Most collections implementations
have an obvious interface that provides a reasonable abstraction of that
implementation. However, one is "missing" for LinkedHashSet, since there's no
interface more specialized than Set. This leaves it undifferentiated from
HashSet. This proposal seems to fill that gap.
I'm having trouble seeing where this would be used in an API, though. A method
could consume an OrderedSet, to prevent somebody from accidentally providing an
unordered set. An OrderedSet as a return type would be good documentation that
the returned set has an order, but this is pretty thin.
One point to consider is: not everything needs to be in the type system. Streams
have a runtime flag that indicates ordering; there's no OrderedStream or
anything like that. It seems to be sufficient.
There's also kind of a clash with SortedSet. Conceptually, SortedSet is an
OrderedSet, but this would force some additional methods into SortedSet (or it
would limit the methods provided by this one, possibly making it not very useful).
Setting aside the types, the added functionality of getting the last element and
iterating in the reverse order seems somewhat useful. Does this justify a whole
new interface?
This boils down to the questions,
- Do we need a new type that indicates that the set has an order?
- Or are we more interested in the added functionality?
This tells me that we should do some thinking about use cases. There was some
Twitter chatter on this topic the other day that I suspect might have led to
this proposal. (The tweets were a bit scattered, as it typical for Twitter, so
I'm not sure it's worth linking to them.) In any case a couple things seemed to
come out of it, namely, the ability to iterate a LHS in reverse, or possibly to
get its last element. As I recall, you even mused about a Deque view of an LHS.
(The methods proposed here seem well aligned with Deque.) It seems like this
approach could be useful, and it's much lighter weight than introducing a new type.
Could you come up with some use cases and see whether and how they could be
satisfied by different approaches?
s'marks
On 4/25/20 9:51 AM, Tagir Valeev wrote:
Hello!
To me, it was always a pity that the internal structure of
LinkedHashMap allows doing more things easily, yet this functionality
is not exposed to the external clients, so if somebody really needs
such things they will need to waste memory/CPU to emulate them. The
functionality I'm speaking about includes:
- getting the last (i.e. most recently added/accessed) entry
- getting the descending iterator going from newest to oldest entries
- getting the iterator that starts at given existing entry
It's easy to do such things with TreeMap and other NavigableMap
implementations. However, LinkedHashMap provides no additional visible
methods in addition to the Map interface (well, except
removeEldestEntry()). The same considerations apply to LinkedHashSet.
So we can provide new interfaces j.u.OrderedMap and OrderedSet that
provide the additional functionality and make
LinkedHashMap/LinkedHashSet implement them. Aside from new
functionality, the interfaces will carry additional semantics: their
iteration order is always predictable and their spliterators return
DISTINCT|ORDERED characteristics, so some clients may require
OrderedMap/Set just to assert that they rely on predictable iteration
order. We can even make existing NavigableMap/Set extend
OrderedMap/Set because NavigableMap/Set are ordered. Some of
NavigableMap/Set methods will naturally fit the OrderedMap/Set
interfaces.
I think I can devote my spare time to write the spec, implementation,
and tests for this enhancement, participate in the API design
discussions and do any other job necessary to make this improvement
happen. However, I'd like to know in advance whether such kind of
change would be met positively. What do you think? Is it possible to
do this or this improvement contradicts the OpenJDK goals and will
likely be rejected? Also, do I need to write a JEP for this?
It's probably too early to discuss the exact API, but for more
specificity of my proposal, here's the quick draft:
package java.util;
public interface OrderedSet<E> extends Set<E> {
// First or NSEE if empty
E first() throws NoSuchElementException;
// Last or NSEE if empty
E last() throws NoSuchElementException;
// Retrieve&remove first or null if empty
E pollFirst();
// Retrieve&remove last or null if empty
E pollLast();
// Iterator starting from element, or NSEE if not found
Iterator<E> iteratorFrom(E fromElement) throws NoSuchElementException;
// Descending
Iterator<E> descendingIterator();
// Descending iterator starting from element, or NSEE if not found
Iterator<E> descendingIteratorFrom(E fromElement) throws
NoSuchElementException;
// TBD: descendingSet()
}
All the methods except iteratorFrom and descendingIteratorFrom are
already present in NavigableSet, so the spec will be mostly copied
from there. For iteratorFrom and descendingIteratorFrom it's easy to
provide a default implementation in NavigableSet. All methods can be
implemented easily in LinkedHashSet and LinkedHashMap::keySet
package java.util;
public interface OrderedMap<K, V> extends Map<K, V> {
// First entry or null if empty
Map.Entry<K,V> firstEntry();
// Last entry or null if empty
Map.Entry<K,V> lastEntry();
// Retrieve&remove first entry or null if empty
Map.Entry<K,V> pollFirstEntry();
// Retrieve&remove last entry or null if empty
Map.Entry<K,V> pollLastEntry();
// First key or NSEE if empty
K firstKey();
// Last key or NSEE if empty
K lastKey();
// Ordered keySet()
OrderedSet<K> orderedKeySet();
// TBD: OrderedSet<K> descendingKeySet();
// TBD: OrderedSet<Entry<K,V>> orderedEntrySet();
}
All the methods except orderedKeySet() are already present in
NavigableMap, and the orderedKeySet() is just an alias for
navigableKeySet(). Also, all of them can be implemented easily in
LinkedHashMap.
Any feedback is welcome.
Thank you in advance,
Tagir Valeev.
