LinkedHashMap/LinkedHashSet enhancement: OrderedMap/OrderedSet

Tagir Valeev amaembo at
Sat Apr 25 16:51:57 UTC 2020


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

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
  // 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

Any feedback is welcome.

Thank you in advance,
Tagir Valeev.

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