Ordered generic Map implementation?

Question

Which library is the best with ordered map implementation?

Implementation is required to be

1) generic

2) accessible by integer index

One from JRE can't be accessed by index: http://docs.oracle.com/javase/7/docs/api/java/util/LinkedHashMap.html

One from Commons-Collections 3.2 is not generic: http://commons.apache.org/proper/commons-collections//javadocs/api-release/org/apache/commons/collections/map/LinkedMap.html

One from Commons-Collection 4.0 is not released yet.

And can't find appropriate class in Guava. Is there one?

Do you realize that any linked map will give you O(N) performance for retrieval by index? You can get identical performance by simply walking the iterator. — parsifal, Mar 19 '13 at 16:17
If you describe the actual problem that you're trying to solve, you might find that there's a better solution. — parsifal, Mar 19 '13 at 16:18
@parsifal I disagree with O(N). Isn't it possible to have two parallel structures in implementation? — Suzan Cioc, Mar 19 '13 at 16:49
It's fine to disagree, but at least you should understand the data structures. If you want list semantics, you have two options: linked or array-backed. A linked list always costs O(N) for indexed retrieval; there's no way around that. An array-backed list gives you O(1) indexed retrieval, but if you delete items from the map you'll need to pay O(N) to update the array. — parsifal, Mar 19 '13 at 16:54

Grzegorz Rożniecki · Answer 1 · 2016-07-29T11:01:20.353

It can be done with ImmutableSortedMap (it's immutable and null hostile):

// use ImmutableSortedMap#copyOf or one of builders - naturalOrder or orderedBy
ImmutableSortedMap<K, V> map = ImmutableSortedMap.copyOf(origMap, comparator);
map.keySet().asList().get(index);

But what problem are you trying to solve? Seems to me like bad code smell...

EDIT:

If you want insertion order instead of using comparator, just use ImmutableMap:

ImmutableMap.copyOf(origMap).keySet().asList().get(index);

Ordered generic Map implementation?

1 Answers1