what's a good persistent collections framework for use in java?

Question

By persistent collections I mean collections like those in clojure.

For example, I have a list with the elements (a,b,c). With a normal list, if I add d, my original list will have (a,b,c,d) as its elements. With a persistent list, when I call list.add(d), I get back a new list, holding (a,b,c,d). However, the implementation attempts to share elements between the list wherever possible, so it's much more memory efficient than simply returning a copy of the original list. It also has the advantage of being immutable (if I hold a reference to the original list, then it will always return the original 3 elements).

This is all explained much better elsewhere (e.g. http://en.wikipedia.org/wiki/Persistent_data_structure).

Anyway, my question is... what's the best library for providing this functionality for use in java? Can I use the clojure collections somehow (other that by directly using clojure)?

Answer 1

Just use the ones in Clojure directly. While obviously you might not want to use the language it's self, you can still use the persistent collections directly as they are all just Java classes.

import clojure.lang.PersistentHashMap;
import clojure.lang.IPersistentMap;

IPersistentMap map = PersistentHashMap.create("key1", "value1");

assert map.get("key1").equals("value1");
IPersistentMap map2 = map.assoc("key1", "value1");

assert map2 != map;
assert map2.get("key1").equals("value1");

(disclaimer: I haven't actually compiled that code :)

the down side is that the collections aren't typed, i.e. there are no generics with them.