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

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.

Answer 2

totallylazy is a very good FP library which has implementations of:

• PersistentList<T>: the concrete implementations are LinkedList<T> and TreeList<T> (for random access)
• PersistentMap<K, V>: the concrete implementations are HashTreeMap<K, V> and ListMap<K, V>
• PersistentSortedMap<K, V>
• PersistentSet<T>: the concrete implementation is TreeSet<T>

Example of usage:

import static com.googlecode.totallylazy.collections.PersistentList.constructors.*;
import com.googlecode.totallylazy.collections.PersistentList;
import com.googlecode.totallylazy.numbers.Numbers;

...

PersistentList<Integer> list = list(1, 2, 3);

// Create a new list with 0 prepended
list = list.cons(0);

// Prints 0::1::2::3
System.out.println(list);

// Do some actions on this list (e.g. remove all even numbers)
list = list.filter(Numbers.odd);
// Prints 1::3
System.out.println(list);

totallylazy is constantly being maintained. The main disadvantage is the total absence of Javadoc.

Answer 3

May want to check out clj-ds. I haven't used it, but it seems promising. Based off of the projects readme it extracted the data structures out of Clojure 1.2.0.

Answer 4

Functional Java implements a persistent List, lazy List, Set, Map, and Tree. There may be others, but I'm just going by the information on the front page of the site.

I am also interested to know what the best persistent data structure library for Java is. My attention was directed to Functional Java because it is mentioned in the book, Functional Programming for Java Developers.

Answer 5

There's pcollections (Persistent Collections) library you can use:

http://code.google.com/p/pcollections/

Answer 6

https://github.com/andrewoma/dexx is a port of Scala's persistent collections to Java. It includes:

• Set, SortedSet, Map, SortedMap and Vector
• Adapters to view the persistent collections as java.util equivalents
• Helpers for easy construction