Functional Data Structures in Java

Question

Does the Java standard library have any functional data structures, like immutable Sets, Lists, etc., with functional update?

Answer 1

Sounds like you're looking for Scala. It compiles to .class, so that's good enough, right?

Answer 2

You don't need scala. Just pass your collection into:

java.util.Collections.unmodifiableCollection(/* Collection<? extends T> c */);
java.util.Collections.unmodifiableSet(Set s);
java.util.Collections.unmodifiableMap(Map m);
java.util.Collections.unmodifiableList(List l);

I just saw this from another SO question:

Google's ImmutableSet

http://google-collections.googlecode.com/svn/trunk/javadoc/com/google/common/collect/ImmutableSet.html

from the docs:

Unlike Collections.unmodifiableSet(java.util.Set), which is a view of a separate collection that can still change, an instance of this class contains its own private data and will never change. This class is convenient for public static final sets ("constant sets") and also lets you easily make a "defensive copy" of a set provided to your class by a caller.

edited to incorporate comment.

Answer 3

Well, there are two possible approaches to "changing" an immutable collection:

• Make a copy of it that includes the "change"

• Create a new, different object that consists of a reference to the original object and a reference to a description of the change.

Clojure takes the latter approach, so it becomes fairly quick to create a lot of siblings of an original collection with minor corrections to each, with reasonable memory requirements. But most Java code tends to go for the first option.

For what it's worth, Google has created a handful of collections that support functional-style programming: http://code.google.com/p/google-collections/ but I haven't looked at them in depth.

Answer 4

Take a look at Google collections.

Answer 5

Strings and numbers are immutable in a functional way, but most collections are not (the immutable collections throw exceptions on add, remove, etc). CopyOnWriteArrayList and CopyOnWriteArraySet are the closest in that sense.

Answer 6

Functional java has Sets, Lists and more interesting abstractions.

Answer 7

It's always nice to see Google Collections plugged, but no, we do not have this. I don't know of any Java library that does. Inside Google, we implemented some functional List structures, and guess what? No one uses them. So they aren't likely to become open-sourced any time soon.

Answer 8

If you are interested in collections manipulation in a functional style give a look to lambdaj