Functional Programming in Java [closed]

Is there a good library for functional programming in Java?

I'm looking for stuff like Predicate and List.Find() (as a static method). Not complicated to implement, but it would be nice to find a reusable library here.

Solution 1:

FunctionalJava is the best known library; it makes use of Java closures (BGGA) for examples:

final Array<Integer> a = array(1, 2, 3);  
final Array<Integer> b = a.map({int i => i + 42});  
arrayShow(intShow).println(b); // {43,44,45}

EDIT

Check also lambdaj.

Further EDIT

BGGA is entirely optional. It just makes for nicer syntax.

Solution 2:

Scala is a functional programming language that is fully compatible with Java (runs through the JVM). It offers a beautiful mix of object-oriented and functional techniques along with many improvements over Java in generics and concurrency. Some even say it could replace Java.

Solution 3:

Java Libraries

There are libraries that can help you do this, by already doing the legwork for you and hiding the arcane things:

Mature / Established Libraries

Functional Java
Google guava
LambdaJ

More Obscure / Experimental Libraries

Fun4J
JCurry
OCaml-Java
Jambda
Bolts

These will allow you to write Java code with a more functional approach and possibly more familiar syntax and semantic, as you'd expect from an FP-competent language. Within reason, that is.

JVM Languages

And obviously, you can implement a functional language on top of Java. So that you can then use that one as your FP language. Which is a bit of a higher-level of abstraction than what you asked for, but relatively within context (though I'm cheating a bit here, granted).

For instance, check out:

Quite Mature Languages

Clojure
Scala

Less Mature or More Obscure Languages

Frege
Jaskell