Functional Programming in Java [closed]

Answer 1

Google Guava has functional:

• collection operations
• concurrency constructs (Futures)

Answer 2

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.

Answer 3

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.

Answer 4

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

Further Reading

You may also want to read or watch these articles or videos:

• Functional Progamming in the Java Language, IBM DeveloperWorks (2004)
• Functional Programming Java, Lambda the Ultimate (2004)
• Functional Programming: a Pragmatic Introduction, InfoQ/CodePalousa (2011)

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Taken from my P.SE answer to "Is Functional Programming Possible in Java?"

Answer 5

Scala was mentioned here, but there's a lot lighter and more Java compatible language: Xtend. It compiles to plain Java and uses the same type system. It has great Eclipse support. You can mix .java and .xtend files in a single project.

Sample code:

def static void main(String[] args) {
    val s = #[1,2,3].map[it+43].join(", ")
    println(s);
}

Answer 6

If you want a pure Java solution check out lambdaj

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

Besides the possibility to define and use closure in a DSL-style, it also allows to manipulate collections in a functional way, without explicitly write closures or loops