What is it?

A tiny interoperability library for passing JRuby/Scala collections back and forth.

https://rubygems.org/gems/jruby-scala-collections

Installation

$ gem install jruby-scala-collections

With bundler:

gem "jruby-scala-collections", ">=0.1.1", require: "jruby/scala_support"

Beware that you have to have scala-library.jar loaded before you load jruby/scala_support. In Rails this generally means it loading in config/boot.rb.

How do you use it?

Each Object has two methods: #to_scala and #from_scala. These can be used to wrap JRuby/Scala collections.

Example:

gem 'jruby-scala-collections'
require 'jruby/scala_support'

r_arr = [1,2,3,4]
scala_arr = scala_object.do_stuff(r_arr.to_scala)
scala_arr.from_scala

• Array#to_scala becomes scala.collection.mutable.Buffer
• Hash#to_scala becomes scala.collection.mutable.Map
• Set#to_scala becomes scala.collection.mutable.Set

Take node that even collections inside collections are wrapped:

> a = [1,[2,3],{4=>5}].to_scala
 => #<Java::JrubyCollection::ListWrapper:0x768bdb> 
> a.apply(1)
 => #<Java::JrubyCollection::ListWrapper:0x884ab9> 
> a.apply(2)
 => #<Java::JrubyCollection::MapWrapper:0x1bb605> 

From Scala side Ruby primitives are converted using default JRuby conversions that are listed in https://github.com/jruby/jruby/wiki/CallingJavaFromJRuby section "Conversion of Types".

So if you expect Array of Fixnums coming to your scala method, it should accept:

// Either
def scalaMethod(args: collection.mutable.Buffer[Long])
// Or
def scalaMethod(args: collection.Seq[Long])

It also adds #Some and None so you could pass values to Scala methods:

scala_object.set(Some(3))
scala_object.get(None)

Disclaimer

This library was written by Artūras 'arturaz' Šlajus for personal usage.

Contributing

#to_scala should work pretty well, however Ruby wrappers in #from_scala may be missing methods. Patches are welcome. jruby-scala-collections uses rubyspec and mspec to test that the behaviour of the Ruby wrappers is correct. Currently only a part of those tests pass. You can contribute by grabbing any of the tests marked as fails in rubyspec/tags/**/*_tags.txt, marking it as focus and implement the needed behaviour. Guard is configured so that bundle exec guard runs all tests that have the focus tag. If you want to run all tests that should be working, use bundle exec mspec -G fails rubyspec. If you have implemented functionality, run bundle exec mspec tag rubyspec to update the list of failing tests and check with a git diff that you haven't broken other functionality.