Java <-> Scala Collection conversions, Scala 2.10 [duplicate]

Question

Looking at the code in JavaConversions and JavaConverters, I am unsure which the "correct" way (with 2.10) to convert between Java and Scala collections (in either direction) is.

There seem to be lots of @deprecated annotations.

Has a definitive answer from the Scala Team (Typesafe?) been published?

Thanks, John

Answer 1

This is the poster child example for the dangers of import JavaConversions._:

scala> val m = Map(1 -> "one")
m: scala.collection.immutable.Map[Int,String] = Map(1 -> one)

scala> m.contains(1)
res0: Boolean = true

scala> m.contains("")
<console>:9: error: type mismatch;
 found   : String("")
 required: Int
              m.contains("")
                         ^

scala> import collection.JavaConversions._
import collection.JavaConversions._

scala> m.contains("")
res2: Boolean = false

Instead of issuing a type error, the compiler converts the Scala Map to to a java.util.Map, which has a looser signature that accepts Object.

Answer 2

I don't know of any such proclamation, but you should just always use JavaConverters, i.e. the ones that require you to indicate conversions with .asScala and .asJava.

As I understand it, JavaConverters were brought in in 2.8.1 because the JavaConversions in 2.8 were dangerous and made it easy to accidentally convert things where you weren't expecting it.