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

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("")
: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.