Scala, doesn't like java collections API?

Question

I'm using asynchttpclient. When building up parameters, one passes a java.util.Map to the setParameters method.

The (Java) signature looks like the following:

setParameters(Map<String,Collection<String>);

Real day 1 stuff. But wow, I'm trying to call it from Scala and I cannot for the life of me create a collection to match that signature.

Here's the mess I've created so far.

var m:java.util.Map[java.lang.String,java.util.Collection[java.lang.String]] = new java.util.HashMap[java.lang.String,java.util.HashSet[java.lang.String]]()
val req = new RequestBuilder().setUrl("http://localhost:1234/").setParameters(m).build

And here's the error message,

Multiple markers at this line
- type mismatch; found : java.util.HashMap[java.lang.String,java.util.HashSet[java.lang.String]] required: 
 java.util.Map[java.lang.String,java.util.Collection[java.lang.String]] Note: java.util.HashSet[java.lang.String] <: java.util.Collection[java.lang.String], but Java-defined 
 **trait Map is invariant in type V. You may wish to investigate a wildcard type such as `_ <: java.util.Collection[java.lang.String]`. (SLS 3.2.10)**
- type mismatch; found : 

Fair enough, I wouldn't have thought I was doing something particularly complicated, but lets give the compiler suggestion a try...

So I change it to the following

 var m:java.util.Map[java.lang.String,_ <: java.util.Collection[java.lang.String]] = new java.util.HashMap[java.lang.String,java.util.HashSet[java.lang.String]]()
 val req = new RequestBuilder().setUrl("http://localhost:1234/").setParameters(m).build

And receive the following lovely error message in response.

Multiple markers at this line
    - overloaded method value setParameters with alternatives: (com.ning.http.client.FluentStringsMap)com.ning.http.client.RequestBuilder <and> 
     (java.util.Map[java.lang.String,java.util.Collection[java.lang.String]])com.ning.http.client.RequestBuilder cannot be applied to (java.util.Map[java.lang.String,_$1])

Back to basic's, I'd also like to mention that my first attempt was as follows.

   import scala.collection.JavaConverters._
   var m = Map[String,Set[String]]()
   val req = new RequestBuilder().setUrl("http://localhost:1234/").setParameters(m.asJava).build

But that produced the following

Multiple markers at this line - overloaded method value setParameters with alternatives: (com.ning.http.client.FluentStringsMap)com.ning.http.client.RequestBuilder (java.util.Map[java.lang.String,java.util.Collection[java.lang.String]])com.ning.http.client.RequestBuilder cannot be applied to (java.util.Map[String,Set[String]])

Edit, thanks to __0, this is now working. Here's my final code:

 def buildReqMap(in: Map[String, String]) = {
   import java.util.{ Map => JMap, Collection => JColl, HashMap => JHashM, HashSet => JHashS }
   val m: JMap[String, JColl[String]] =
     new JHashM[String, JColl[String]]()
   in.fold(m) { (a, b) =>
     {
       val s = new JHashS[String]()
       s.add(b.asInstanceOf[String])
       m.put(a.asInstanceOf[String], s)
       m
     }
   }
   m
 }

 def main(args: Array[String]): Unit = {
   val m = buildReqMap(
     Map(
       ("delimited" -> "length"),
       ("track" -> "binarytemple,music,kittens")))

   val req = new RequestBuilder().setUrl("http://localhost:1234/").setParameters(m).build
   val fut = asyncHttpClient.executeRequest(req, handler).get

Answer 1

Well, the first error message made it clear— since java.util.Map is mutable, it can't handle variance. So you cannot cast a Map[_,Set[_]] to a Map[_,Collection[_]].

val m: java.util.Map[String, java.util.Collection[String]] = 
   new java.util.HashMap[String,java.util.Collection[String]]()
val s = new java.util.HashSet[String]()
s.add("welt")
m.put("hallo", s)