Question

I’m new to Scala and our project mixes Java and Scala code together (using the Play Framework). I’m trying to write a Scala method that can take a nested Java Map such as:

LinkedHashMap<String, LinkedHashMap<String, String>> groupingA = new LinkedHashMap<String, LinkedHashMap<String,String>>();

And have that java object passed to a Scala function that can loop through it. I have the following scala object definition to try and support the above Java nested map:

Seq[(String, Seq[(String,String)])]

Both the Java file and the Scala file compile fine individually, but when my java object tries to create a new instance of my scala class and pass in the nested map, I get a compiler error with the following details:

[error]  ..... overloaded method value apply with alternatives:
[error]   (options: java.util.List[String])scala.collection.mutable.Buffer[(String, String)] <and>
[error]   (options: scala.collection.immutable.List[String])List[(String, String)] <and>
[error]   (options: java.util.Map[String,String])Seq[(String, String)] <and>
[error]   (options: scala.collection.immutable.Map[String,String])Seq[(String, String)] <and>
[error]   (options: (String, String)*)Seq[(String, String)]
[error]  cannot be applied to (java.util.LinkedHashMap[java.lang.String,java.util.LinkedHashMap[java.lang.String,java.lang.String]])

Any ideas here on how I can pass in a nested Java LinkedHashMap such as above into a Scala file where I can generically iterate over a nested collection? I’m trying to write this generic enough that it would also work for a nested Scala collection in case we ever switch to writing our play framework controllers in Scala instead of Java.

Answer 1

Seq is a base trait defined in the Scala Collections hierarchy. While java and scala offer byte code compatibility, scala defines a number of its own types including its own collection library. The rub here is if you want to write idiomatic scala you need to convert your java data to scala data. The way I see it you have a few options.

• You can use Richard’s solution and convert the java types to scala types in your scala code. I think this is ugly because it assumes your input will always be coming from java land.

• You can write beautiful, perfect scala handler and provide a companion object that offers the ugly java conversion behavior. This disentangles your scala implementation from the java details.

• Or you could write an implicit def like the one below genericizing it to your heart’s content.

.

import java.util.LinkedHashMap
import scala.collection.JavaConversions.mapAsScalaMap

object App{
  implicit def wrapLhm[K,V,G](i:LinkedHashMap[K,LinkedHashMap[G,V]]):LHMWrapper[K,V,G] = new LHMWrapper[K,V,G](i)

  def main(args: Array[String]){
    println("Hello World!")
    val lhm = new LinkedHashMap[String, LinkedHashMap[String,String]]()
    val inner = new LinkedHashMap[String,String]()
    inner.put("one", "one")
    lhm.put("outer",inner);
    val s = lhm.getSeq()
    println(s.toString())
  }
  class LHMWrapper[K,V,G](value: LinkedHashMap[K,LinkedHashMap[G,V]]){
    def getSeq():Seq[ (K, Seq[(G,V)])] = mapAsScalaMap(value).mapValues(mapAsScalaMap(_).toSeq).toSeq
  }
}