How to convert scala.collection.immutable.List[scala.collection.mutable.MutableList[String]] to java.util.List[java.util.List[String]]

Question

I have a Scala method being used in java class throwing the error below. For some reason the implicit conversions are not working for List of Lists but it does work for a List . (for ex: mutable.MutableList to util.List )

Error:(124, 143) type mismatch;
    found: scala.collection.immutable.List[scala.collection.mutable.MutableList[String]]
     required: java.util.List[java.util.List[String]]

or

Error:(124, 143) type mismatch;
    found: scala.collection.immutable.List[scala.collection.mutable.MutableList[String]]
     required: scala.collection.immutable.List[java.util.List[String]]

Answer 1

The inner list will not be converted unless you explicitely convert it.

import scala.collection.JavaConverters._
import java.util.{List=>JavaList}
import scala.collection.immutable.{List => ScalaList}
import scala.collection.mutable.{MutableList => ScalaMutableList}

val a : ScalaList[ScalaMutableList[String]] = List(MutableList())
val b: ScalaList[JavaList[String]]= a.map(_.asJava)
val c: JavaList[JavaList[String]] = b.asJava

I made the last conversion (c= b.asJava ) explicit and I would recommend to keep it so to make the code easier to ready for future readers.

Answer 2

Java knows nothing about scala existence. So whole scala<->java interoperability should be done on scala side.

This java code:

import java.util.List;

public class JavaMethod {
    public static void main(String[] args){
        List<Integer> listInteger = null;
        List<Integer> resultListInteger = ScalaMethod.argListInteger(listInteger);

        List<List<Integer>> listListInteger = null;
        List<List<Integer>> resultListListInteger = ScalaMethod.argListListInteger(listListInteger);
    }
}

compiles well with this scala code:

import scala.collection.JavaConverters._

object ScalaMethod {
  def argListInteger(listInteger: java.util.List[Integer]): java.util.List[Integer] = {
    val scalaList = listInteger.asScala

    //Do whatever you want
    scalaList
      .filter(e => true)
      .map(e => e)
    //And convert back to Java
      .asJava
  }

  def argListListInteger(listListInteger: java.util.List[java.util.List[Integer]]) = {
    val scalaListListInteger = listListInteger.asScala.map(_.asScala)

    //Do whatever you want
    scalaListListInteger
      .filter(e => true)
      .map(e => e)
    //And convert back to Java
      .map(_.asJava)
      .asJava
  }
}