scala: classcast exception: java.util.HashMap cannot be cast to scala.collection.immutable.Map

Question

I have a scala code snippet given below:

// country code to nameToPlaces traversable
val input : Traversable[(Int, java.util.Map[String, java.util.Set[String]])] ... 
val nameToPlacesMap : Map[String, Set[String]] = input.toMap.
            values.
            map (x => x.toSeq).    // throws a class cast exception
            reduce((x,y) => x ++ y).
            groupBy(_._1).
            mapValues(_.map(_._2).
            reduce((x,y)=>x.toSet ++ y.toSet))

nameToPlacesMap is expected to collate all nameToPlaces from all countries. However, while running this code, i am getting a classcastexception

 java.util.HashMap cannot be cast to scala.collection.immutable.Map

I have tried adding asScala but of no use. I am also importing import scala.collection.JavaConverters._ Any help will be appreciated.

NOTE: I don't see this question as a duplicate of a generic classcast exception as noted below: Please revise your judgement.

Your code doesn't compile (too many argument to Traversable). Please edit your post and correct it. Also, it would be useful to know where you put .asScala. — The Archetypal Paul
– The Archetypal Paul, Commented Oct 4, 2015 at 7:33
Is there a special reason why you are mixing Java and Scala collections? If your program is only Scala, then don't use Java collections. — Jesper
– Jesper, Commented Oct 4, 2015 at 7:45
Could you try input.flatMap(_._2.asScala).groupBy(_._1).mapValues(_.map(_._2.toSet).reduce(_ ++ _)) ? You need to import both JavaConversions._ and JavaConverters._ for that. — Kolmar
– Kolmar, Commented Oct 4, 2015 at 11:23

Sascha Kolberg · Accepted Answer · 2015-10-05 07:27:07Z

0

You have several java collections in your outer map, you have to make sure to convert them all. As far as I can see, the following should work for you:

import java.util

import scala.collection.JavaConverters._
import scala.collection.mutable

val input : Traversable[(Int, java.util.Map[String, java.util.Set[String]])] =
  Seq((1, new util.HashMap[String, java.util.Set[String]]()))

val nameToPlacesMap : Map[String, mutable.Set[String]] = input.toMap.mapValues(
  // convert inner Map
  _.asScala.mapValues(
    // convert java.util.Set
    mutable.Set.empty ++ _.asScala)
  ).values.
            map (x => x.toSeq).    // hopefully not anymore :: throws a class cast exception
            reduce((x,y) => x ++ y).
  groupBy(_._1).
  mapValues(_.map(_._2).
    reduce((x,y)=> x ++ y))

edited Oct 5, 2015 at 7:27

answered Oct 4, 2015 at 7:32

Sascha Kolberg

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5 Comments

Amm Sokun Over a year ago

Hi Sasche, i get the compiling error: value asScala is not a member of Traversable

Sascha Kolberg Over a year ago

I was not so sure about the outermost traversable, just try to leave out the first 'asScala'

Sascha Kolberg Over a year ago

@AmmSokun I edited my answer, to treat the outer most Traversable as scala collection

Amm Sokun Over a year ago

it still doesn't compile, after removing the outmost asScala, next one gives the error and fails to compile

Sascha Kolberg Over a year ago

@AmmSokun I missed some confusion about mutable and immutable Set as well as import of required converters. I edited my answer to a version, that at least on my machine, compiles.

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