Cannot cast java ArrayList to scala immutable List

Question

I have the following data structure:

java.util.Map[List[String],List[String]] = {[10, 20]=[1500], [5, 7]=[1400]}

I am trying to extract the numbers 10 20 5 and 7 using Scala. The way I was looking to achieve this is:

map.head._1 -> to extract 10 (map.head returns a tuple)
map.head._2 -> to extract 20 (map.head returns a tuple)

However, I am getting the following exception:

java.lang.ClassCastException: java.util.ArrayList cannot be cast to scala.collection.immutable.List

I have read about importing import scala.collection.JavaConversions._ however, this did not fix anything.

Thanks, any help is highly appreciated!

The piece of code that tries to achieve this is:

 def getTokenRangeForKeys(params: String): java.util.Map[List[String], List[String]] = {
    invokeOperation[java.util.Map[List[String], List[String]]]("XXX", "YYY", Array(params))
  }

The above method returns my map, which looks like this:

  map = java.util.Map[List[String],List[String]] = {[10, 20]=[1500], [5, 7]=[1400]}

What I have tried so far:

map.head._1 -> java.lang.ClassCastException: java.util.ArrayList cannot be cast to scala.collection.immutable.List

scalaMap = map.asScala
m.headOption match {
  case Some((h1, h2)) => println((h1, h2)) -> java.util.ArrayList cannot be cast to scala.collection.immutable.List
  case None => ...
}

Show your code - post a complete piece of code that reproduces the problem, not just a single statement snippet (like map.head._1 etc.), that will make it much easier to tell what exactly is wrong. — Jesper
– Jesper, Commented Jan 11, 2017 at 9:28
This is my entire code related to this. A method returns a java Map that needs to be worked with in a Scala script. — Adrian Daniel Culea
– Adrian Daniel Culea, Commented Jan 11, 2017 at 9:38

Suma · Accepted Answer · 2019-08-01 22:56:18Z

3

I think your declaration of what comes from Java world should be:

java.util.Map[java.util.List[String], java.util.List[String]]

In the current form of java.util.Map[List[String], List[String]] you declare a java Map of Scala Lists, which is probably not what you want. JVM is not complaining when you pass your Java types because only top level type is checked as a part of function signature check - this is called type erasure.

On this you should use JavaConverters asScala to convert to corresponding Scala types as written in the Reactormonk answer:

import scala.collection.JavaConverters._
val m = map.asScala.map{case (h, k) => (h.asScala, k.asScala)}

edited Aug 1, 2019 at 22:56

answered Jan 11, 2017 at 9:59

Suma

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1 Comment

Adrian Daniel Culea Over a year ago

This is indeed a solution. It works as expected now!

Reactormonk · Accepted Answer · 2017-01-11 09:45:31Z

0

Don't use JavaConversions.

import scala.collection.JavaConverters._
val m = map.asScala.map({case (h, k) => (h.asScala, k.asScala)})
m.headOption match {
  case Some((h1, h2)) => ...
  case None => ...
}

edited Jan 11, 2017 at 9:45

answered Jan 11, 2017 at 9:28

Reactormonk

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8 Comments

Adrian Daniel Culea Over a year ago

I see what you are trying to do, but I can't get the syntax write I guess. It's my first ever scala project, could you please provide the additional code that achieves this?

Reactormonk Over a year ago

On the pattern matching, I'm using headOption so you handle the case of an empty Map.

Adrian Daniel Culea Over a year ago

I get back the same exception. I tried to just print (h1, h2) like this: ... => println((h1, h2))

Adrian Daniel Culea Over a year ago

h.asScala and k.asScala throw an error: value asScala is not a memeber of List[String]

Adrian Daniel Culea Over a year ago

map.getClass() -> Class[_ <: java.util.Map[List[String],List[String]]] = class java.util.HashMap

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