Best solution for converting java.util.List to Scala list

Question

I am trying to convert a java.util.List to a Scala list. There are many possibilities presented in post 674713 so I would like to know which one is the the best. I am using Play 2.1.1. My model is :

case class Page[T](
    var data: java.util.List[T],
    var previous: String,
    var next: String,
    var totalPageCount: Int)(implicit val tWrites: Writes[T])

object Page {

    implicit def pageWrites[T: Writes]: Writes[Page[T]] = (
        (__ \ 'data).write[java.util.List[T]] and
        (__ \ 'previous).write[String] and
        (__ \ 'next).write[String] and
        (__ \ 'totalPageCount).write[Int])(unlift(Page.unapply[T]))
}

This code does not work because I need to add a writer for generic java.util.List[T] type.

I have added this to my object Page :

implicit def listWrites[T](implicit fmt: Writes[T]): Writes[List[T]] = new Writes[List[T]] {
    def writes(ts: List[T]) = JsArray(ts.map(t => Json.toJson(t)(fmt)))
}

But this generates an error when executing my project ("MatchError: null" on listWrites). Therefore I would like to convert my java.util.List to a Scala one to avoid using this writer that does not work. Any solution ?

possible duplicate of How to Convert java util list to Scala list — user908853
– user908853, Commented Apr 22, 2014 at 14:16

Andreas Neumann · Accepted Answer · 2014-04-22 17:46:45Z

5

As a default I'd start of with:

import scala.collection.JavaConverters._
data.asScala //var data: java.util.List[T]

This will return a buffer, as the JavaList is mutable. To convert to a List you will have to also call the toList method

data.asScala.toList //List[T]

This would make the conversion explicit, is short readable and is bound to work throughout different versions because it's part of the standard library.

Here is a solution by adding an implicit Conversion. You now should be able to call data.toScalaList if you import the conversion.

object Example {
  implicit class Data[T](data: java.util.List[T]) {
    import scala.collection.JavaConverters._
    def toScalaList: List[T] = data.asScala.toList
  }
}

//just import it with
import Example._

edited Apr 22, 2014 at 17:46

answered Apr 22, 2014 at 14:15

Andreas Neumann

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7 Comments

Andreas Neumann Over a year ago

Works for me: scala> new java.util.LinkedList[String].asScala res3: scala.collection.mutable.Buffer[String] = Buffer(). WHich Version of Scala are you using. The example is based on 2.10.

Christophe Over a year ago

I am working with Play 2.1.1. So I think it is Scala 2.1 too.

Andreas Neumann Over a year ago

The converters are part of Scala since 2.8. Have a look at your build.sbt. THere should be a key scalaVersion := <VERSION NUMBER> which denotes the version.

Christophe Over a year ago

I don't have a build.sbt (as I am using Play maybe ?) but I have the converters as I have access to asScalaMapConverters method for example which is part of JavaConverters. I am just missing asScala.

Christophe Over a year ago

I have "AsJava" with an uppercase so I am not sure this is what I should use. And in fact when I use it I don't have the toList method on it.

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