How to convert Scala List to Java ArrayList

Question

I am using a Java API that requires ArrayList as parameter. Now in Scala, I have a List[String]. How can I convert the List[String] to ArrayList in Scala?

I have tried :

import scala.collection.JavaConverters._
val scalaList = List("a","b","c")
scalaList.asJava

Result is:

java.util.List[String] = [a, b, c]

The above does not work because I want the result to be

java.util.ArrayList[String]

instead of

java.util.List[String] = [a, b, c]

Answer 1

It totally makes sense to return java.util.List[T] which is a java interface and can have ArrayList[T] or LinkedList[T] as implementation.

scala> val scalaList = List(1,2,3) 
scalaList: List[Int] = List(1, 2, 3)

scala> import scala.collection.JavaConverters._  
import scala.collection.JavaConverters._

scala> scalaList.asJava
res22: java.util.List[Int] = [1, 2, 3]

In my opinion it needs to be converted to ArrayList[T] or LinkedList[T] whatever you want by yourself.

One way I could think of is

scala> new java.util.ArrayList[Int](scalaList.asJava)
res27: java.util.ArrayList[Int] = [1, 2, 3]

or

scala> new java.util.LinkedList[Int](scalaList.asJava)
res28: java.util.LinkedList[Int] = [1, 2, 3]

Better version of this would be

scala> def toArrayList[T](input: List[T]): java.util.ArrayList[T] = new java.util.ArrayList[T](input.asJava)
toArrayList: [T](input: List[T])java.util.ArrayList[T]

scala> toArrayList(List(1000, 2000, 3000))
res33: java.util.ArrayList[Int] = [1000, 2000, 3000]

And the best version would be, write implicit method

class AsArrayList[T](input: List[T]) {
  def asArrayList : java.util.ArrayList[T] = new java.util.ArrayList[T](input.asJava)
}

implicit def asArrayList[T](input: List[T]) = new AsArrayList[T](input)

List(1000, 2000, 3000).asArrayList