How to convert a scala.List to a java.util.List?

How to convert Scala's scala.List into Java's java.util.List?

Solution 1:

Not sure why this hasn't been mentioned before but I think the most intuitive way is to invoke the asJava decorator method of JavaConverters directly on the Scala list:

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
res11: java.util.List[Int] = [1, 2, 3]

Solution 2:

Scala List and Java List are two different beasts, because the former is immutable and the latter is mutable. So, to get from one to another, you first have to convert the Scala List into a mutable collection.

On Scala 2.7:

import scala.collection.jcl.Conversions.unconvertList
import scala.collection.jcl.ArrayList
unconvertList(new ArrayList ++ List(1,2,3))

From Scala 2.8 onwards:

import scala.collection.JavaConversions._
import scala.collection.mutable.ListBuffer
asList(ListBuffer(List(1,2,3): _*))
val x: java.util.List[Int] = ListBuffer(List(1,2,3): _*)

However, asList in that example is not necessary if the type expected is a Java List, as the conversion is implicit, as demonstrated by the last line.