Scala convert List[Int] to a java.util.List[java.lang.Integer]

Apparently you need both conversions. However, you can group them in a single implicit conversion:

implicit def toIntegerList( lst: List[Int] ) =
  seqAsJavaList( lst.map( i => i:java.lang.Integer ) )

Example:

scala> def sizeOf( lst: java.util.List[java.lang.Integer] ) = lst.size

scala> sizeOf( List(1,2,3) )
res5: Int = 3