Scalaの「::」と「:::」

package sample

object Sample {
    
  def main(args: Array[String]): Unit = {
    val a = List(1,2,3)
    val b = List(4,5,6)
    println(a :: b)
    println(a ::: b)
  }
}

List(List(1, 2, 3), 4, 5, 6)
List(1, 2, 3, 4, 5, 6)

JavaでいうList#add()とList#addAll()だね。