Problem compiling 2.9.1 scala program

Rotor.scala

/**
  * Created by Jeremy on 3/27/2016.
  */
import collection.mutable.ListBuffer

class Rotor(var rotorMap: List[(Char, Char)], val notch: Char){
  def this(alphabet: String, beginPos: Int, notch: Char) = this(
    Rotor.rotate((Alphabets.realAlphabet zip alphabet).toList, beginPos), notch)

  def this(alphabet: String, beginPos: Int) = this(alphabet, beginPos, 'Z')

  def showing =  rotorMap.head._1

  def isAtNotch = rotorMap.head._1 == notch

  def mirror = new Rotor(rotorMap.map(x => (x._2, x._1)), notch)

  def translate(windowPos: Int): Int = {
    rotorMap.indexWhere(t => t._1 == rotorMap(windowPos)._2)
  }

  def rotate : Unit = {
    rotorMap = Rotor.rotate(rotorMap, 1)
  }

  object Rotor {
    def rotate(l: List[(Char, Char)], nr: Int): List[(Char, Char)] = {
      val b: ListBuffer[(Char, Char)] = ListBuffer.empty
      b.appendAll(l.drop(nr))
      b.appendAll(l.take(nr))
      b.toList
    }
  }

}