Design patterns exercise (observer + strategy)

Clock.scala

import java.text.SimpleDateFormat
import java.util.Date

object Clock {
  val TIME_24 = 0
  val TIME_US = 1
  val TIME_UNIX = 2

  def main(args: Array[String]): Unit = {
    val c = new Clock
    c.setDisplayType(TIME_24)
    new Thread(c).start()
    // After a few seconds, switch to a different display type
      Thread.sleep(3000)
    c.setDisplayType(TIME_UNIX)
  }
}

class Clock extends Runnable {
  private var displayType = Clock.TIME_24

  def setDisplayType(dt: Int): Unit = {
    displayType = dt
  }

  override def run(): Unit = {
    while (true) {
      var time = ""
      if (displayType == Clock.TIME_24) time = new SimpleDateFormat("HH:mm:ss").format(new Date)
      else if (displayType == Clock.TIME_US) time = new SimpleDateFormat("hh:mm:ss a").format(new Date)
      else if (displayType == Clock.TIME_UNIX) time = String.valueOf(System.currentTimeMillis / 1000L)
      System.out.println(time)
      Thread.sleep(1000)
    }
  }
}