Clock.scala

package com.balopat.timeinjection

import java.util.Date

class RealClock extends Clock {
  def getTime() = new Date()
}

trait Clock {
  def getTime(): Date
}

Wall.scala

package com.balopat.timeinjection

import java.util.Date

class Wall(clock: Clock) { //injection!!!
    var messages = List[Message]()

    def message(message: String) {
       messages ::= Message(message, clock.getTime()) //we are using clock as a service to get time
    }
}

case class Message(message: String, createdAt: Date)