Queue.scala

abstract class IntQueue {
  def get(): Int
  val buf = new ArrayBuffer[Int]
  def put(x: Int) = {
    println("IntQueue.put")
    println("x=" + x)
    buf += x
  }
}

class BasicIntQueue extends IntQueue {
  def get() = 1
  override def put(x: Int) = {
    println("BasicIntQueue.put")
    println("x=" + x)
    super.put(x)
  }
}

trait Doubling extends IntQueue {
  abstract override def put(x: Int) = super.put(x * 2)
}

object Main extends App {
  val p = new BasicIntQueue with Doubling
  p.put(1)
  println(p.get())
}

Result:

BasicIntQueue.put
x=2
IntQueue.put
x=2