UpperBound.scala

trait GenericCache[Key, Value] {
  def retrieve(key: Key): Value
  def insert(key: Key, value: Value): Unit
}

case class FastCache[Key, Value]() extends GenericCache[Key, Value] {
  private var m = Map[Key, Value]() //excuse mutability for illustration purposes

  def retrieve(key: Key): Value = m(key)

  def insert(key: Key, value: Value) {
    m = m + (key -> value)
  }
}

case class SlowCache[Key, Value]() extends GenericCache[Key, Value] {
  private var m = Map[Key, Value]() //excuse mutability for illustration purposes

  def retrieve(key: Key): Value = {
    Thread.sleep(1000)
    m(key)
  }

  def insert(key: Key, value: Value) {
    Thread.sleep(1000)
    m = m + (key -> value)
  }
}

object Test {
    def test() {
        val fc: FastCache[String, Int] = useCache("a", 1, FastCache[String, Int]())
        val sc: SlowCache[Char, Int]   = useCache('b', 2, SlowCache[Char, Int]())
    }

    def useCache[Key, Value, Cache <: GenericCache[Key, Value]](a: Key, b: Value, cache: Cache): Cache = {
        cache.insert(a, b)
        println("getting " +  cache.retrieve(a))
        cache
    }
}