covariant

gistfile1.scala

class Father {
  def say() {
    Console.println("I am father")
  }
}

class Son extends Father {
  override def say() {
    Console.println("I am son")
  }
}
def speak(speakers:List[Father]) = speakers map (_.say) 
val sFathers = List(new Father)
val sSons = List(new Son)

import java.util.ArrayList
def javaSpeak(speakers:ArrayList[Father]) = speakers.get(0).say
val jFathers = new java.util.ArrayList[Father]()
jFathers.add(new Father)
val jSons = new java.util.ArrayList[Son]()
jSons.add(new Son)

----------------------------------------------
speak(sFathers) //OK
speak(sSons) //OK
javaSpeak(jFathers) //OK
javaSpeak(jSons)   //wrong!