When defining a variable of a generic type we can use a wild card instead of specifying the exact secondary type the generic type uses.
object HelloSample { def main(args:Array[String]):Unit = { val a:MyStack[SportCar] = new MyStack[SportCar]; val b:MyStack[_ <: Car] = a; } } class Car class SportCar extends Car class MyStack[T]
The following video clip shows the execution of this code sample and explains it.