We've seen an example where we were allowed to create AircraftSeat for passengers. The example looked like the following:
From what we know so far, Aircraft is contravariant in its type parameter T. But the thing is, when it comes to creating instances of AircraftSeat, it can be created for any type of T. What's expected is that this type parameter can only be of the Passengers type or it's subtype. So to achieve that we can introduce a type bound, in our case we'll use an upper-type bound. The reason for this is because we want to specify the type that's on the top of the inheritance hierarchy, in our case it's Passengers.
It'll look as follows:
class AircraftSeat[-T <: Passengers]
Here, the notation ...