Generics in Kotlin: Covariance and Contravariance 

same for me, the contravariance it's difficult to grasp because there is not any great example to recognize the pattern

Confusing in and out. Yep my understanding is same as you.

yeah she didn't explain it detailed enough

What I've understood from the Kotlin in Action book is that when we use 'in' variant we can pass supertypes also and when we use 'out' variant we can pass subtypes.

You are correct. in means that a type parameter can only be used as an input, and out means that a type parameter can only be used as an output. List is defined like this: interface List : Collection { .... }, because we will make a new List, and that's an output. One can assign subtype instance to super type variable, the other once vice versa. We can memorize the relation using List, which is an example of out/covariance, and we can assign List/supperType with an instance of ArrayList()/subType; and for in/contravariance, it's the inverse; and without either in or out keyword, then it's called invariance, between the supertype and subtype there is no assignment allowed. For example, since List is using out/covariance, the first is correct, because Int is Number. But if List is in/contravariance, then the second will be correct; without in or out/invariance, then none of these is correct. (1) var numList: List = listOf() (2) var intList: List = listOf()

@@vakho30thank you! :)

yeh, you think she would have, at least, removed it from the video.