<@U9SNELT4G> your `Box&lt;T&gt;` is invariant, tha...

# getting-started

[Andreas Sinz]

@kiptoo your Box<T> is invariant, that means Box<Int> is not a subtype of Box<Number> https://proandroiddev.com/understanding-generics-and-variance-in-kotlin-714c14564c47

[kiptoo]

hi @Andreas Sinz i now get the general concept (i actually went thru the article you shared earlier, it's excellent). i can tell the compiler that my intention is for IntBox to be a subtype of Box<T> by doing: open class Box<out T>(t: T) {}, but what if some of my functions in Box need to take in parameters of type (or subtype) T, e.g Box.createBox(box: Box<T>)?

[Andreas Sinz]

@kiptoo you can only have Contravariance or Covariance, meaning T only occurs in the in-position or out-position, not both at the same time, otherwise you could create code that fails at runtime

[kiptoo]

good point. looks like i'll have to settle for covariance in this case

[kiptoo]

thanks!