Covariance in Kotlin?

Answer

Covariance allows you to use a subtype in place of a supertype. In Kotlin generics, it is denoted by the out keyword.

Producer (out)

• If a generic class only returns (produces) type T, it can be covariant.
• List<out T> is covariant. A List<String> IS A List<Any>.

Copy codeval strings: List<String> = listOf("A", "B")
val objects: List<Any> = strings // Valid because List is covariant (out T)

Contravariance (in)

• If a generic class only consumes type T, it is contravariant.
• Denoted by in.
• Comparable<in T>. A Comparable<Any> can compare Strings.