In and out type variant of Kotlin

If you ever defined generic in Kotlin, you’ll notice many a times, it would propose to use the in or out keyword to define the generic. It puzzles me at a start on when which is used, and for what.

Formally, this is a way to define contravariance and covariant. It took me a while to learn about it. I’ll dive in here to explain what how I understand and memorize their different.

In & Out easily remembered

Out (covariant type)

If your generic class only use the generic type as output of it’s function/s, then out is used i.e.

interface Production<out T> {
    fun produce(): T
}

I call it production class/interface, as it is mainly to produce output of the generic type. Hence very simple one could remember

produce = output = out.

In (contravariance type)

If your generic class only use the generic type as input of it’s function/s, then in is used i.e.

interface Consumer<in T> {
    fun consume(item: T)
}

I call it consumer class/interface, as it is mainly consuming the generic type. Hence very simple one could remember

consume = input = in.