Kotlin Generics: Understanding in, out, and where Keywords

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In Kotlin, the generics system provides powerful features to ensure type safety while allowing flexibility. This blog post will explore the <in, out, where> keywords in Kotlin generics with examples ranging from basic to advanced.

The out Keyword: Covariance

The out keyword is used to mark a type parameter as covariant. It means that the type parameter can only be produced (returned) by the class or function, and not consumed (passed as an argument). Here's a simple example:

abstract class Producer<out T> {
    abstract fun produce(): T
}

class StringProducer : Producer<String>() {
    override fun produce(): String = "Hello, world!"
}

fun main() {
    val producer: Producer<Any> = StringProducer()
    println(producer.produce())
}

In this example, the Producer class has a covariant type parameter T. As a result, we can use a StringProducer instance where a Producer<Any> is expected.

The in Keyword: Contravariance

The in keyword is used to mark a type parameter as contravariant. It means that the type parameter can only be consumed (passed as an argument) by the class or function, and not produced (returned). Here's an example:

interface Consumer<in T> {…