Variance in out in Kotlin

Introduction

Variance in out is a fundamental concept every Kotlin developer should understand. Generics let you write type-safe containers and algorithms that work with many types while keeping compile-time checks.

Out makes type parameter covariant (producer). In this tutorial you will learn the syntax, walk through a complete example program, study the sample output, and review best practices so you can apply the concept confidently in your own projects.

Definition

• out makes type parameter covariant (producer).
• in makes type parameter contravariant (consumer).
• Variance controls subtype relationships for generics.

Syntax

interface Source<out T>

Variance in out in Kotlin Example Program in Kotlin

interface Producer<out T> {
    fun produce(): T
}
class StringProducer : Producer<String> {
    override fun produce() = "Kotlin"
}

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

Sample Output

Kotlin

When to use

Use generics when building reusable containers, parsers, or algorithms that must work with more than one type.

How it works

1. The program starts with a main function — the entry point that runs when you execute the file.

2. override fun produce() = "Kotlin" assigns or updates a value used later in the program.

3. val producer: Producer<Any> = StringProducer() assigns or updates a value used later in the program.

4. The println(producer.produce()) statement writes a line to the console — this produces part of the sample output below.

5. Out makes type parameter covariant (producer).

6. Run the program in IntelliJ IDEA, Android Studio, or with the Kotlin command-line compiler (kotlinc / kotlin). Compare your console output with the sample output shown below.

Best Practices

Common Mistakes

Key Points

• out makes type parameter covariant (producer).
• in makes type parameter contravariant (consumer).
• Variance controls subtype relationships for generics.
• Test the example locally and verify the output matches the sample.
• Experiment by changing input values to see how behaviour changes.

Notes

• Semicolons at the end of statements are optional in Kotlin.