Out variance (covariance) in Kotlin - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a covariant interface in Kotlin.

Kotlin

interface Producer<[1] T> {
    fun produce(): T
}

Drag options to blanks, or click blank then click option'

Aout

Bin

Cvar

Dval

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'in' instead of 'out' which declares contravariance.

Using 'var' or 'val' which are not variance modifiers.

In Kotlin, out keyword declares covariance, allowing the type to be returned (produced) but not consumed.

2fill in blank

medium

Complete the function signature to accept a covariant Producer.

Kotlin

fun <T> consume(producer: Producer<[1] T>) {
    val item = producer.produce()
    println(item)
}

Drag options to blanks, or click blank then click option'

Aout

Bvar

Cin

Dval

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'in' which is for contravariance and not compatible here.

Omitting variance keyword causing type mismatch.

The parameter type uses out to accept a covariant Producer, ensuring safe usage.

3fill in blank

hard

Fix the error in the assignment by adding the correct variance modifier.

Kotlin

val stringsProducer: Producer<String> = object : Producer<String> {
    override fun produce() = "Hello"
}

val anyProducer: Producer<[1] Any> = stringsProducer

Drag options to blanks, or click blank then click option'

Ain

Bval

Cout

Dvar

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'in' which is contravariant and causes type mismatch.

Not using any variance modifier causing compilation error.

Using out variance allows assigning Producer to Producer safely.

4fill in blank

hard

Fill both blanks to create a covariant list producer and consume it safely.

Kotlin

interface ListProducer<[1] T> {
    fun produceList(): List<T>
}

fun printFirst(producer: ListProducer<[2] String>) {
    println(producer.produceList().first())
}

Drag options to blanks, or click blank then click option'

Aout

Bin

Cvar

Dval

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'in' which is for consuming, not producing.

Mixing variance keywords causing type errors.

Both blanks use out to declare covariance, allowing safe production and consumption of lists.

5fill in blank

hard

Fill all three blanks to define a covariant producer interface, a function consuming it, and a variable assignment.

Kotlin

interface Box<[1] T> {
    fun get(): T
}

fun <T> useBox(box: Box<[2] T>) {
    println(box.get())
}

val stringBox: Box<String> = object : Box<String> {
    override fun get() = "Kotlin"
}

val anyBox: Box<[3] Any> = stringBox

Drag options to blanks, or click blank then click option'

Aout

Bin

Cvar

Dval

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'in' which is for consumers, causing type errors.

Omitting variance causing assignment errors.

Using out in all three places declares covariance, allowing safe assignment and usage.