In variance (contravariance) in Kotlin - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a contravariant generic interface using the correct keyword.

Kotlin

interface Consumer<[1] T> {
    fun consume(item: T)
}

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

Avar

Bin

Cout

Dval

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'out' instead of 'in' for contravariance.

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

In Kotlin, in is used to declare contravariance, allowing the generic type to be used only as input.

2fill in blank

medium

Complete the function parameter type to accept a contravariant generic Consumer.

Kotlin

fun feed(consumer: Consumer<[1] Animal>) {
    consumer.consume(Dog())
}

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

Ain

Bout

Cvar

Dval

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'out' which is for covariance, not contravariance.

Omitting the variance keyword causing type errors.

The in keyword allows the Consumer to accept subtypes of Animal, enabling contravariance.

3fill in blank

hard

Fix the error in the function call by adding the correct variance modifier.

Kotlin

val consumer: Consumer<[1] Dog> = object : Consumer<Animal> {
    override fun consume(item: Animal) {
        println("Consumed: $item")
    }
}

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

Ain

Bout

Cvar

Dval

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'out' which causes type mismatch.

Not using any variance modifier causing compilation error.

Declaring Consumer allows assigning a Consumer to it because of contravariance.

4fill in blank

hard

Complete the code to create a contravariant function type parameter and call it correctly.

Kotlin

fun process(consumer: ([1] String) -> Unit) {
    consumer("Hello")
}

process -> { println(it) }

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Ain

Bout

C->

D->>

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'out' instead of 'in' for the input parameter.

Using '->>' which is invalid syntax.

The function parameter uses in for contravariance on input type, and the arrow -> separates parameter and return type.

5fill in blank

hard

Fill all three blanks to declare a contravariant generic class, implement it, and use it.

Kotlin

class Printer<[1] T> {
    fun print(item: T) {
        println(item)
    }
}

val printer: Printer<[2] Any> = Printer<[3] String>()

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

Ain

Bout

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'out' which is covariance, not contravariance.

Mixing variance modifiers causing type mismatch.

Using in declares contravariance, allowing Printer to be assigned to Printer.