In variance (contravariance) in Kotlin - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does in keyword mean in Kotlin generics?

The in keyword marks a generic type as contravariant. It means the type can only be used as input (consumer) and not as output (producer).

beginner

Explain contravariance with a real-life example.

Imagine a mail sorter who can accept any type of mail but never sends mail. Contravariance means you can use a sorter for a more general mail type when a more specific one is expected.

intermediate

How does Kotlin enforce contravariance in function parameters?

Kotlin allows contravariance on generic types used only as function parameters (inputs) by marking them with <code>in</code>. This prevents returning that type from the function.

intermediate

What happens if you try to use a contravariant type as a return type in Kotlin?

Kotlin will give a compile-time error because contravariant types (marked with in) cannot be used as output types to keep type safety.

beginner

Show a simple Kotlin example of contravariance using in keyword.

interface Consumer<in T> { fun consume(item: T) } // Consumer<Any> can be used where Consumer<String> is expected because of contravariance.

What does the in keyword indicate in Kotlin generics?

A Covariance - type used only as output

B Invariant - type used as input and output

C Contravariance - type used only as input

D No variance - type is unrestricted

The in keyword marks a generic type as contravariant, meaning it can only be used as input.

Which of these is a valid use of a contravariant type parameter in T?

A As a function parameter type

B As a function return type

C As a property type that returns T

D As a type for a variable that produces T

Contravariant types can only be used as input parameters, not as return types or producers.

If Consumer<in T> is contravariant, which assignment is allowed?

A Consumer&lt;Any&gt; = Consumer&lt;String&gt;

B Consumer&lt;String&gt; = Consumer&lt;Any&gt;

C Consumer&lt;String&gt; = Consumer&lt;String&gt;

D Consumer&lt;Any&gt; = Consumer&lt;Any&gt;

Contravariance allows assigning a consumer of a more general type (Any) to a consumer of a more specific type (String).

Why does Kotlin restrict using contravariant types as return types?

A To allow more flexible code

B Because return types must always be invariant

C Because contravariant types are deprecated

D To prevent type safety issues

Using contravariant types as return types can break type safety, so Kotlin forbids it.

Which keyword would you use to declare a covariant generic type in Kotlin?

A out

B in

C var

D val

The out keyword marks a type as covariant, meaning it can only be used as output.

Describe what contravariance means in Kotlin generics and give a simple example.

Think about how a type can be used only as input.

Explain why Kotlin restricts contravariant types from being used as return types.

Consider what could go wrong if a contravariant type was returned.