Explain Kotlin's declaration-site variance with in and out
This tests your grasp of generic type safety and API design. Explain that out marks covariant producers (e.g., List<out E>) and in marks contravariant consumers, then contrast this cleaner declaration-site model with Java's repetitive use-site wildcards.
What's really being asked
This question tests your deep understanding of generic type systems, specifically the trade-offs between Kotlin's declaration-site variance and Java's use-site variance (wildcards). The interviewer wants to see if you can move beyond simple definitions and explain why this feature exists: to create safer, more flexible, and cleaner generic APIs by embedding variance rules directly into the type's definition.
The full answer
A strong answer connects the keywords to their roles and contrasts them with Java. First, define covariance: out makes a type a producer, allowing subtypes to be used (a Source<String> is a Source<Any>). Second, define contravariance: in makes a type a consumer, allowing supertypes to be used (a Comparator<Any> is a Comparator<String>). Third, explain this is "declaration-site" variance because it's part of the class/interface declaration, like List<out E>. Fourth, contrast this with Java's "use-site" variance, where callers must repeatedly use wildcards like List<? extends Object>, making Kotlin APIs simpler for consumers.
The mistakes people make
The most common red flag is confusing in and out. A candidate who says out is for consumers immediately signals a shallow understanding. Another mistake is conflating variance with immutability; a covariant collection (List<out T>) is not necessarily immutable, you can still call methods like clear() that don't involve the generic type T. Finally, senior candidates are expected to provide the "why"—failing to contrast declaration-site with use-site variance and explain the benefit to the API consumer is a missed opportunity to show depth.
What usually comes next
Be ready for: "When is a generic type invariant, and why is that the default?" (When it's both a producer and consumer of T, like MutableList<T>). "What is type projection and when is it needed?" (It's Kotlin's equivalent of a use-site wildcard, used to handle types that aren't declared as covariant/contravariant, e.g., passing a MutableList<String> to a function expecting MutableList<out Any>).
A concrete example
Kotlin's List<out E> interface is covariant. Because it's a producer (you only get elements out of it via get()), the compiler allows you to assign a List<Int> to a variable of type List<Number>. Conversely, the Comparator<in T> interface is contravariant. It's a consumer (it only consumes T for comparison), so you can assign a Comparator<Number> to a variable of type Comparator<Int>, because a comparator that can handle any number can certainly handle an integer.
Interview question
To maximize type flexibility for a generic interface DataSink<T> that exclusively consumes elements of type T, which variance modifier should be used?
- a.T (no modifier)
- b.in TCorrect
- c.reified T
- d.out T
Why? this is the answer
The 'in' modifier is used for contravariant types that act as consumers, allowing a supertype (e.g., DataSink<Any>) to be used where a subtype (e.g., DataSink<String>) is expected, thus maximizing flexibility. The 'out' modifier is for producers, which would be incorrect for a consumer interface.
Just read this? Test yourself on what you have been reading.
Read the original → kotlinlang.org
- #kotlin
- #generics
- #api design
- #type system
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