Interview questions: Kotlin

How to answer

Use a simple structure: one-line definition, why it exists, a two-line example, and one common pitfall. Speak slowly and stop when you have answered; long answers often wander into mistakes.

Q1. val versus var?

Model answer: "val is a read-only reference that cannot be reassigned; var can be reassigned. I use val by default because values that do not change make code easier to reason about. Note that val does not make the object immutable: a val list = mutableListOf() can still be changed."

Q2. How does Kotlin handle null?

Model answer: "Types are non-null by default; String? is nullable. The compiler forces safe handling: ?. for safe calls, ?: (Elvis) for defaults, smart casts after checks, and let for running code only when not null. !! forces a value and throws if null, so I avoid it."

Q3. What does a data class give you?

Model answer: "equals and hashCode based on the primary-constructor properties, toString, copy for creating changed copies, and componentN functions for destructuring. It is ideal for models and immutable UI state."

Q4. Sealed class versus enum?

Model answer: "An enum has a fixed set of single instances with the same properties. A sealed class or interface has a fixed set of subtypes, each of which can hold different data and have many instances. I use sealed types for UI states like Loading, Success(data), Error(message), and the compiler checks when expressions are exhaustive."

Q5. Extension functions and scope functions?

Model answer: "Extensions add functions to existing types without inheritance, resolved statically. Scope functions run a block on an object: let and also refer to it as it, run, with and apply as this; apply and also return the object, let, run and with return the block result. I use apply for configuration and ?.let for null checks."

Q6. Coroutines versus threads?

Model answer: "Threads are operating-system resources and heavy; coroutines are lightweight tasks that suspend instead of blocking, so thousands can run on a few threads. Suspend functions pause without blocking the thread, and structured concurrency ties coroutines to scopes so they are cancelled with their owner."

Q7. launch versus async?

Model answer: "launch starts a coroutine for work without a result and returns a Job. async returns a Deferred, and await gives the result. I use async to run independent calls concurrently, starting all before awaiting. Exceptions in async surface at await."

Q8. Flow versus StateFlow versus SharedFlow?

Model answer: "Flow is cold: it runs for each collector when collected. StateFlow is hot and always holds a current value, which suits UI state. SharedFlow is hot without a required current value and suits events. A ViewModel exposes StateFlow and keeps MutableStateFlow private."

Q9. A quick output question

Expect small snippets such as: val list = listOf(1, 2, 3, 4); println(list.filter { it % 2 == 0 }.map { it * it }). The answer is [4, 16]. Read every step aloud and work it through rather than guessing.

Common interview mistakes

• Saying "val means immutable". It means the reference cannot be reassigned.
• Saying coroutines "are threads". They run on threads but are not threads.
• Forgetting that ?. returns a nullable result.
• Memorised answers with no example. Always give a tiny example from your own project.