Top Kotlin Interview Questions and Answers

Here is a comprehensive list of 30 Kotlin interview questions with answers for experienced developers (5-8 years), covering Core Kotlin, Coroutines, Android Development, Functional Programming, and Advanced Topics.

1. Kotlin Basics

1. What are the key differences between val and var in Kotlin?

• val (immutable): Read-only, cannot be reassigned.
• var (mutable): Can be reassigned.

val name = "Kotlin"  // Immutable
var age = 30         // Mutable

2. How does Kotlin handle null safety?

• ? denotes nullable types: val name: String? = null
• Safe calls (?.), Elvis operator (?:), and !! (force unwrap).

val length = name?.length ?: 0  // Avoids NullPointerException

3. What are the differences between == and === in Kotlin?

• == checks structural equality (content comparison).
• === checks referential equality (same object).

2. Object-Oriented Concepts

4. What is the difference between open and final in Kotlin?

• open: Allows inheritance.
• final: Prevents further extension.

5. How do you create a Singleton in Kotlin?

• Use the object keyword.

object Singleton {
    fun show() = println("Singleton instance")
}

6. What is the difference between object and companion object?

• object: Standalone singleton.
• companion object: Singleton inside a class (like static in Java).

class Example {
    companion object {
        fun show() = println("Companion Object")
    }
}

7. What is a data class in Kotlin?

• Automatically generates equals(), hashCode(), copy(), toString().

data class User(val name: String, val age: Int)

3. Functional Programming

8. What are extension functions in Kotlin?

• Allows adding methods to existing classes.

fun String.removeSpaces() = this.replace(" ", "")

9. What is the difference between let, apply, run, with, and also?

10. What are higher-order functions in Kotlin?

• Functions that take other functions as parameters.

fun operation(x: Int, y: Int, op: (Int, Int) -> Int) = op(x, y)

4. Coroutines & Concurrency

11. What are coroutines in Kotlin?

• Lightweight threads for async programming using suspend.

12. Difference between launch and async?

• launch: Does not return a value.
• async: Returns a value (Deferred<T>).

13. What is suspend in Kotlin?

• Marks a function as suspendable, allowing non-blocking execution.

suspend fun fetchData() { /*...*/ }

14. What are Dispatchers.IO, Dispatchers.Main, Dispatchers.Default?

• IO: Background tasks.
• Main: UI thread.
• Default: CPU-intensive work.

5. Collections & Generics

15. Difference between List, MutableList, Set, and Map in Kotlin?

• List: Immutable.
• MutableList: Can be modified.
• Set: Unique elements.
• Map: Key-value pairs.

16. What is reified in Kotlin?

• Allows access to generic type information at runtime.

inline fun <reified T> getType() = T::class.java

17. Explain covariance and contravariance in Kotlin?

• out: Covariant (only return values).
• in: Contravariant (only accept values).

6. Android Development with Kotlin

18. How is LiveData different from StateFlow?

19. How do you use Kotlin Coroutines with ViewModel?

class MyViewModel : ViewModel() {
    private val viewModelScope = CoroutineScope(Dispatchers.Main)
}

20. How does Kotlin improve Android development?

• Null safety, concise syntax, coroutine support, extension functions.

7. Advanced Topics

21. What are sealed classes in Kotlin?

• Used for restricted class hierarchies.

sealed class Result {
    class Success(val data: String) : Result()
    class Error(val error: String) : Result()
}

22. What is Kotlin’s delegation pattern?

interface Printer {
    fun printMessage()
}

class PrinterImpl : Printer {
    override fun printMessage() = println("Printing...")
}

class PrinterDelegate(printer: Printer) : Printer by printer

23. How do inline functions improve performance?

• Avoids creating function objects and lambda expressions.

8. Serialization & Networking

24. How do you use Kotlin with Retrofit?

interface ApiService {
    @GET("users")
    suspend fun getUsers(): List<User>
}

25. What are Kotlin Serialization and Gson?

• Kotlin Serialization: Built-in JSON handling.
• Gson: Java-based JSON library.

9. Testing & Performance Optimization

26. How do you write unit tests in Kotlin?

• Use JUnit and MockK for testing.

27. How does lazy initialization work in Kotlin?

• Delays initialization until the first access.

val data: String by lazy { "Initialized on first use" }

10. Miscellaneous

28. What is typealias in Kotlin?

• Creates an alias for a type.

typealias UserMap = Map<String, List<String>>

29. How do you optimize Kotlin code for performance?

• Use inline, avoid reflection, use sealed classes, prefer lazy.

30. How does Kotlin handle immutability?

• Use val, data class, copy(), and immutable collections.