How to Pass a Kotlin Interview: Top Questions and Answers

How to Pass a Kotlin Interview: Breakdown of Top Questions with Examples

Kotlin has firmly established itself as a modern language for Android development and JVM backend. If you are preparing for a technical interview, it's important not just to know the syntax, but also to understand the language's philosophy: safety, conciseness, and Java compatibility. In this article, we will break down the key questions most often asked in Kotlin interviews, with real code examples.

1. Null Safety: How Kotlin Solves the null Problem

One of Kotlin's main selling points is its built-in protection against NullPointerException. In an interview, you will definitely be asked about nullable and non-null types.

Question: What is the difference between String? and String? How to work safely with nullable?

Answer: The String type guarantees that the variable will never be null. String? allows null. For safe access, the operators ?., ?: (Elvis), and !! (only if you are 100% sure) are used.

fun main() {    val name: String? = null        // Safe call    println(name?.length) // null        // Default value    val length = name?.length ?: 0    println(length) // 0        // Force unwrap (risk of NPE)    // println(name!!.length) // Error!}

Additional question: What are lateinit and lazy?

• lateinit — for variables that are initialized later (only var).
• lazy — for delegated initialization (only val).

class User {    lateinit var email: String        val config by lazy { loadConfig() }        fun init() {        email = "test@example.com"    }}

2. Coroutines: Asynchrony Without Pain

Coroutines are a must-have for modern Kotlin. Interviewers check understanding of suspend, launch, async, and dispatchers.

Question: What is the difference between launch and async?

Answer: launch starts a coroutine and returns a Job (without a result). async returns a Deferred, from which you can get a result via await().

suspend fun fetchData(): String {    delay(1000)    return "Data loaded"}

fun main() = runBlocking {    // launch: without result    val job = launch {        println("Task 1 completed")    }        // async: with result    val deferred = async {        fetchData()    }        println(deferred.await()) // "Data loaded"    job.join()}

Question: What dispatchers (Dispatchers) do you know?

• Dispatchers.Main — for UI (Android).
• Dispatchers.IO — for input/output, network.
• Dispatchers.Default — for CPU-intensive tasks.
• Dispatchers.Unconfined — not tied to a specific thread (rarely used).

3. Extension Functions and Sealed Classes: Expanding Capabilities

Kotlin allows adding new functions to existing classes without inheritance. This is often asked in the context of architecture.

Question: What are extension functions? Give an example.

Answer: These are functions that are declared outside a class but are called as its methods. They do not modify the class, but merely add syntactic sugar.

// Extension function for Stringfun String.isEmail(): Boolean {    return this.contains("@") && this.contains(".")}

fun main() {    val email = "user@example.com"    println(email.