Kotlin Interview Question & Answer

Kotlin Interview Questions and Answers

Top 100 Kotlin Interview Questions for Freshers

Here’s the revised version tailored for Machine Learning:

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To help you succeed, we have compiled a list of the Top 100 Machine Learning Interview Questions along with their answers. Mastering these will give you a strong edge in cracking Machine Learning interviews at IDM TechPark.

What is Kotlin?
Answer: Kotlin is a statically typed, modern programming language that runs on the JVM and is officially supported for Android development by Google. It is concise, safe, and interoperable with Java.
What are the advantages of Kotlin over Java?
Answer:
- Concise syntax
- Null safety
- Extension functions
- Coroutines for asynchronous programming
- Interoperability with Java
What is the difference between val and var in Kotlin?
Answer:
- val (immutable): Read-only variable (like final in Java).
- var (mutable): Can be reassigned.
What is a data class in Kotlin?
Answer: A data class is a class used to hold data. It automatically provides equals(), hashCode(), copy(), and toString().

data class User(val name: String, val age: Int)
What is the difference between == and === in Kotlin?
Answer:
- == checks structural equality (equivalent to .equals()).
- === checks referential equality (whether they point to the same object).
What is an extension function in Kotlin?
Answer: A function that extends a class without modifying its source code.

fun String.hello() = "Hello, $this"
What is the primary constructor in Kotlin?
Answer: A constructor declared in the class header.

class Person(val name: String, val age: Int)
What is the difference between an open class and a final class?
Answer:
- open: Can be inherited.
- final (default in Kotlin): Cannot be inherited.
What is an object declaration in Kotlin?
Answer: Used to create a singleton.

object Singleton { fun show() = "I am a singleton" }
What are sealed classes in Kotlin?
Answer: A restricted class hierarchy used for representing restricted types.

sealed class Result { class Success(val data: String) : Result() class Error(val message: String) : Result() }
How does Kotlin handle null safety?
Answer: By using nullable (?) and non-nullable (!) types.

var name: String? = null // Nullable
What is the Elvis operator (?:) in Kotlin?
Answer: Provides a default value when encountering null.

val length = name?.length ?: 0
What are Kotlin collections, and how do they differ from Java?
Answer:
- Kotlin has mutable (MutableList, MutableSet, MutableMap) and immutable collections (List, Set, Map).
How do you filter a list in Kotlin?
Answer:

val numbers = listOf(1, 2, 3, 4, 5) val even = numbers.filter { it % 2 == 0 }
How do you convert a list to a mutable list in Kotlin?
Answer:

val mutableList = listOf(1, 2, 3).toMutableList()
What are coroutines in Kotlin?
Answer: Lightweight threads that help in asynchronous programming.
What is suspend in Kotlin?
Answer: Used to define a coroutine function that can be paused and resumed.

suspend fun fetchData() { /* ... */ }
What is a higher-order function?
Answer: A function that takes another function as a parameter or returns a function.

fun operation(a: Int, b: Int, action: (Int, Int) -> Int): Int { return action(a, b) }
What are inline functions in Kotlin?
Answer: Functions that are expanded at the call site, reducing overhead.

inline fun greet(name: String) = "Hello, $name"
What is the difference between launch and async in coroutines?
Answer:
- launch: Starts a coroutine that does not return a result.
- async: Starts a coroutine that returns a result (Deferred).
How is Kotlin interoperable with Java?
Answer:
- Call Java from Kotlin and vice versa.
- Use @JvmStatic, @JvmOverloads, and @JvmField for smooth interop.
How do you call a Kotlin function from Java?
Answer: Use static methods via companion objects or top-level functions.
What are Android KTX extensions?
Answer: Kotlin extensions that simplify Android development.
What is lateinit in Kotlin?
Answer: Allows a non-null variable to be initialized later.

lateinit var name: String
What is by lazy in Kotlin?
Answer: Delayed initialization until first use.

val data: String by lazy { "Hello, World!" }

1. How do you implement a Singleton in Kotlin?

Answer: Use an object declaration.

object Singleton { fun show() = "I am a Singleton" } println(Singleton.show())

2. How to create a Kotlin DSL?

Answer: Use extension functions and lambdas.

class Config { var url: String = "" } fun setup(block: Config.() -> Unit): Config = Config().apply(block) val config = setup { url = "https://example.com" } println(config.url)

3. How to implement the Strategy Pattern using Kotlin functions?

Answer: Use function types.

typealias Strategy = (Int, Int) -> Int val add: Strategy = { a, b -> a + b } val multiply: Strategy = { a, b -> a * b } fun executeStrategy(a: Int, b: Int, strategy: Strategy) = strategy(a, b) println(executeStrategy(5, 3, multiply))

4. What is the difference between flow and channel in Kotlin?

Answer:

Flow emits values reactively (cold stream).
Channel is hot and can send values to multiple consumers.

Example:

import kotlinx.coroutines.flow.* val flowData = flow { emit(1); emit(2) } flowData.collect { println(it) }

5. How to implement a Producer-Consumer pattern with coroutines?

Answer:

import kotlinx.coroutines.* import kotlinx.coroutines.channels.* fun main() = runBlocking { val channel = Channel<Int>() launch { for (x in 1..5) channel.send(x) channel.close() } for (msg in channel) println(msg) }

6. How to implement a thread-safe lazy singleton?

Answer: Use lazy {}.

object SafeSingleton { val instance by lazy { SafeSingleton } }

7. How to create a Kotlin metaprogramming function?

Answer: Use reflection.

fun <T> printProperties(obj: T) { obj!!::class.members.forEach { println(it.name) } } printProperties(User("John", 25))

8. How to implement a custom delegate in Kotlin?

Answer:

import kotlin.reflect.KProperty class CustomDelegate { private var value: String = "Default" operator fun getValue(thisRef: Any?, property: KProperty<*>) = value operator fun setValue(thisRef: Any?, property: KProperty<*>, newValue: String) { value = newValue } } class Example { var data by CustomDelegate() } val obj = Example() obj.data = "Hello" println(obj.data)

9. How to perform function composition in Kotlin?

Answer:

fun double(x: Int) = x * 2 fun square(x: Int) = x * x val composed = { x: Int -> double(square(x)) } println(composed(3))

10. How to make a Kotlin function memoized?

Answer:

fun <T, R> memoize(fn: (T) -> R): (T) -> R { val cache = mutableMapOf<T, R>() return { input -> cache.getOrPut(input) { fn(input) } } } val fib = memoize { n: Int -> if (n <= 1) n else fib(n - 1) + fib(n - 2) } println(fib(10))

11. How to handle multiple coroutines with select?

Answer:

import kotlinx.coroutines.* import kotlinx.coroutines.selects.* suspend fun selectExample() { val ch1 = Channel<String>() val ch2 = Channel<String>() launch { delay(100); ch1.send("Message 1") } launch { delay(200); ch2.send("Message 2") } val result = select<String> { ch1.onReceive { it } ch2.onReceive { it } } println(result) }

12. How to use the invoke operator in Kotlin?

Answer:

class Invokable { operator fun invoke() = "Invoked!" } val obj = Invokable() println(obj()) // Calls invoke()

13. How to create an infix function in Kotlin?

Answer:

infix fun Int.powerOf(exponent: Int) = this.toDouble().pow(exponent).toInt() println(2 powerOf 3)

14. How to implement reified type parameters?

Answer:

inline fun <reified T> genericFunc() = T::class println(genericFunc<String>())

15. How to use yield in coroutines?

Answer:

import kotlinx.coroutines.* import kotlinx.coroutines.yield fun main() = runBlocking { launch { repeat(5) { println("Task 1"); yield() } } launch { repeat(5) { println("Task 2"); yield() } } }

16. How to use asSequence() for lazy evaluation?

Answer:

val result = listOf(1, 2, 3, 4).asSequence().map { it * 2 }.filter { it > 5 }.toList() println(result) // [6, 8]

17. How to perform operator overloading?

Answer:

data class Vector(val x: Int, val y: Int) { operator fun plus(other: Vector) = Vector(x + other.x, y + other.y) } val v1 = Vector(1, 2) val v2 = Vector(3, 4) println(v1 + v2) // Vector(x=4, y=6)

18. How to work with function references?

Answer:

fun add(a: Int, b: Int) = a + b val ref: (Int, Int) -> Int = ::add println(ref(3, 5))

19. How to implement a custom coroutine scope?

Answer:

import kotlinx.coroutines.* val myScope = CoroutineScope(Dispatchers.IO) myScope.launch { println("Running in custom scope") }

20. How to implement a cache using delegates?

Answer:

class Cache { val data by lazy { loadData() } fun loadData() = "Fetched Data" } println(Cache().data)

21. How to implement a retry mechanism in Kotlin?

Answer:

suspend fun retry(times: Int, block: suspend () -> Unit) { repeat(times) { try { block(); return } catch (e: Exception) { delay(1000) } } }

22. How to implement a circuit breaker in Kotlin?

Answer:

class CircuitBreaker(var threshold: Int = 3) { private var failures = 0 fun execute(action: () -> Unit) { if (failures >= threshold) println("Circuit open") else try { action() } catch (e: Exception) { failures++ } } }

23. How to use reflection to instantiate a class dynamically?

Answer:

val clazz = Class.forName("com.example.MyClass").kotlin val instance = clazz.createInstance()

24. How to create a rate limiter using coroutines?

Answer:

val rateLimiter = Semaphore(2) suspend fun limitedAction() = rateLimiter.withPermit { println("Executing") }

25. How to handle coroutine cancellation?

Answer:

val job = launch { delay(5000) } job.cancel()

These questions cover advanced Kotlin topics with practical coding examples. Let me know if you need more! 🚀

1. How to implement a Kotlin Coroutine Dispatcher for Custom Thread Pools?

Answer:

import kotlinx.coroutines.* import java.util.concurrent.Executors val myDispatcher = Executors.newFixedThreadPool(2).asCoroutineDispatcher() fun main() = runBlocking { launch(myDispatcher) { println("Running on custom thread: ${Thread.currentThread().name}") } }

2. How to Implement a Custom operator invoke() Function?

Answer:

class Logger { operator fun invoke(message: String) = println("Log: $message") } val log = Logger() log("This is a log message")

3. How to Implement a Kotlin Custom Annotation and Process it?

Answer:

@Target(AnnotationTarget.CLASS) annotation class CustomAnnotation(val message: String) @CustomAnnotation("Test Class") class MyClass fun main() { val annotation = MyClass::class.annotations.find { it is CustomAnnotation } as? CustomAnnotation println(annotation?.message) // Output: Test Class }

4. How to Implement a Custom Exception in Kotlin?

Answer:

class CustomException(message: String) : Exception(message) fun validate(value: Int) { if (value < 0) throw CustomException("Value cannot be negative") } try { validate(-1) } catch (e: CustomException) { println(e.message) }

5. How to Implement an Immutable Data Structure in Kotlin?

Answer:

data class ImmutableList<T>(private val list: List<T>) { fun get(index: Int) = list[index] } val immutableList = ImmutableList(listOf(1, 2, 3)) println(immutableList.get(1)) // Output: 2

6. How to Perform Reflection on Kotlin Functions?

Answer:

fun sampleFunction() = println("Hello, Reflection!") fun main() { val function = ::sampleFunction function.call() }

7. How to Implement a Custom DSL in Kotlin?

Answer:

class Config { var url: String = "" } fun setup(block: Config.() -> Unit): Config = Config().apply(block) val config = setup { url = "https://example.com" } println(config.url)

8. How to Implement a Custom Sequence Generator?

Answer:

val customSequence = sequence { yield(1) yield(2) yieldAll(listOf(3, 4, 5)) } println(customSequence.toList())

9. How to Implement a Memoization Function in Kotlin?

Answer:

fun <T, R> memoize(fn: (T) -> R): (T) -> R { val cache = mutableMapOf<T, R>() return { input -> cache.getOrPut(input) { fn(input) } } } val factorial = memoize { n: Int -> if (n <= 1) 1 else n * factorial(n - 1) } println(factorial(5))

10. How to Implement a Custom Lazy Delegate in Kotlin?

Answer:

class CustomLazy<T>(private val initializer: () -> T) { private var value: T? = null operator fun getValue(thisRef: Any?, property: kotlin.reflect.KProperty<*>): T { if (value == null) value = initializer() return value!! } } class Example { val data by CustomLazy { "Lazy Loaded" } } println(Example().data)

11. How to Implement an Event Bus in Kotlin?

Answer:

object EventBus { private val listeners = mutableListOf<(String) -> Unit>() fun subscribe(listener: (String) -> Unit) { listeners.add(listener) } fun publish(event: String) { listeners.forEach { it(event) } } } fun main() { EventBus.subscribe { println("Received event: $it") } EventBus.publish("Test Event") }

12. How to Implement Function Composition?

Answer:

fun double(x: Int) = x * 2 fun square(x: Int) = x * x val composed = { x: Int -> double(square(x)) } println(composed(3))

13. How to Implement Retry Mechanism in Coroutines?

Answer:

suspend fun retry(times: Int, block: suspend () -> Unit) { repeat(times) { try { block(); return } catch (e: Exception) { delay(1000) } } }

14. How to Use reified for Type-Safe Reflection?

Answer:

inline fun <reified T> typeInfo() = T::class println(typeInfo<String>())

15. How to Implement Circuit Breaker in Kotlin?

Answer:

16. How to Implement a Parallel Map Function?

Answer:

suspend fun <T, R> List<T>.parallelMap(transform: suspend (T) -> R): List<R> = coroutineScope { map { async { transform(it) } }.awaitAll() }

17. How to Implement a Custom Rate Limiter?

Answer:

val rateLimiter = Semaphore(2) suspend fun limitedAction() = rateLimiter.withPermit { println("Executing") }

18. How to Implement a Custom Operator Overloading?

Answer:

data class Vector(val x: Int, val y: Int) { operator fun plus(other: Vector) = Vector(x + other.x, y + other.y) } val v1 = Vector(1, 2) val v2 = Vector(3, 4) println(v1 + v2) // Vector(x=4, y=6)

19. How to Implement an Observer Pattern in Kotlin?

Answer:

class Observable { private val observers = mutableListOf<() -> Unit>() fun subscribe(observer: () -> Unit) { observers.add(observer) } fun notifyChange() { observers.forEach { it() } } }

20. How to Implement an Efficient Job Queue?

Answer:

val jobQueue = ArrayDeque<suspend () -> Unit>() suspend fun addJob(job: suspend () -> Unit) { jobQueue.add(job) } suspend fun processJobs() { while (jobQueue.isNotEmpty()) jobQueue.removeFirst().invoke() }

21. How to Implement a Finite State Machine in Kotlin?

Answer:

enum class State { IDLE, RUNNING, STOPPED } class StateMachine { var state = State.IDLE }

22. How to Implement Tail Recursion?

Answer:

tailrec fun factorial(n: Int, acc: Int = 1): Int { return if (n == 1) acc else factorial(n - 1, acc * n) }

23. How to Implement a Custom Coroutine Scope?

Answer:

val myScope = CoroutineScope(Dispatchers.IO) myScope.launch { println("Running in custom scope") }

24. How to Implement a Custom Thread-Safe Singleton in Kotlin?

Answer:

class ThreadSafeSingleton private constructor() { companion object { @Volatile private var instance: ThreadSafeSingleton? = null fun getInstance(): ThreadSafeSingleton { return instance ?: synchronized(this) { instance ?: ThreadSafeSingleton().also { instance = it } } } } } val singleton = ThreadSafeSingleton.getInstance()

25. How to Implement a Lightweight Dependency Injection Framework in Kotlin?

Answer:

class Injector { private val instances = mutableMapOf<Class<*>, Any>() fun <T : Any> provide(clazz: Class<T>, instance: T) { instances[clazz] = instance } fun <T : Any> get(clazz: Class<T>): T { return instances[clazz] as? T ?: throw IllegalStateException("No instance provided for ${clazz.simpleName}") } } // Usage val injector = Injector() injector.provide(String::class.java, "Hello, DI!") val message: String = injector.get(String::class.java) println(message) // Output: Hello, DI!

1. How does Kotlin handle null safety?

Answer:
Kotlin uses nullable (?) and non-nullable types to prevent NullPointerException.

var name: String? = null // Nullable var age: Int = 25 // Non-nullable

Using ?., !!, or ?: ensures safe access.

println(name?.length ?: "Default value") // Safe call with Elvis operator

2. How to create a Singleton in Kotlin?

Answer:

object Singleton { fun show() = println("I am a singleton") } Singleton.show()

This is thread-safe and lazy-initialized.

3. How does lateinit differ from lazy?

Answer:

lateinit is for mutable properties, initialized later.
lazy is for read-only properties, initialized on first use.

lateinit var name: String val value: String by lazy { "Initialized on first use" }

4. How to use reified in inline functions?

Answer:
reified allows using generic types at runtime.

inline fun <reified T> getType() = T::class println(getType<String>()) // Output: class kotlin.String

5. How to handle exceptions in Kotlin?

Answer:

try { val result = 10 / 0 } catch (e: ArithmeticException) { println("Exception: ${e.message}") } finally { println("Execution finished") }

6. How to create a Kotlin extension function?

Answer:

fun String.firstChar(): Char = this[0] println("Hello".firstChar()) // Output: H

7. How to implement a custom delegate in Kotlin?

Answer:

8. How to use higher-order functions in Kotlin?

Answer:

fun operate(a: Int, b: Int, operation: (Int, Int) -> Int): Int { return operation(a, b) } val sum = operate(5, 3) { x, y -> x + y } println(sum) // Output: 8

9. What is with vs apply in Kotlin?

Answer:

with: Operates on an object, returns the last expression.
apply: Configures an object, returns the same object.

data class Person(var name: String, var age: Int) val person = Person("John", 30) with(person) { println("$name is $age years old") } val newPerson = person.apply { age = 31 }

10. How to use coroutines for background tasks?

Answer:

import kotlinx.coroutines.* fun main() = runBlocking { launch(Dispatchers.IO) { println("Running in background thread") } }

11. How to create a sealed class in Kotlin?

Answer:

sealed class Response class Success(val data: String) : Response() class Error(val message: String) : Response() fun handleResponse(response: Response) { when (response) { is Success -> println("Data: ${response.data}") is Error -> println("Error: ${response.message}") } }

12. How to define a custom infix function?

Answer:

infix fun Int.add(other: Int) = this + other println(5 add 3) // Output: 8

13. How to implement operator overloading in Kotlin?

Answer:

data class Point(val x: Int, val y: Int) { operator fun plus(other: Point) = Point(x + other.x, y + other.y) } val p1 = Point(2, 3) val p2 = Point(1, 1) println(p1 + p2) // Output: Point(x=3, y=4)

14. How to implement memoization in Kotlin?

Answer:

fun <T, R> memoize(fn: (T) -> R): (T) -> R { val cache = mutableMapOf<T, R>() return { input -> cache.getOrPut(input) { fn(input) } } }

15. How to use data class destructuring?

Answer:

data class User(val name: String, val age: Int) val (name, age) = User("Alice", 25) println("$name is $age years old")

16. How to use Flow in Kotlin?

Answer:

import kotlinx.coroutines.flow.* fun fetchData(): Flow<Int> = flow { for (i in 1..5) { emit(i) delay(100) } } fun main() = runBlocking { fetchData().collect { println(it) } }

17. How to implement custom annotations?

Answer:

@Target(AnnotationTarget.CLASS) annotation class CustomAnnotation(val message: String) @CustomAnnotation("Test Class") class MyClass

18. What is inline class in Kotlin?

Answer:

@JvmInline value class UserId(val id: Int) val userId = UserId(123) println(userId.id) // Output: 123

19. How to create an immutable list in Kotlin?

Answer:

val numbers = listOf(1, 2, 3) // Read-only list

20. How to handle function composition in Kotlin?

Answer:

fun double(x: Int) = x * 2 fun square(x: Int) = x * x val composed = { x: Int -> double(square(x)) } println(composed(3)) // Output: 18

21. How to handle concurrency using async and awaitAll?

Answer:

suspend fun parallelTasks() = coroutineScope { val results = listOf( async { "Task 1" }, async { "Task 2" } ).awaitAll() println(results) }

22. How to use Channel in Kotlin?

Answer:

import kotlinx.coroutines.channels.Channel val channel = Channel<Int>() launch { channel.send(10) } launch { println(channel.receive()) }

23. How to use Mutex for synchronization?

Answer:

import kotlinx.coroutines.sync.Mutex val mutex = Mutex() suspend fun criticalSection() { mutex.lock() try { println("Critical Section") } finally { mutex.unlock() } }

24. How to implement a MapReduce in Kotlin?

Answer:

val numbers = listOf(1, 2, 3, 4) val sum = numbers.map { it * 2 }.reduce { acc, i -> acc + i } println(sum)

25. How to implement a rate limiter using Semaphore?

Answer:

import kotlinx.coroutines.sync.Semaphore val rateLimiter = Semaphore(2) suspend fun limitedAction() = rateLimiter.withPermit { println("Executing") }

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Kotlin Interview Questions and Answers

Top 100 Kotlin Interview Questions for Freshers

"Deep Concepts to Elevate Your Career"

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Kotlin Interview Questions and Answers

Top 100 Kotlin Interview Questions for Freshers

"Deep Concepts to Elevate Your Career"

Contact Us

IDM Techpark Erode 1st floor, 33/15 vasavi complex, nalli hospital road, annamalai layout, near bus stand, erode 638011

+91-95853-05700

idmtechpark@gmail.com

Software Courses

Full Stack Developer

IDM Techpark Erode
1st floor, 33/15 vasavi complex, nalli
hospital road, annamalai layout, near bus
stand, erode 638011