Top 20+ Kotlin Interview Questions and Answers

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Kotlin is a modern programming language created by JetBrains – the same company behind IntelliJ IDEA. It was first released in 2011 and named after Kotlin Island in Russia. Kotlin became popular for building Android apps because it is cleaner and safer than Java. In 2017, Google announced official support for Kotlin in Android development, which made it even more popular. If you are preparing for a job in Android or backend development, knowing Kotlin can give you an advantage. Here are 20+ common Kotlin interview questions to help you get ready.

Fun Fact – Kotlin continues to power over 95% of the top 1,000 Android apps.

Basic Kotlin Interview Questions (For Freshers)

Here are some commonly asked Kotlin interview questions and answers to help you build a strong foundation.

1. What is Kotlin? How does it run on the JVM?

Kotlin is a statically typed programming language developed by JetBrains. It runs on the Java Virtual Machine (JVM), which means Kotlin code is compiled into Java bytecode. This allows Kotlin to work in any environment where Java is used, including Android development.

2. Compare val, var, const, and lazy declarations.

Kotlin lets you declare variables in different ways depending on mutability, scope, and initialization timing.

Here’s a simple comparison –

Keyword	Mutability	Scope	Initialization Time	Use Case
val	Immutable	Local / class-level	At runtime	For values that don’t change
var	Mutable	Local / class-level	At runtime	For values that need updates
const	Immutable	Top-level / object	At compile time	For fixed compile-time constants
lazy	Immutable (with val)	Class-level only	When first accessed	For expensive or delayed initialization

3. Explain the when expression and where you would use it.

when in Kotlin replaces the traditional switch statement. It allows cleaner branching. You use it to match values or types and return results without multiple if-else blocks.

4. What are Kotlin’s basic data types?

Kotlin provides a rich set of basic data types. All of them are objects but are compiled to efficient JVM primitives when possible.

Numeric Types – Byte (8-bit), Short (16-bit), Int (32-bit), Long (64-bit) / Float (32-bit), Double (64-bit)

Boolean – Represents true or false

Char – Represents a single character, e.g., ‘A’

String – Immutable sequence of characters enclosed in double quotes

Arrays – Use arrayOf() or specific types like intArrayOf()

In Kotlin, these are all objects, but the compiler optimizes them to Java primitives when possible.

5. Describe null safety in Kotlin and how it prevents NullPointerException.

In Kotlin, variables are non-nullable by default. To allow null, you use ?. Safe calls (?.), Elvis operators (?:), and not-null assertions (!!) help manage nullable values. This design reduces runtime crashes.

6. How do extension functions work in Kotlin?

Extension functions let you add new functions to existing classes without modifying them. 

For example, fun String.removeSpaces(): String adds a custom method to the String class. It improves readability and keeps code organized.

Advanced Kotlin Android Interview Questions (For Experienced Professionals)

Here are some advanced-level Android Kotlin interview questions that test your deep knowledge of Android development using Kotlin.

7. Explain @JvmStatic, @JvmField, and @JvmOverloads annotations.

• @JvmStatic makes a method callable as a static method in Java.
• @JvmField exposes a property directly as a field to Java, skipping the getter.
• @JvmOverloads generates overloads for functions with default parameters, useful when calling from Java where default arguments don’t apply.

8. How can you convert Kotlin source to Java via Android Studio tools?

Go to Tools > Kotlin > Show Kotlin Bytecode in Android Studio. 

Then click the Decompile button. 

It will show the Java equivalent of your Kotlin code. It helps when you want to understand how Kotlin compiles behind the scenes.

9. What are the differences in behavior between == and ===?

• == checks for structural equality (like .equals() in Java).
• === checks if two variables point to the exact same memory reference.

For primitives, both usually behave the same. For objects, they differ.

10. How do you implement a singleton in Kotlin?

You use the object keyword.

object NetworkClient {

   fun connect() = println(“Connected”)

}

This approach is thread-safe and creates only one instance. I have used it for API managers in Android apps.

11. What are scope functions (let, run, apply, also, with) and when would you use each?

• let: Use for null checks or value transformations.
• run: Great for combining configuration and execution.
• apply: Best for setting properties on an object.
• also: Use when you want to perform extra operations without changing the object.
• with: Good when you don’t need to return a different result.

12. Compare List and Array types in Kotlin.

In Kotlin, both Array and List are used to store collections, but they differ in flexibility and usage.

Feature	Array	List
Size	Fixed	Can be fixed or dynamic (mutable list)
Mutability	Mutable	Immutable by default, mutable with mutableListOf()
Type Support	One specific type	Can hold mixed types (with List<Any>)
Syntax	arrayOf(1, 2, 3)	listOf(1, 2, 3) / mutableListOf()
Preferred Use	Simple fixed-size collections	Commonly used in Android development

Also Read - Top 50+ Android Interview Questions and Answers

Kotlin Interview Questions for Senior Developer

These Android interview questions for senior developer Kotlin roles focus on architecture, performance, and problem-solving.

13. Explain a suspend function and its role in coroutines.

A suspend function is a Kotlin function that can pause and resume. It doesn’t block the thread. Instead, it suspends execution until the result is ready. You can only call it from another suspend function or coroutine scope. It is used for long-running tasks like API calls.

14. How do launch and async differ when used in coroutines?

In Kotlin coroutines, launch and async both start coroutines but are used for different purposes.

Feature	launch	async
Return Type	Job (no result)	Deferred<T> (returns a result)
Usage Purpose	Fire-and-forget tasks	Tasks where a result is needed
Result Handling	No return value	Use .await() to get the result
Exception Handling	Can use try-catch inside coroutine	Exceptions thrown when .await() is called
Common Use Case	Updating UI, logging, database writes	API requests, background calculations

15. What is a sealed class, and how does it differ from an enum?

A sealed class lets you define a restricted class hierarchy. All subclasses must be in the same file. Unlike enums, sealed classes can hold different data types and custom logic. It is useful in when expressions for type safety without needing an else branch.

16. Describe infix, inline, and reified functions in Kotlin.

• infix allows cleaner syntax for functions with one parameter (a times b instead of a.times(b)).
• inline tells the compiler to replace the function call with the function body. It helps with lambdas.
• reified is used with inline to access generic type info at runtime, which is normally erased.

17. How do delegated properties (Delegates.observable, vetoable) work?

observable lets you react to changes in a property.

vetoable lets you reject updates based on conditions.

Both are useful for tracking state or adding validation logic. I’ve used observable for UI state sync in MVVM.

Kotlin Coroutines Interview Questions

This section covers key Coroutines Kotlin interview questions to test your understanding of asynchronous programming in Kotlin.

18. What is a coroutine and why are coroutines used over threads?

A coroutine is a lightweight task that runs asynchronously. It is cheaper than a thread. Coroutines suspend without blocking the thread, making them ideal for network calls, database access, or delays. They help you write cleaner async code.

19. What are coroutine scopes, and why are they important?

Coroutine scopes manage the lifecycle of coroutines. Each scope ties coroutines to a specific job or component. In Android, I use viewModelScope to cancel coroutines when the ViewModel clears. It prevents memory leaks.

20. How do exceptions propagate in coroutines and how can they be handled?

Exceptions in coroutines follow structured concurrency. If one child fails, its parent and siblings may cancel. Use try-catch inside a coroutine or set a CoroutineExceptionHandler. In supervisorScope, failures don’t cancel other children.

Kotlin Coding Interview Questions

Let’s go through some practical Kotlin interview questions that test your coding skills.

21. Write a data class and show how to use its copy() method.

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

val original = User(“Aman”, 25)

val updated = original.copy(age = 30)

// updated is User(name=”Aman”, age=30)

22. Implement a function that uses when for control flow.

fun grade(score: Int): String {

    return when (score) {

        in 90..100 -> “A”

        in 75..89 -> “B”

        in 60..74 -> “C”

        in 0..59 -> “F”

        else -> “Invalid”

    }

}

23. Create an extension function for a view visibility toggle (e.g., show/hide).

fun View.show() { visibility = View.VISIBLE }

fun View.hide() { visibility = View.GONE }

24. Demonstrate string concatenation using interpolation, +, and StringBuilder.

val name = “Kotlin”

val ver = 1.9

val msg1 = “$name $ver”                   // Interpolation

val msg2 = name + ” version ” + ver       // Using +

val msg3 = StringBuilder().apply {

    append(name)

    append(” version “)

    append(ver)

}.toString()

Tips to Crack Your Kotlin Interview

Kotlin interviews focus on how you solve problems, not just writing correct syntax. Here are some tips to help you succeed –

• Understand how coroutines work. Know when to use launch vs async.
• Practice writing data classes, extension functions, and sealed classes.
• Be clear on how Kotlin is different from Java.
• Know how to avoid null pointer issues using null safety.
• Try small Kotlin coding problems on a whiteboard or notepad.
• Read your past projects in Kotlin. Be ready to talk about what you wrote.

Wrapping Up

And there you have it – the top 20+ Kotlin interview questions and answers to help you prepare. These questions reflect what companies actually ask in technical rounds, so practicing them can give you a real edge.

Looking for jobs that need Kotlin skills? Visit Hirist – a one-stop platform to find the best IT jobs in India, including roles that require Kotlin.

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