Jenna Inouye | 15 Dec, 2022

40 Top Kotlin Interview Questions and Answers in 2024

Kotlin is a modern, object-oriented programming language that has become increasingly popular. Suppose you’re looking to land a job as an entry-level or even senior developer using Kotlin. You’ll need to prepare for a wide variety of challenging Kotlin interview questions.

Even if you’re experienced in the programming language, consider this. You may not realize you don't know enough about coroutines until you specifically encounter Kotlin coroutines interview questions.

Let's look at some basic, intermediate, and advanced questions and answers for Kotlin interviews.

Why Study Kotlin Interview Questions and Answers?

What follows are some of the most common questions an interviewer might ask. These questions and answers will give you invaluable pre-interview practice — and highlight areas in which you need to improve.

Kotlin Interview Questions and Answers for Beginners

At the most basic level, you should be familiar with common terminology and concepts such as Kotlin's null safety features, its support for functional programming, and how it compares to other languages such as Java. You should also be able to write simple Kotlin code and understand common libraries and frameworks.

Here are some of the most common Kotlin basic interview questions.

1. What is Kotlin, and what are its key features?

Kotlin is a modern, statically typed programming language that runs on the JVM. Its key features include null safety, support for functional programming, and Java compatibility.

Here’s “Hello, world!” in Kotlin:

fun main(args : Array<String>) {
   println("Hello, World!")

Kotlin offers several advantages over Java, including greater flexibility, readability, and more concise syntax. Additionally, Kotlin supports modern programming paradigms such as functional programming, which Java doesn’t fully support.

2. How does Kotlin compare to other languages such as Java?

Here’s “Hello, world!” in Java:

class HelloWorld {
   public static void main(String[] args) {
       System.out.println("Hello, World!");

Some of the most common libraries and frameworks for Kotlin include Anko, RxJava, and Ktor. Each of these libraries provides important functionality for building complex applications in Kotlin.

3. What are some common libraries and frameworks that you would use with Kotlin?

Ktor homepage screenshot

4. What is null safety in Kotlin, and why is it important?

Null safety is a feature of Kotlin that ensures that variables cannot be assigned a null value. This is important because it helps avoid NullPointerException errors, which can be difficult to debug.

5. What are some of the basic syntax rules for Kotlin?

Some of the basic syntax rules for Kotlin include the use of semicolons to terminate statements, the use of curly braces to delimit blocks of code, and the use of the val keyword to declare immutable variables.

6. What is the difference between a var and a val in Kotlin?

A var is a mutable variable, meaning its value can be changed. A val is an immutable variable, which means that its value can not be changed. Think of them as variables and constants.

7. What is the difference between an interface and an abstract class in Kotlin?

An interface is a type that specifies a set of abstract methods that must be implemented by any class that implements the interface. An abstract class is a type that defines both abstract and concrete methods, which subclasses can inherit.

8. What is the difference between a data class and a regular class in Kotlin?

A data class is a class intended to hold data. A regular class is a class that can perform arbitrary operations. Data classes are typically simpler and more efficient than regular classes.

9. What is a lambda expression in Kotlin?

A lambda expression is an anonymous function that can concisely represent a function with a single parameter. Lambda expressions are often used in conjunction with higher-order functions, such as map and filter.

10. What is the difference between a function and a method in Kotlin?

A function is a named code block invoked from other locations within the source code. A method is a function associated with an object and can be invoked from other code with the dot notation.

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11. What is the difference between a class and an object in Kotlin?

A class is a template for creating objects. An object is an instance of a class. Classes can contain properties and methods, while objects contain only data.

12. What is the difference between a constructor and an initializer in Kotlin?

A constructor is a special method invoked when an object is created. An initializer is a special method you can use to initialize an object before its first use. Both constructors and initializers are typically declared with the unit keyword.

13. How do you declare a default argument in Kotlin?

To declare a default argument in Kotlin, use the default keyword when defining a function parameter. This will specify a default value for that argument, which will be used if no actual value is supplied when calling the function.

14. How do you define an object in Kotlin?

To define an object in Kotlin, simply declare a class and instantiate it with the new keyword. This will create a new class instance, which can perform various actions.

val newObject= object {
   val one = "Hello"
   val two = "World"
   override fun toString() = "$one $two"

The above would print “Hello World.”

15. How do you declare a function in Kotlin?

To declare a function in Kotlin, use the fun keyword followed by the name of the function and the parameters that it accepts. You can then define the function body within curly braces.

16. How do you invoke a function in Kotlin?

To invoke a function in Kotlin, simply use its name followed by the parentheses operator and any necessary arguments. This will cause the function to execute with the specified arguments.

17. What is a higher-order function in Kotlin?

A higher-order function takes one or more functions as arguments or returns a function as its result. Higher-order functions are often used in conjunction with lambda expressions to create concise and powerful code.

Kotlin Interview Questions and Answers for Experienced Users

At the intermediate level, you should be able to discuss more advanced topics such as concurrency, Lambdas, and coroutines. You should also be able to write code that uses Kotlin's more advanced features.

18. What is the difference between a suspend function and a regular function?

A suspend function can be suspended, meaning that you can pause its execution and resume it at a later time. A regular function cannot be suspended and will always execute to completion.

19. What are coroutines in Kotlin?

Coroutines are Kotlin features that allow for concurrent or parallel execution of code. Coroutines can improve application performance by taking advantage of multiple cores in a processor.

20. What is the difference between a thread and a coroutine?

A thread is an execution unit that can run independently from other threads. A coroutine is a unit of execution that can be suspended and resumed, allowing it to share resources with other concurrent or parallel executions.

21. How do you launch a coroutine in Kotlin?

Call the launch function on the CoroutineScope object to launch a coroutine, passing in the function you wish to execute. This will create a new coroutine and launch it immediately.

22. How do you cancel a coroutine in Kotlin?

Call the cancel function on the CoroutineScope object to cancel a coroutine in Kotlin. This will cancel the coroutine and free up any resources.

23. How do you pass data between coroutines in Kotlin?

One of the best ways to pass data between coroutines in Kotlin is with channel objects, which allow for safe and synchronized communication between separate threads or processes. To create a channel object, simply use the Channel constructor and define any channels you want to send data through.

24. How do you create a lambda expression in Kotlin?

First, you must define the parameters the expression accepts using the parentheses operator. You can then provide an executable block of code within curly braces and use the arrow operator to indicate that this code is the body of the lambda expression.

val items = listOf(1, 2, 3, 4, 5)
items.fold(0, {
   acc: Int, i: Int ->
   print("acc = $acc, i = $i, ")
   val result = acc + i
   println("result = $result")

To pass a lambda expression as an argument to a function, include it within parentheses after the function name and any necessary arguments. This will cause the code within that lambda expression to be executed whenever the function is called.25. How do you pass a lambda expression as an argument in Kotlin?

26. What is the difference between a lambda expression and an anonymous function?

A lambda expression is a function that can be passed as an argument to another function. An anonymous function is a function that does not have a name and cannot be passed as an argument to another function. Thus, they’re actually opposites.

27. What are inline functions in Kotlin?

Inline functions are expanded inline at the call site, meaning that the function code is copied and pasted into the body of the code where it is called. This can improve performance by eliminating the need for a function call while also increasing the readability of the code.

28. What is the difference between an extension function and a regular function?

An extension function is a function defined for a specific type and can be called on variables of that type. A regular function is not defined for a specific type and can be called on any type of variable.

Kotlin Interview Questions for Advanced Developers

These Kotlin interview questions for senior developer roles will test your knowledge and experience. At the advanced level, you should be able to discuss topics such as compiler plugins, type inference, and reified types. You should also be able to write code that uses Kotlin's more advanced features in complex and real-world scenarios.

29. What is type inference in Kotlin?

Type inference is the process of automatically determining the type of a variable or expression based on its value. In Kotlin, type inference determines the type of variables when they are first declared and the return type of functions.

30. What are reified types in Kotlin?

Reified types can be accessed at runtime rather than just at compile time. In Kotlin, reified types provide metadata about a type at run time, such as its name or the names of its members. They are also commonly used in reflection and generic programming. It is an advanced feature not commonly used by beginners to the Kotlin language.

31. How do you create a compiler plugin in Kotlin?

To create a compiler plugin in Kotlin, you first need to create an abstract class that extends the CompilerPlugin class. This class defines several functions that should be implemented by your plugin, including load and init. After this, you can use the CompilerInstance class to access the Kotlin compiler and use its API.

32. What is the kotlin c compiler?

Thekotlinccompiler is a command-line tool that can compile Kotlin source code into JVM bytecode or JavaScript. You can also use it to run Kotlin code directly without compiling it first.

33. What are the major limitations of Kotlin?

The major limitations of Kotlin are its lack of support for operator overloading and variable arguments. Additionally, Kotlin does not have any built-in string interpolation or formatting features.

34. When wouldn't you want to use Kotlin?

There are a few scenarios where you might not want to use Kotlin. If you need to write code compatible with Java 6 or earlier, Kotlin will not be an option. Additionally, if you are targeting the Android platform, you will need to use Java 8 or higher to use Kotlin. Finally, if you are working on a project with a large codebase, Kotlin’s slow compile times makes it a poor choice.

Kotlin Technical Interview Questions

Experience-based interview questions are common in Kotlin interviews. However, you might also run into Kotlin coding interview questions or even more specific questions, like Android Kotlin interview questions.

Here are some common coding interviews for Kotlin.

35. How do you sort a list in Kotlin?

There are two main ways to sort a list in Kotlin. The first is via the sort method, which sorts the list in place based on natural ordering. The second is the sortedBy function, which sorts the list in place and returns a new sorted version.

val numbers = listOf("one", "two", "three", "four")
println("Sorted ascending: ${numbers.sorted()}")
println("Sorted descending: ${numbers.sortedDescending()}")

There are a few different ways to find an item or items within a list in Kotlin. You can use the find function or the filter function. You can also index items directly with indexOf or lastIndexOf, but this is less useful. Why? The entire point is discovering the index associated with a given value.

36. How do you find an item or items within a list in Kotlin?

var colors: List<String> = listOf("Red", "Orange","Yellow","Blue","Green","Indigo","Violet")
val favoriteColor = rainbow.find { color -> "Blue".equals(color) }

You can remove items from a list in Kotlin in a few different ways. The simplest is the filter method, which returns all elements that satisfy a given predicate except for those that match the specified element. Additionally, you can use the partition function, which splits the provided list into two new lists: one that matches the element and one that does not. Finally, just use the remove function like so:

37. How do you remove an item or items from a list in Kotlin?


38. How do you generate a sequence of data in Kotlin?

You can generate number sequences or data in a few different ways. The simplest is to use the range function, which returns a sliding window of integers based on a starting and ending value. Alternatively, you can use the generate Sequence function, which allows for a more complex generation via a seed value and function that produces the next element in the sequence.

39. How do you group data by key in Kotlin?

To group data by key in Kotlin, you must first convert the data into a map. From there, you can use the groupBy function to group all of the data in the map by key. Alternatively, you can use the mapAndFlattenToList function to return a flat list of values grouped by key.

40. Can you use IntArray and Array<Int> interchangeably in Kotlin?

In most cases, you can use IntArray and Array<Int> interchangeably. This could lead you to believe that they are interchangeable, which is not the case — and using them as such is bad practice.

IntArray is an int[] type, whereas Array<Int> is an Integer[] type. They will be treated differently during calls such as Integer.valueOf() and you will receive a type mismatch error.

Tips for Your Next Kotlin Interview

While the above Kotlin interview questions can help prepare you for most types of technical interviews, there are a few tips and best practices that will make it easier to succeed. Here are some tips to keep in mind as you prepare:

  • Thoroughly review your experience and relevant projects and be prepared to discuss them in detail.
  • Create a portfolio of the projects you’ve completed in Kotlin thus far and prepare to explain them and your development process.
  • Be familiar with the Kotlin standard library nd prepare to write code snippets on the spot.
  • Have a solid understanding of functional programming concepts and general fundamental programming concepts, such as sorting and finding.
  • Practice your Kotlin coding skills by solving problems or taking a Kotlin boot camp or class.
  • Prepare to discuss your thoughts on the Kotlin language, its features, and its ecosystem.

The more you prepare for your interview, the better. And you're not just preparing for one interview. By reviewing the above questions and answers, you can prepare for all your Kotlin interviews at once—and brush up on essential Kotlin knowledge.

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Frequently Asked Questions

1. What are Kotlin Interview Questions?

Kotlin interview questions are a set of questions commonly asked during technical interviews for positions involving Kotlin programming. These questions typically focus on functional programming, data structures and algorithms, object-oriented design principles, and the Kotlin language itself.

2. Can We Execute Kotlin Code Without JVM?

Yes, Kotlin can be compiled to JavaScript and run in a web browser. Additionally, various tools and frameworks allow the execution of Kotlin code without requiring a JVM, such as the Ktor framework.

3. What Kinds of Programming Types Does Kotlin Support?

Kotlin supports both object-oriented and functional programming paradigms. Additionally, it has support for metaprogramming, which allows for the generation of code at compile time.

4. What is the Use of Kotlin in Android?

Kotlin is a statically typed programming language designed specifically for the Java Virtual Machine (JVM). As such, you can use it to write Android applications, as it provides several benefits over Java, including enhanced readability and increased code safety. Its popularity for Android applications prompts an entire category for Kotlin interview questions Android developers should know.

The most popular Kotlin frameworks and tools include Ktor and the Kotlin Android Extensions library.

5. Is Kotlin Frontend or Backend?

Kotlin is a backend programming and frontend programming language, although it works a little differently depending on how it’s being used. Frontend programming leans on Kotlin/JavaScript, whereas backend programming runs on the JVM.

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Kotlin is a versatile and robust language, but it has certain complexities. There’s always more to learn, regardless of how long you’ve been in the game. Before you start interviewing for your next Kotlin developer position, remember to review the Kotlin interview questions in this guide. You might also consider taking a course or certification to refresh and read firm your Kotlin skills.

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By Jenna Inouye

Jenna Inouye currently works at Google and has been a full-stack developer for two decades, specializing in web application design and development. She is a tech expert with a B.S. in Information & Computer Science and MCITP certification. For the last eight years, she has worked as a news and feature writer focusing on technology and finance, with bylines in Udemy, SVG, The Gamer, Productivity Spot, and Spreadsheet Point.

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