Top 30 Most Common Kotlin Interview Questions You Should Prepare For

Introduction

Preparing for a Kotlin interview requires a solid understanding of the language's core concepts, modern features, and how it differs from or interoperates with Java. As Kotlin continues to grow in popularity for Android development, backend systems, and multiplatform projects, employers are increasingly seeking developers proficient in this expressive and concise language. Mastering common kotlin interview questions is essential to showcase your skills and land your desired role. This guide covers thirty fundamental questions frequently asked in technical interviews, providing insights into what interviewers look for and how to articulate your knowledge effectively. By understanding these key areas, you can approach your next Kotlin interview with confidence and demonstrate your readiness to build robust, efficient applications.

What Are Kotlin Interview Questions

Kotlin interview questions are technical questions designed to evaluate a candidate's proficiency with the Kotlin programming language. These questions cover a wide range of topics, from fundamental syntax like variable declaration (val vs var) and null safety (?, ?., ?:, !!) to more advanced concepts like coroutines, sealed classes, extension functions, and data classes. Interviewers use these questions to gauge a candidate's understanding of Kotlin's unique features, its advantages over other languages, how to write idiomatic Kotlin code, and their ability to solve problems using the language. They often assess practical skills through questions about collections, concurrency, interoperability with Java, and memory management.

Why Do Interviewers Ask Kotlin Interview Questions

Interviewers ask Kotlin interview questions to assess a candidate's foundational knowledge and practical experience with the language. They want to ensure you understand why Kotlin is used (conciseness, safety, interoperability), how its core features like null safety prevent common bugs, and how to leverage modern paradigms such as functional programming elements (lambdas, higher-order functions) and asynchronous programming (coroutines). Questions about val vs var, when, sealed classes, and object singletons reveal your grasp of basic syntax and design patterns. Discussion on extension functions, data classes, and scope functions indicates your ability to write idiomatic, maintainable code. Ultimately, these questions help interviewers determine if you can effectively contribute to a Kotlin project and harness the language's benefits.

Preview List

1. What is Kotlin and why is it used?

2. How do you declare variables in Kotlin?

3. What is the difference between val and var?

4. What are nullable types and how does Kotlin handle null safety?

5. Explain the when keyword in Kotlin.

6. Can Kotlin code run without JVM?

7. What is a sealed class in Kotlin?

8. How do you create a singleton in Kotlin?

9. How do you initialize an array in Kotlin?

10. What are extension functions?

11. What is the difference between == and === in Kotlin?

12. How does Kotlin support higher-order functions and lambdas?

13. What are data classes?

14. What are coroutines in Kotlin?

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

16. How do you handle null pointer exceptions in Kotlin?

17. What is the purpose of the companion object in Kotlin?

18. Explain the use of inline functions.

19. How is exception handling done in Kotlin?

20. What are sealed classes vs enums?

21. What is smart casting?

22. Can you explain Kotlin’s collection types?

23. How does Kotlin support default arguments?

24. What is destructuring declaration?

25. Explain Kotlin’s visibility modifiers.

26. What is type inference?

27. How does Kotlin interoperate with Java?

28. What are inline classes?

29. How do you declare a function in Kotlin?

30. What is the use of the lateinit modifier?

1. What is Kotlin and why is it used?

Why you might get asked this:

This fundamental question assesses your basic knowledge of Kotlin's purpose and benefits, showing you understand why projects adopt it over alternatives like Java.

How to answer:

Define Kotlin and mention its key platforms (JVM, Android, JS, Native). Highlight its core advantages like conciseness, safety (null safety), interoperability, and reduced boilerplate compared to Java.

Example answer:

Kotlin is a statically typed, pragmatic language developed by JetBrains, running on JVM, Android, JS, and Native. It's used for its conciseness, null safety to prevent NPEs, strong Java interoperability, reduced boilerplate, and features like coroutines, which boost productivity and code quality.

2. How do you declare variables in Kotlin?

Why you might get asked this:

Tests your understanding of basic syntax and the distinction between mutable and immutable variables, which is key to writing safer code.

How to answer:

Explain using val for read-only (immutable) variables and var for mutable variables. Mention type inference but also show explicit type declaration syntax.

Example answer:

Variables in Kotlin are declared using val for immutable references (value cannot be reassigned) and var for mutable references (value can be reassigned). You can explicitly state the type (val name: String = "Bob") or let Kotlin infer it (var age = 25).

3. What is the difference between val and var?

Why you might get asked this:

This directly checks your grasp of mutability, a core concept impacting code safety and maintainability. It's a very common first question.

How to answer:

Clearly state that val declares a read-only property that cannot be reassigned after initialization, while var declares a mutable property whose value can be changed. Emphasize favoring val where possible.

Example answer:

The key difference is mutability. val (from "value") creates an immutable variable; once assigned, you can't change its reference. var (from "variable") creates a mutable variable that can be reassigned to a different value at any time. We prefer val for thread safety and predictable state.

4. What are nullable types and how does Kotlin handle null safety?

Why you might get asked this:

Null safety is a cornerstone of Kotlin. This tests your understanding of how Kotlin prevents null pointer exceptions, a frequent source of bugs in other languages.

How to answer:

Explain nullable types using the ? suffix. Describe how Kotlin enforces null safety at compile time. Mention the safe call operator (?.), the Elvis operator (?:), and the non-null assertion operator (!!).

Example answer:

Kotlin's type system distinguishes between nullable (String?) and non-nullable (String) types. This prevents NPEs at compile time. Null safety is handled using operators like ?. (safe call), ?: (Elvis operator for default value), and !! (non-null assertion, use with caution).

5. Explain the when keyword in Kotlin.

Why you might get asked this:

Checks your knowledge of Kotlin's enhanced control flow structures, which are more flexible and powerful than Java's switch.

How to answer:

Describe when as a replacement for switch statements, allowing pattern matching on values or arbitrary boolean expressions. Mention it can be used as a statement or an expression.

Example answer:

when is a powerful conditional expression or statement. It's like a flexible switch statement that can match values, ranges (in), types (is), or even arbitrary boolean conditions without an argument. It can return a value when used as an expression.

6. Can Kotlin code run without JVM?

Why you might get asked this:

Evaluates your awareness of Kotlin's multiplatform capabilities beyond just Android/JVM development.

How to answer:

State that Kotlin can run on platforms other than the JVM, specifically mentioning Kotlin/JS, Kotlin/Native, and Kotlin Multiplatform.

Example answer:

Yes, absolutely. While Kotlin is popular on the JVM and Android, it also targets JavaScript (Kotlin/JS) for web frontends and native code (Kotlin/Native) for platforms like iOS, macOS, and embedded systems, enabling true code sharing via Kotlin Multiplatform.

7. What is a sealed class in Kotlin?

Why you might get asked this:

Tests your understanding of restricted hierarchies, a feature useful for representing states or fixed sets of types, often used with when.

How to answer:

Explain that a sealed class restricts inheritance to subclasses defined within the same file or module, creating a closed hierarchy. Mention its benefit in exhaustive when expressions.

Example answer:

A sealed class defines a restricted class hierarchy. All direct subclasses must be declared within the same file or module as the sealed class itself. This makes when expressions using sealed class instances exhaustive, meaning the compiler ensures all possible subclasses are covered, reducing errors.

8. How do you create a singleton in Kotlin?

Why you might get asked this:

Checks your knowledge of how Kotlin simplifies common design patterns like the singleton compared to Java.

How to answer:

Explain the use of the object keyword to declare a singleton instance. Mention that it's thread-safe and initialized lazily upon first access.

Example answer:

You can create a thread-safe singleton easily using the object keyword. Declaring object MySingleton { ... } creates a single instance of the class upon first access, which is globally available. This is much simpler than traditional Java singleton patterns.

9. How do you initialize an array in Kotlin?

Why you might get asked this:

Tests basic data structure handling and syntax.

How to answer:

Show the common ways: using arrayOf() for a fixed list of elements or using the Array constructor with size and a lambda for computed values. Mention primitive arrays like IntArray.

Example answer:

Arrays can be initialized using arrayOf(element1, element2, ...) or the Array(size) { initialization_lambda } constructor. For performance with primitives, use specialized classes like IntArray, CharArray, etc., initialized with intArrayOf() or their specific constructors.

10. What are extension functions?

Why you might get asked this:

Evaluates your understanding of a powerful feature that enhances existing classes without modifying their source code, promoting code readability.

How to answer:

Define extension functions as a way to add new functions to a class without inheritance or using design patterns like Decorator. Explain the syntax: fun ReceiverType.functionName(params).

Example answer:

Extension functions allow you to add new functions to an existing class without modifying its source code. You define them outside the class using fun ClassName.functionName(...). This enhances readability and utility, like adding a lastChar() function directly to String.

11. What is the difference between == and === in Kotlin?

Why you might get asked this:

Tests your understanding of equality checks, crucial for avoiding subtle bugs related to object comparison.

How to answer:

Explain that == checks structural equality (like Java's .equals() by default), comparing the contents of objects. === checks referential equality, comparing whether two references point to the exact same object in memory.

Example answer:

== checks for structural equality, comparing if the contents or values of two objects are the same (it calls the equals() method internally). === checks for referential equality, determining if two references point to the exact same object instance in memory.

12. How does Kotlin support higher-order functions and lambdas?

Why you might get asked this:

Checks your knowledge of functional programming paradigms supported by Kotlin, which are common in modern Android development and collection processing.

How to answer:

Define higher-order functions as functions that can accept functions as parameters or return functions. Define lambdas as anonymous functions. Show basic syntax examples.

Example answer:

Kotlin fully supports higher-order functions, which can take functions as arguments or return them. Lambdas are anonymous functions, concise blocks of code often passed to higher-order functions. Example: sorting a list with a lambda for comparison.

13. What are data classes?

Why you might get asked this:

Assesses your knowledge of a convenience feature that simplifies creating classes primarily holding data.

How to answer:

Explain that data classes are designed to hold data and the compiler automatically generates useful methods like equals(), hashCode(), toString(), copy(), and componentN() functions for destructuring.

Example answer:

Data classes are concise classes used primarily to hold data. By marking a class with data, the compiler auto-generates standard functions like equals(), hashCode(), toString(), and copy(), saving significant boilerplate code compared to Java POJOs.

14. What are coroutines in Kotlin?

Why you might get asked this:

Tests your knowledge of Kotlin's structured concurrency solution, essential for modern asynchronous programming on Android and backend.

How to answer:

Describe coroutines as lightweight threads for asynchronous programming. Explain they allow writing non-blocking code in a sequential style using suspend functions, managing concurrency without complex callbacks.

Example answer:

Coroutines are a concurrency design pattern used for asynchronous programming. They are lightweight, stackless, and managed by Kotlin. They allow writing non-blocking code that looks sequential using suspend functions, simplifying asynchronous tasks compared to traditional threads or callbacks.

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

Why you might get asked this:

A classic OOP question adapted to Kotlin, testing your understanding of abstraction mechanisms.

How to answer:

Explain that both can define abstract methods. Key differences: interfaces can't hold state (fields), abstract classes can; interfaces can't have constructors, abstract classes can; a class can implement multiple interfaces but inherit only one abstract class.

Example answer:

Both define abstract contracts. An interface can't store mutable state or have a constructor, and a class can implement multiple interfaces. An abstract class can have state, a constructor, and can only be extended by one class. Interfaces are more flexible for defining capabilities.

16. How do you handle null pointer exceptions in Kotlin?

Why you might get asked this:

Reinforces your understanding of Kotlin's core null safety feature from a practical perspective.

How to answer:

Reiterate the use of nullable types (?) and the operators ?. (safe call), ?: (Elvis), and !! (non-null assertion) as primary mechanisms for handling potential null values safely or explicitly stating intent.

Example answer:

Kotlin prevents NPEs at compile time via its type system distinguishing nullable types (String?). We handle potential nulls using safe calls (?.), the Elvis operator (?:) for default values, or the !! operator to assert non-null (risky, use sparingly).

17. What is the purpose of the companion object in Kotlin?

Why you might get asked this:

Tests your knowledge of how Kotlin handles static-like members associated with a class rather than an instance.

How to answer:

Explain that a companion object is a singleton object declared inside a class, used to hold members (properties and functions) that belong to the class itself, accessible via the class name, similar to static members in Java.

Example answer:

A companion object is a single instance associated with a class, declared using the companion object keyword inside the class body. It holds members accessible directly using the class name (like ClassName.memberName), serving a similar purpose to static members in Java.

18. Explain the use of inline functions.

Why you might get asked this:

Checks your understanding of a performance optimization technique, often used with higher-order functions and lambdas.

How to answer:

Explain that inline is a hint to the compiler to copy the function's bytecode directly to the call site instead of generating a function call. This is mainly used to reduce the overhead of lambda expressions.

Example answer:

The inline modifier requests the compiler to substitute the function body directly into where it's called. This is especially useful for higher-order functions using lambdas, reducing the performance overhead of creating and invoking a function object for each call.

19. How is exception handling done in Kotlin?

Why you might get asked this:

Tests your knowledge of error handling patterns, comparing Kotlin's approach to languages like Java (checked vs unchecked exceptions).

How to answer:

Explain that Kotlin uses try, catch, and finally blocks similar to Java. Crucially, mention that Kotlin does NOT have checked exceptions, simplifying error handling by treating all exceptions as unchecked.

Example answer:

Exception handling in Kotlin is done using try, catch, and finally blocks, similar to Java. However, Kotlin does not have checked exceptions; all exceptions are unchecked. This means you are not forced to declare exceptions in function signatures, simplifying code but requiring careful consideration of potential runtime errors.

20. What are sealed classes vs enums?

Why you might get asked this:

Compares two related language features for representing restricted choices, highlighting their different strengths.

How to answer:

Explain that enums represent a fixed set of named constants. Sealed classes represent a restricted hierarchy where subclasses can hold state and have different behaviors, offering more flexibility than simple constants.

Example answer:

Enums are for fixed sets of simple constant values (e.g., directions like North, South). Sealed classes represent a restricted hierarchy of types, where each subclass can be an object or a data class with its own state and behavior, offering more flexibility for complex states or events.

21. What is smart casting?

Why you might get asked this:

Evaluates your understanding of a convenience feature that reduces boilerplate and improves code readability by performing implicit type casts.

How to answer:

Explain that Kotlin's compiler automatically casts a variable to a specific type after a type check (is or !is) within the same scope, avoiding the need for explicit casts (as).

Example answer:

Smart casting is a compiler feature where, after a type check (like using is or !is), Kotlin automatically casts the variable to that specific type within that scope. This eliminates redundant explicit cast calls (as Type), making the code cleaner and safer.

22. Can you explain Kotlin’s collection types?

Why you might get asked this:

Tests your understanding of standard library data structures, including the distinction between mutable and immutable collections.

How to answer:

Describe the key collection interfaces: List, Set, Map. Emphasize that Kotlin distinguishes between immutable interfaces (List, Set, Map) and mutable implementations (MutableList, MutableSet, MutableMap), promoting immutability by default.

Example answer:

Kotlin provides standard collections like List, Set, and Map. It distinguishes between immutable interfaces (read-only, like List) and mutable concrete classes (modifiable, like MutableList). Favoring immutable collections by default helps prevent unintended state changes and improves thread safety.

23. How does Kotlin support default arguments?

Why you might get asked this:

Checks your knowledge of features that reduce function overloads and improve call-site readability.

How to answer:

Explain that function parameters can be assigned default values directly in the function signature, allowing callers to omit those arguments when the default value is acceptable.

Example answer:

Kotlin allows defining default values for function parameters directly in the function declaration: fun greeting(name: String = "Guest"). Callers can then call the function without providing that argument (greeting()), and the default value is used. This reduces the need for function overloads.

24. What is destructuring declaration?

Why you might get asked this:

Tests your knowledge of a syntactic convenience feature that allows extracting values from objects (like data classes) into separate variables.

How to answer:

Describe destructuring as a way to "unpack" an object into multiple variables in a single statement. Mention it works automatically for data classes and can be implemented for other classes using componentN() functions.

Example answer:

Destructuring declaration lets you assign properties of an object to multiple variables concisely, like val (name, age) = person. It works out-of-the-box for data classes and can be applied to any class that defines componentN() operator functions.

25. Explain Kotlin’s visibility modifiers.

Why you might get asked this:

Evaluates your understanding of access control, crucial for encapsulation and module design.

How to answer:

List and explain the four modifiers: public (visible everywhere, default), private (visible within the same class/file), protected (visible within the same class and its subclasses), and internal (visible within the same module).

Example answer:

Kotlin has public (default, visible everywhere), private (visible within the declaring scope - class, function, file), protected (visible in the class and subclasses - only for class members), and internal (visible within the same module).

26. What is type inference?

Why you might get asked this:

Checks your understanding of a core language feature that reduces verbosity by allowing the compiler to deduce types.

How to answer:

Explain that type inference is the ability of the Kotlin compiler to automatically determine the data type of a variable or expression based on its initial value or context, reducing the need for explicit type declarations.

Example answer:

Type inference is the compiler's ability to automatically figure out the type of a variable or expression without you explicitly stating it. For example, val count = 10 infers count is an Int. This reduces boilerplate while maintaining static typing.

27. How does Kotlin interoperate with Java?

Why you might get asked this:

Crucial for real-world projects that often involve mixed Java and Kotlin codebases.

How to answer:

Explain that Kotlin has excellent, seamless interoperability with Java. Kotlin code can call Java code, and Java code can call Kotlin code. Mention that Kotlin compiles to JVM bytecode, making this possible.

Example answer:

Kotlin offers excellent two-way interoperability with Java. You can call Java code from Kotlin and Kotlin code from Java seamlessly within the same project. Kotlin compiles to JVM bytecode, allowing easy integration with existing Java libraries and frameworks.

28. What are inline classes?

Why you might get asked this:

Tests knowledge of a specific feature for creating lightweight wrappers with type safety.

How to answer:

Describe inline classes as a way to create a value-based class that wraps a single value, avoiding the overhead of creating a new object instance at runtime. They provide type safety without memory allocation costs for the wrapper itself.

Example answer:

Inline classes (now value classes in newer Kotlin) are lightweight classes wrapping a single value, marked with value class. The compiler avoids creating a wrapper object instance at runtime, using just the underlying value. This provides type safety (e.g., data class Milliseconds(val value: Long)) with performance similar to using the raw type.

29. How do you declare a function in Kotlin?

Why you might get asked this:

A basic syntax question, foundational to writing any Kotlin code.

How to answer:

Explain the syntax using the fun keyword, followed by the function name, parameters in parentheses (name: Type), an optional return type after a colon, and the function body in curly braces or using an equals sign for single-expression functions.

Example answer:

Functions are declared using the fun keyword. The basic syntax is fun functionName(param1: Type1, ...): ReturnType { / body / }. For single-expression functions, you can use fun functionName(params): ReturnType = expression.

30. What is the use of the lateinit modifier?

Why you might get asked this:

Tests your understanding of managing non-nullable properties that cannot be initialized immediately, common in frameworks like Android (dependency injection, view initialization).

How to answer:

Explain lateinit is used for var properties (not val, not primitive types) that are non-nullable but cannot be initialized in the constructor. It defers initialization, but you must ensure it's initialized before accessing it, or it will throw a UninitializedPropertyAccessException.

Example answer:

lateinit is used with mutable (var), non-nullable properties (not primitive types) that you promise to initialize later, typically outside the constructor (e.g., via dependency injection or setup methods). It avoids needing nullable types or initial dummy values but will throw an exception if accessed before initialization.

Other Tips to Prepare for a Kotlin Interview

Beyond practicing these common kotlin interview questions, enhance your preparation by writing actual Kotlin code. Work on small projects or contribute to open source to gain practical experience with features like coroutines, flows, and testing frameworks. Understand common libraries used in the domain you're applying for (e.g., Android Jetpack components, Ktor for backend). Be ready to discuss architectural patterns (MVC, MVP, MVVM, MVI) and how Kotlin features can be applied. According to industry experts, "understanding the 'why' behind Kotlin features is as crucial as knowing the 'how'." Practice explaining your thought process clearly, especially for coding problems. Consider using tools like Verve AI Interview Copilot https://vervecopilot.com to simulate interview scenarios and get personalized feedback on your answers and coding skills. Incorporating Verve AI Interview Copilot into your study routine can help you refine your communication and approach to common challenges. Remember, "Confidence comes from preparation," so leverage resources like Verve AI Interview Copilot to feel fully ready.

Frequently Asked Questions

Q1: Is Kotlin better than Java? A1: It offers advantages like conciseness and null safety, often preferred for new development, but "better" is subjective to project needs.
Q2: What is a scope function? A2: Functions like let, run, with, apply, also that execute a code block on an object, often used for concise operations or null handling.
Q3: What is the Elvis operator? A3: The ?: operator, providing a default value if the expression on its left is null, e.g., nullableValue ?: defaultValue.
Q4: How do you write a coroutine? A4: Using coroutine builders like launch or async within a coroutine scope, calling suspend functions.
Q5: Can you use Kotlin on the backend? A5: Yes, frameworks like Ktor and Spring support Kotlin for backend development on the JVM.
Q6: What is Flow? A6: An asynchronous stream that emits multiple values, built on coroutines for handling asynchronous data sequences.