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Top Kotlin Android interview questions

13 min read
Mar 04, 2025
content
Common Kotlin interview questions
Advanced Kotlin interview preparation topics
1. Kotlin multiplatform
2. Delegation in Kotlin
3. Kotlin type system
4. Extension functions
5. Advanced collections handling
6. Data classes and destructuring
7. Kotlin reflection
8. Kotlin’s object-oriented features
9. Kotlin scripting
10. Kotlin annotations
Next steps

Did you know Kotlin is the official language for Android development and has been adopted by over 70% of professional Android developers? With its concise syntax, powerful features, and seamless interoperability with Java, Kotlin has become a preferred language for Android development. As Kotlin is the primary language for modern Android development, mastering it is crucial for Android and coding interviews. If you are preparing for coding interviews at FAANG companies and want to attempt the interview in Kotlin, you must be well-versed in its features, best practices, and advanced topics.

In this blog, you’ll learn:

  • Commonly asked Kotlin interview questions with code examples and explanations

  • Advanced Kotlin interview preparation topics

The Ultimate Guide to Kotlin Programming

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The Ultimate Guide to Kotlin Programming

Kotlin has been growing in popularity among developers for some time. It was given even more recognition when Google announced in 2019 that Kotlin was now their preferred language for Android development. In this comprehensive course, you’ll start by learning the fundamentals, such as: how Java and Kotlin differ, how to work with functions, and how to utilize collections, something you’ll work with extensively in Kotlin. In the latter half of the course, you’ll be introduced to more advanced concepts like lambdas, fluency in Kotlin, and asynchronous programming. In the last section of the course, you’ll take what you’ve learned and build out an Android application that talks to a backend service. By the time you’re done with this course, you’ll have a thorough mastery of this modern JVM language. This course is based on the PragProg book, Programming Kotlin, written by the extraordinary, experienced programming-language enthusiast: Venkat Subramaniam.

20hrs
Beginner
170 Playgrounds
34 Quizzes

Common Kotlin interview questions#

Following are the Kotlin interview questions commonly asked in the interviews:

  1. What is Kotlin, and how does it differ from Java?
    Kotlin is a statically typed, modern programming language developed by JetBrains, designed to work seamlessly with Java. It has gained prominence due to its concise syntax, advanced features, and ability to simplify Android development while maintaining full interoperability with Java. Some of the key features include:

    1. Concise syntax: Reduces boilerplate code for better readability.

    2. Null safety: Minimizes null pointer exceptions at compile-time.

    3. Interoperability with Java: Works seamlessly with existing Java codebases.

    4. Advanced features: Includes higher-order functions, coroutines, and more.

  2. What are the key differences between Kotlin and Java?
    Following are the key differences between Kotlin and Java programming languages:

    1. Concise syntax:
      Kotlin reduces boilerplate code, making programs more readable and maintainable compared to Java.

    2. Null safety:
      Kotlin provides built-in null safety by distinguishing between nullable and non-nullable types, preventing null pointer exceptions (NPEs) at compile-time.

    3. Extension functions:
      Kotlin allows developers to extend functionality to existing classes without modifying their source code.

    4. Coroutines for asynchronous programming:
      Kotlin includes coroutines, which simplify asynchronous and concurrent programming compared to Java’s threading model.

    5. Interoperability with Java:
      Kotlin integrates seamlessly with Java, allowing developers to call Java code from Kotlin and vice versa without additional overhead.

    6. Default values for function parameters:
      Kotlin supports default parameter values, reducing the need for overloading methods.

Kotlin minimizes verbose code, ensuring faster development and easier maintenance. It enhances safety with null safety features, boosts productivity through modern tools, and is officially supported by Google for Android development.

For those looking for a quick and effective way to get up to speed with Kotlin or prepare for a Kotlin coding interview, Kotlin Crash Course for Programmers offers a comprehensive introduction to the language, covering all essential concepts in a concise format.

  1. Explain null safety in Kotlin. How does Kotlin handle null values?
    The null safety is a core feature designed to prevent null pointer exceptions (NPEs), which are common runtime errors in many programming languages, including Java. Kotlin achieves this by distinguishing between nullable and non-nullable types at the language level.
    Here’s how Kotlin handles null values:

    • Non-nullable types:
      By default, all types in Kotlin are non-nullable. This means that variables cannot hold null unless explicitly specified. For example, a variable of type String cannot be assigned null without a compile-time error.

    • Nullable types:
      To declare a variable that can hold null, Kotlin uses a special syntax by appending a ? to the type. For instance, String? indicates that the variable can hold either a String or null.

// Declaring a string that can hold null values
val name: String? = null

    • Safe calls (?.):
      Kotlin provides the safe call operator ?. to safely access properties or methods of nullable types. If the object is null, it will return null rather than throwing an exception.

// Returns null if name is null
val length = name?.length

    • Elvis operator (?:):
      The Elvis operator helps provide a default value when a nullable type is null.

// Returns 0 if name is null
val length = name?.length ?: 0

    • Not-null assertion (!!):
      If you are certain that a nullable variable is not null, you can use the !! operator to assert that it is not null. If it is null, a NullPointerException will be thrown.

// Throws an exception if name is null
val length = name!!.length

Kotlin’s approach to null safety is designed to catch potential null pointer issues at compile-time, thus improving the reliability and safety of your code. This makes Kotlin a safer alternative compared to Java, where null pointer exceptions are often only detected at runtime.

  1. What are the different types of constructors in Kotlin?
    There are two main types of constructors:
    1. Primary constructor: The primary constructor is part of the class declaration and is used for basic initialization. It can take parameters and directly initialize properties. Initialization logic can be added in the init block.
    2. Secondary constructor: A class can have additional constructors, defined using the constructor keyword. These allow more flexible initialization, including calling the primary constructor or other secondary constructors.

Here's an example of a Book class using both primary and secondary constructors:

class Book(val title: String, val author: String, var yearPublished: Int) {
    // Primary constructor initializes title, author, and yearPublished
    
    init {
        println("Book details: Title - $title, Author - $author, Year Published - $yearPublished")
    }
    // Secondary constructor with a default yearPublished
    constructor(title: String, author: String) : this(title, author, 2020) {
        println("Book details (default year): Title - $title, Author - $author, Year Published - $yearPublished")
    }
    // Method to check if the book is a classic (published before 1980)
    fun isClassic(): Boolean {
        return yearPublished < 1980
    }
    // Method to update the publication year
    fun updateYear(newYear: Int) {
        yearPublished = newYear
    }
}
fun main() {
    // Using primary constructor
    val book1 = Book("1984", "George Orwell", 1949)
    
    // Using secondary constructor with a default year
    val book2 = Book("The Great Gatsby", "F. Scott Fitzgerald")
    
    // Checking if the books are classics
    println("Is '${book1.title}' a classic? ${book1.isClassic()}")
    println("Is '${book2.title}' a classic? ${book2.isClassic()}")
    
}

In the code above:

  • book1 is initialized using the primary constructor with all three parameters: title, author, and yearPublished.

  • book2 is initialized using the secondary constructor, which only requires the title and author, with the yearPublished defaulting to 2020.

  1. What is the purpose of the companion object in Kotlin?
    A companion object is used to define members (like methods or properties) that are tied to the class itself rather than to instances of the class. Essentially, it allows you to define static-like members, which are not directly tied to an object but can be accessed via the class itself. This is similar to static members in Java, but Kotlin doesn’t have a static keyword.

Here are the key purposes of a companion object:

  • Access to class-level members: A companion object allows you to create class-level members that can be accessed without creating an instance of the class.

  • Factory methods: It’s often used to implement factory methods, allowing you to create instances of the class in a specific way.

  • Initialization and constants: Companion objects can be used for constants and initialization logic, similar to static variables in Java.

  1. How are exceptions handled in Kotlin?
    Here’s a concise list of key points on exception handling in Kotlin:

    1. Try-catch blocks: Handle exceptions using try-catch blocks, where catch specifies the type of exception to be handled.

    2. Multiple catch clauses: You can handle different exceptions in separate catch blocks.

    3. Finally block: The finally block always runs, regardless of whether an exception was thrown or not.

    4. Throwing exceptions: You can use the throw keyword to manually throw exceptions.

    5. No checked exceptions: Kotlin does not require explicit handling or declaration of checked exceptions, simplifying error handling.

    6. Null safety and exception handling: Kotlin enforces null safety, but NullPointerException can still occur if explicitly forced.

    7. Custom exceptions: You can create custom exceptions by extending the Exception class in Kotlin.

  2. What are coroutines in Kotlin, and why are they used?
    In Kotlin, coroutines are a powerful feature for handling asynchronous programming and concurrency. They allow code to run asynchronously without blocking threads, making it easier to write non-blocking, concurrent code in a sequential style.

    • Non-blocking execution: Coroutines enable asynchronous operations without blocking the main thread, improving performance, especially in mobile and UI applications.

    • Lightweight: Coroutines are lighter than threads, allowing thousands of concurrent operations without a significant performance hit.

    • Structured concurrency: Kotlin ensures that coroutines are properly managed within a defined scope, reducing the risk of memory leaks and other concurrency issues.

    • Sequential code style: Coroutines allow asynchronous code to be written in a sequential manner, making it more readable and maintainable than traditional callback-based approaches.

In the following code example, the delay function simulates a long-running task without blocking the thread, and the program continues executing while the coroutine runs.

import kotlinx.coroutines.*
fun main() = runBlocking {
    launch {
        // Simulate a long-running task
        delay(1000L)
        println("Task completed")
    }
    println("Waiting for task to finish...")
}
Coroutines in Kotlin

For those looking to deepen their understanding of Kotlin’s asynchronous programming capabilities, the course Mastering Kotlin Coroutines is highly recommended for an in-depth exploration of coroutines and their role in building efficient, concurrent applications.

  1. What are val and var variables in Kotlin?
    In Kotlin, val and var are used to declare variables, but they differ in how they handle mutability.

    1. val (immutable variable)

      1. A variable declared with val is immutable, meaning its value cannot be changed once it is assigned.

      2. It is similar to a final variable in Java.

      3. The reference itself is immutable, but if the value is an object, the object’s properties can still be modified (if the object itself is mutable).

    2. var (mutable variable)

      1. A variable declared with var is mutable, meaning its value can be reassigned after the initial assignment.

      2. It allows both the reference and the value to be changed.

Here is an example using both val and var in Kotlin, demonstrating the difference between immutable and mutable variables:

fun main() {
    // Immutable variable 'val'
    val name = "John"  // The value of 'name' cannot be reassigned
    println("Name: $name")
    // Mutable variable 'var'
    var age = 25  // The value of 'age' can be reassigned
    println("Age: $age")
    // Reassigning 'age'
    age = 30
    println("Updated Age: $age")
    // Example with an object (mutable property)
    val person = Person("John")
    println("Person's Name: ${person.name}")
    // Modifying the property of the mutable object
    person.name = "Doe"
    println("Updated Person's Name: ${person.name}")
}
class Person(var name: String)

  1. What are higher-order functions in Kotlin? Provide an example.
    The higher-order functions are functions that can either take other functions as parameters or return a function as a result. This is a powerful feature of Kotlin that allows for more flexible and reusable code. For example, in the following code example, operateOnNumbers is a higher-order function because it takes another function (operation) as an argument. The function operation accepts two integers and returns an integer. In the main function, the add function is passed as an argument to operateOnNumbers.

// A simple higher-order function that takes a function as a parameter
fun operateOnNumbers(a: Int, b: Int, operation: (Int, Int) -> Int): Int {
    return operation(a, b)
}
// A simple function that can be passed as an argument
fun add(x: Int, y: Int): Int = x + y
fun main() {
    val result = operateOnNumbers(5, 3, ::add) // Passing the 'add' function as an argument
    println("Result of addition: $result")
}

The higher-order functions enable functional programming in Kotlin and help write more modular and maintainable code. Their common use cases include:

    1. Lambdas: Higher-order functions are often used with lambda expressions for cleaner, more concise code.

    2. Collection operations: Functions like map, filter, and reduce are examples of higher-order functions in Kotlin that operate on collections

The following code example multiplies two numbers using a lambda function instead of a predefined function like add. Also, the map function is used on the numbers list to apply the multiply by 2 operation to each element.

fun operateOnNumbers(a: Int, b: Int, operation: (Int, Int) -> Int): Int {
    return operation(a, b)
}
fun main() {
    val result = operateOnNumbers(5, 3) { x, y -> x * y }  // Lambda passed as argument
    println("Result of multiplication: $result")
    
    val numbers = listOf(1, 2, 3, 4, 5)
    
    // Mapping each number in the list by multiplying it by 2
    val mappedResult = numbers.map { it * 2 }
    println("Mapped result (multiply by 2): $mappedResult")
  
}

  1. What is the purpose of using sealed classes in Kotlin?

A sealed class is a special class that restricts class hierarchies. The key point is that sealed classes can only have subclasses defined within the same file. This ensures that the set of possible subclasses is limited and known, and inheritance is restricted to specific classes.

  • Restricting inheritance: Sealed classes allow only a specific set of subclasses. All subclasses must be defined within the same file, preventing external extensions. This helps create a closed type hierarchy, ensuring safety in type handling.

  • Type safety: Sealed classes allow you to represent a limited set of types that can be used in a when expression. This provides a safer way to handle different cases, as the compiler can check if all possibilities are covered at compile-time, preventing runtime errors.

  • Modeling states or outcomes: Sealed classes are often used to represent complex states or outcomes that may have multiple possibilities (like success or failure). It’s particularly useful for representing things like API responses, navigation states, and different UI states in Android development.

  • Better code readability and maintainability: By using sealed classes, developers can clearly define all possible subclasses, improving the clarity of code and reducing ambiguity.

In the code example below, the Animal class is sealed, which means that only Dog, Cat, and Fish can inherit from it. If someone tries to create a new subclass of Animal outside this file, they will get a compiler error. The when expression is exhaustive, handling all possible subclasses of Animal, which helps prevent unhandled cases during compilation.

// Sealed class
sealed class Animal
// Only classes in the same file can inherit from Animal
data class Dog(val name: String) : Animal()
data class Cat(val name: String) : Animal()
// This is allowed since it's in the same file
object Fish : Animal()
fun printAnimalSound(animal: Animal) {
    when (animal) {
        is Dog -> println("Woof! I am ${animal.name}")
        is Cat -> println("Meow! I am ${animal.name}")
        Fish -> println("Glub glub!")
    }
}
fun main() {
    val dog = Dog("Buddy")
    val cat = Cat("Whiskers")
    val fish = Fish
    
    printAnimalSound(dog)
    printAnimalSound(cat)
    printAnimalSound(fish)
}

Having covered common Kotlin interview questions, let’s now explore advanced Kotlin interview preparation topics that will help you tackle more challenging questions and demonstrate your deep understanding of Kotlin.

Advanced Kotlin interview preparation topics#

When preparing for advanced Kotlin and Android development interview positions, there are numerous essential Kotlin advanced features that candidates must focus on to demonstrate a strong mastery of the language. In addition to understanding fundamental concepts, it’s crucial to explore advanced Kotlin features, optimizations, and language-specific nuances. Below is a comprehensive list of advanced topics that should be studied to excel in Kotlin interviews for senior or specialized roles.

1. Kotlin multiplatform#

  • What is Kotlin multiplatform, and how can it be used to share code across different platforms (Android, iOS, and web)?

  • How do you structure a Kotlin Multiplatform project?

  • Discuss the key differences between Kotlin for Android and Kotlin for iOS in a multiplatform project.

  • What challenges do developers face while working with Kotlin Multiplatform?

2. Delegation in Kotlin#

  • What is delegation in Kotlin, and how is it implemented using by keyword?

  • Explain the difference between delegated properties and class delegation in Kotlin.

  • How do you create custom property delegates?

3. Kotlin type system#

  • Explain variance in Kotlin with examples of covariant (out) and contravariant (in) types.

  • How does Kotlin handle null safety and Nullable types?

  • Discuss the difference between nullable types and optional types in Kotlin.

4. Extension functions#

  • What are extension functions, and how do they extend existing classes without modifying their source code?

  • Provide examples where extension functions are useful for improving code readability and reducing boilerplate code.

  • How do extension functions differ from traditional inheritance in Kotlin?

5. Advanced collections handling#

  • How do you handle immutable and mutable collections in Kotlin? Explain with examples of List, Set, and Map.

  • What is the significance of lazy collections in Kotlin?

  • How do Kotlin’s built-in collection operators (e.g., fold, groupBy) work, and when should you use them?

6. Data classes and destructuring#

  • What are Kotlin data classes, and what are the key benefits of using them?

  • How does destructuring work in Kotlin with data classes and non-data classes?

7. Kotlin reflection#

  • What is Kotlin reflection, and how does it differ from Java reflection?

  • How can you use reflection to inspect class metadata and perform runtime operations?

8. Kotlin’s object-oriented features#

  • Explain Kotlin’s approach to object-oriented programming, focusing on inheritance, interfaces, and abstract classes.

  • How do Kotlin’s open and final keywords impact inheritance?

  • How are companion objects used, and what is their significance?

9. Kotlin scripting#

  • What is Kotlin scripting, and how can Kotlin be used as a scripting language?

  • Discuss the role of KotlinScript and how Kotlin scripts differ from regular Kotlin code.

10. Kotlin annotations#

  • What are annotations in Kotlin, and how do they differ from Java annotations?

  • How do you use custom annotations and process them using reflection?

Next steps#

To take your Kotlin skills to the next level and prepare thoroughly for advanced Kotlin interviews, here are some excellent courses to consider:

  1. The Ultimate Guide to Kotlin Programming: This course covers everything from the basics to advanced Kotlin concepts. It's perfect for building a solid foundation and refining your skills.

  2. Kotlin Design Patterns and Best Practices: This course delves into the practical application of design patterns in Kotlin. It’s ideal for understanding how to write clean, efficient, and scalable code using Kotlin.

  3. Mastering Kotlin Essentials: This course is designed to help you become proficient with Kotlin’s core features, including its syntax, collections, and object-oriented programming techniques.

For coding interview preparation specifically in Kotlin, check out:

Another great starting point is exploring the How to learn Kotlin: A guide for beginners & Android developers blog, which offers guidance on building a strong foundation for learning Kotlin.

These resources will help you not only master Kotlin but also gain the expertise required to perform well in Kotlin-based coding interviews.

Frequently Asked Questions

Is it possible to execute Kotlin code without JVM?

Yes, Kotlin code can run without the JVM using Kotlin/Native or Kotlin/JS. Kotlin/Native compiles to native binaries for platforms like iOS and Linux, while Kotlin/JS compiles to JavaScript for use in browsers or Node.js.

Is Kotlin enough to get a job?

Will Kotlin overtake Java?

Is Kotlin easy if you know Java?

Written By:
Ishrat Fatima
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