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30+ Kotlin Interview Questions And Answers

Kotlin is a modern, statically typed language used largely for Android app development, as well as server-side and web applications. It is designed to be fully interoperable with Java, allowing for the seamless integration of Kotlin code with existing Java codebases. Kotlin simplifies programming tasks, making code more concise and readable. With its growing popularity, proficiency in Kotlin is highly sought after in developers. Preparing for an interview entails understanding both basic and advanced Kotlin concepts, which is why below we have listed essential interview questions aimed at both beginners and advanced developers.


Most asked Kotlin interview questions



What is Kotlin and how does it interoperate with Java?

Kotlin is a modern programming language that runs on the JVM (Java Virtual Machine) and can be used anywhere Java is used. It interops with Java seamlessly, meaning you can call Java code from Kotlin and vice versa without any problems. This is crucial in Android development where many libraries are written in Java.


Explain the 'val' and 'var' keywords in Kotlin

In Kotlin, val is used to declare a read-only (final) variable that can be assigned only once. On the other hand, var declares a mutable variable that can be changed after assignment.


What is a data class in Kotlin?

In Kotlin, a data class is used to hold data/state. It automatically generates field getters, setters, equals(), hashCode(), and toString() methods among others. It’s intended to save you boilerplate code for classes that are used to carry data.


Can you explain the difference between '==' and '===' in Kotlin?

'==' is used to compare the values of two objects, while '===' checks for referential equality, meaning it will only return true if both references point to the same object.


What does the ?. operator do in Kotlin?

The ?. operator is known as the safe call operator in Kotlin. It allows you to call a method or access a property of an object that could potentially be null, without causing a NullPointerException.


What are extension functions in Kotlin? Provide an example.

Extension functions allow you to add new functions to an existing class without altering its code structure. For example, adding a function to print a custom string for List would be:

fun List.printCustomString() {
val myList = listOf("a", "b", "c")
myList.printCustomString() // Prints: a-b-c


What are sealed classes in Kotlin?

Sealed classes allow you to define restricted class hierarchies, where a value can only be of one of the given types. Think of them as an advanced form of enum classes which can hold state and have methods.


Explain the use of the 'lateinit' keyword in Kotlin

The lateinit keyword is used to declare non-null properties that will be initialized after the object’s constructor has completed. It helps avoid nullable types and the need for non-null assertion operators.


What is a companion object in Kotlin?

A companion object is an object that is common to all instances of that class. Similar to static methods in Java, they can be called on the class itself rather than an instance.


What does the 'apply' function do in Kotlin?

The 'apply' function calls the specified function block with 'this' value as its receiver and returns 'this'. It's usually used for initializing or configuring an object.


How do you create a singleton in Kotlin?

You can create a singleton in Kotlin using the 'object' keyword. This defines a type that will have only one instance throughout the application.


What is a high-order function?

A high-order function is a function that can take functions as parameters or return a function.


What are coroutines in Kotlin and can you name some advantages?

Coroutines in Kotlin are a way to write asynchronous, non-blocking code. They help in managing background tasks more efficiently and are less resource-intensive compared to threads.


Guess what does the following code do:

fun printSum(a: Int, b: Int) {
    println("sum of $a and $b is ${a + b}")

The code defines a function that takes two integers as parameters and prints their sum.


How do default parameters work in Kotlin functions?

Default parameters allow you to specify default values for function parameters. These can be omitted when the function is called if the default is sufficient.

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Explain inline functions and when should you use them?

Inline functions are used in Kotlin to reduce overhead from lambdas. Inlining substitutes the body of the function into the place where it is called, which can help with performance. Use them carefully, as they can increase the compiled code size if overused.


Discuss the differences between 'List', 'Set', and 'Map' in Kotlin.

'List' in Kotlin is an ordered collection with access to elements by indices. 'Set' is a collection of unique elements and 'Map' is a set of key-value pairs. Keys are unique, and each key maps to exactly one value.


How does Kotlin handle null safety and what is the Elvis operator?

Kotlin has null safety features built into its type system. The Elvis operator, ?:, allows for providing a default value if an expression evaluates to null, which prevents null pointer exceptions.


Can you demonstrate how 'reified type parameters' are used in inline functions?

Reified type parameters allow you to access the type at runtime, which is normally erased due to type erasure in Java. This is useful in inline functions since the type can be inlined as well.

inline fun  myGenericFun(value: Any) {
    if (value is T) {
        // Do something


Explain the difference between 'const' and 'val'.

'const' is used to define compile-time constants, meaning their value has to be assigned during compile time. Unlike 'val', which can be assigned a value during runtime, 'const' can only hold primitives and String types.


What is destructuring in Kotlin and give an example?

Destructuring allows you to declare multiple variables at once and initialize them with the properties of an object. For instance, with a pair you can do:

val (a, b) = Pair(1, "x")
// a = 1, b = "x"


What is a 'typealias' in Kotlin?

A 'typealias' is a feature that lets you provide alternate names for existing types, making the code more readable and understandable, especially when dealing with complex generic types.


Explain the difference between a coroutine and a thread in Kotlin.

Coroutines are lighter than threads as they don’t map one-to-one with the underlying threads. They can suspend and resume without blocking the thread, whereas threads can consume more resources and cause an overhead when there's a large number of concurrent tasks.


How would you implement dependency injection in Kotlin without a framework?

You can implement dependency injection in Kotlin by using constructor injection, property injection, or method injection manually without relying on a framework. However, using a DI framework (like Dagger or Koin) is usually more convenient.


Guess what does the following function returns:

fun  T.check(): T where T : Number, T : Comparable {
    return if (this >= 0) this else throw IllegalArgumentException("Number not valid")

The function returns the object itself if it's a non-negative number and implements the Comparable interface with T. If the number is negative, an IllegalArgumentException is thrown.


Explain the concept of smart casts in Kotlin.

Smart casts automatically cast types if they have been checked in a control structure. For instance, if you check a variable is of a certain type, Kotlin allows you to use that variable as that type without explicit casting.


What is the function of 'with' and 'apply' scope functions?

'with' is used to call multiple functions on an object within a block of code. 'apply' works similarly but returns the object itself after executing the block, usually used for configuration.


What are inline properties in Kotlin and what are their benefits?

Inline properties optimize your code by inlining the property's getter and setter methods wherever they are accessed instead of actually calling the method. This can reduce overhead.


Explain how to handle checked exceptions in Kotlin.

Kotlin doesn't have checked exceptions, which means you don't need to catch or declare any exceptions. However, you can still handle exceptions using try/catch blocks for better error management.


Demonstrate a use case for 'let' and 'run' scope functions with code examples.

val list: List? = getList()
list?.let { nonNullList ->

'let' is often used for null-checks and executing a block of code if the object is not null. It is also useful when you want to limit the scope of variables. 'run' is similar to 'let' but it's for scenarios when the receiver is already nullable or if you need to return a result from a block of code while using 'this' as the receiver. {

Kotlin Interview Tips

Understand the Problem

  • Regardless of the difficulty, break down the question into manageable parts. Listen carefully, ask clarifying questions, and don't rush your answer. Use the information given to align your response with what the interviewer is seeking. Assure them that your approach will be rational and phased.

Keep Calm and Demonstrate Confidence

  • Your composure during an interview is as informative as your answers. Being struck by a tough question can unsettle anyone, but maintaining calmness is key to navigating the challenge. Show confidence in your knowledge, and if unsure, tackle the question to the best of your understanding, showing openness to learn.

Refer to Past Experiences

  • Drawing parallels from your experience to the issue at hand can reflect your practical knowledge. Talk about similar situations you’ve faced and how you handled them. This reassures interviewers of your applied skills and problem-solving approach. Always be specific and relevant to the topic.

Show Enthusiasm and Curiosity

  • Conveying enthusiasm for the challenge a question presents can turn the situation in your favor. Showing that you're passionate about problem-solving and enthusiastic about tough tasks can distinguish you from other candidates. Additionally, expressing curiosity and a desire to delve deeper into the subject matter will indicate a continuous learning mindset.

Structure Your Answer

  • For tough questions, structuring your response can immensely improve its coherence. Start by summarizing the question, outline your understanding, and then present your answer in a clear, logical sequence. This will demonstrate your analytical abilities and ensure the interviewer follows your thought process easily.


How do I hire a Kotlin developer?

To hire a Kotlin developer, simply contact FireHire, and we'll connect you with pre-vetted, skilled developers matching your project requirements swiftly. Read more

What is the hourly rate of Kotlin developer?

The starting hourly rate for a Kotlin developer at FireHire is $45, with variations depending on experience and project complexity. Read more

What is the salary of Kotlin developer?

The salary for a Kotlin developer can vary widely by region and experience, but we offer competitive rates and flexible hiring models to fit your budget. Read more

Why using FireHire for hiring Kotlin developers is the best choice?

Using FireHire ensures access to top-tier Kotlin developers, an efficient hiring process, a 30-day replacement guarantee, and a vast network of tech expertise for your project.

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