2023 Top Kotlin Interview Questions and Answers - IQCode

Kotlin: A Powerful Programming Language

Kotlin is a dynamically typed, general-purpose, open-source programming language that is interoperable with Java. It is used in all projects where Java is applied, including the creation of Android apps, server-side applications and other programs. The language blends object-oriented programming with functional programming capabilities, making it a unique and powerful tool for developers. JetBrains team launched Kotlin in 2016, an improvement from Java, using the Apache 2.0 license.

One of the fantastic things about Kotlin is that it does not require semicolons, thus resulting in highly readable and easy-to-understand code. Here are some of the most notable Kotlin features:

  • Compact Code: Kotlin's code lines are 40 percent shorter than Java, making it the best choice for software development.
  • Open Source: Kotlin for Android is open-source and combines the benefits of object-oriented and functional programming.
  • Simple Language: It is straightforward to compile the code when working with Kotlin, resulting in improved performance for Android development, along with the ability to use data types throughout the code.
  • Large Number of Extensions: Kotlin supports a wide range of extension functions, making existing code more lovely and appealing to developers.
  • Full Java Interoperability: Kotlin and Java can work together. The use of Java based on the Kotlin environment is also possible.
  • Smart Cast: Kotlin uses typecasting or immutable data to handle programming efficiently, thereby reducing app cost and enhancing efficiency.
  • Low Learning Curve: The ease of learning Kotlin makes it an excellent choice for developers, particularly those having experience in programming.

Here is an example of one of the critical Kotlin interview questions for Freshers:

Question 1: What are the various data types available in Kotlin? Explain them.

In Kotlin, there are two types of data types:

  1. Primitive Data Types: These are the basic data types that can store only one value and have no secondary values. Kotlin supports the following primitive data types: int, double, float, short, long, char, and boolean.
  2. Non-Primitive Data Types: These are object data types with built-in functions. For example, classes, interfaces, and objects are non-primitive data types.

Kotlin is indeed an excellent choice for software development, and one can start learning it by exploring its official documentation.

Variable Declaration and Types in Kotlin

// Variable declaration in Kotlin
var age: Int = 27
val name: String = "John"

// Types of variables in Kotlin
// 1. Mutable Variables
var x: Int = 5
x = 10

// 2. Immutable Variables
val pi: Double = 3.14
val language: String = "Kotlin"

In Kotlin, variables are declared using the `var` and `val` keywords. The `var` keyword is used for declaring mutable variables while the `val` keyword is used for declaring immutable variables.

Mutable variables are those whose values can be changed while immutable variables are those whose values remain constant throughout their lifetime.

In the above code snippet, `age` and `name` are examples of variable declaration in Kotlin where `age` is a mutable variable holding an integer value of 27 and `name` is an immutable variable holding a string value of "John".

The `var` keyword is also used to declare and initialize a mutable variable `x` with a value of 5. We can then reassign the value of `x` to 10. On the other hand, the `val` keyword is used to declare an immutable variable such as `pi` holding a double value of 3.14 and `language` holding a string value of "Kotlin".

In summary, Kotlin supports both mutable and immutable variables, which can hold a variety of data types such as integers, strings, doubles, and so on.

What are Data Classes in Kotlin?

Data classes are a special kind of class in Kotlin that are primarily used to hold data. They make it easy to create classes that are meant to store and manipulate large amounts of data by automatically generating commonly used functions such as equals(), toString(), hashCode() and copy() methods. In simple terms, they are used when the sole purpose of the class is to store data.

Let’s take a look at an example to better understand the concept of data classes:

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

In the above example, we have created a data class Person with two properties: name and age. The “data” keyword before the class name tells the Kotlin compiler to generate certain methods for us.

Some of these automatically generated methods are:

  • getters/setters for the properties
  • hashCode()
  • equals()
  • toString()
  • copy()

With the help of the generated functions, we can easily compare and manipulate objects of this data class.

Concept of Null Safety in Kotlin

Null safety in Kotlin is a feature that helps prevent NullPointerExceptions (NPEs) in code. In Java, variables can be assigned a null value, which can lead to runtime exceptions such as NPEs. However, in Kotlin, null safety is built into the language, with the idea of "nullability" in mind. Variables and parameters in Kotlin must be explicitly marked as either nullable or not nullable. Non-nullable types are guaranteed to never hold a null value, preventing NPEs at runtime.

For example, declaring a variable as follows means it cannot hold a null value:

    var myVariable: String = "Hello world"

If you try to assign a null value to the variable above, a compilation error will occur. However, if you want to allow the variable to hold a null value, you can declare it as follows:

    var myNullableVariable: String? = null

The question mark after the data type indicates that the variable can hold a null value. When accessing the value of a nullable variable, Kotlin forces you to use a null check to avoid NPEs:


The safe call operator "?." prevents NPEs by returning null if the variable is null, instead of throwing an NPE at runtime. Null safety is a powerful feature of Kotlin that greatly improves the safety and reliability of code.

Explaining Safe Call, Elvis, and Not Null Assertion Operator in Kotlin

Kotlin has three operators that simplify working with null values: Safe call operator (?), Elvis operator (?:), and Not Null assertion operator (!!). Here's a brief explanation of each:

1. Safe call operator (?.): This operator checks for null values before accessing properties or functions of a variable. If the variable is null, the expression returns null, and the program continues executing without throwing a NullPointerException.


val text: String? = null val length = text?.length // length will be null

2. Elvis operator (?:): This operator is used to provide a default value when a variable is null. If the variable has a non-null value, that value is used. Otherwise, the expression after the operator (the default value) is used.


val text: String? = null val length = text?.length ?: 0 // length will be 0 because text is null

3. Not Null assertion operator (!!): This operator is used to tell the compiler that you know a variable is not null. If the variable is null, a NullPointerException will be thrown at runtime.


val text: String? = null val length = text!!.length // throws NullPointerException because text is null

These operators make it easier to work with null values and reduce the risk of NullPointerExceptions in your code. However, it's still important to use them appropriately and handle null values when necessary to ensure the stability and reliability of your program.

Differences Between Kotlin and Java

Kotlin and Java are both programming languages used to develop software applications. However, there are some major differences between them:

// Kotlin Code
fun main(args: Array<String>) {
    val message: String = "Hello World!"

// Java Code
public class Main {
    public static void main(String[] args) {
        String message = "Hello World!";

One of the major differences is that Kotlin is a statically typed programming language whereas Java is a dynamically typed programming language. In Kotlin, you have to declare the variable data type whereas in Java, the data type is inferred automatically.

Another key difference is that Kotlin is more concise and requires less boilerplate code compared to Java. For example, in Kotlin, you can define a function with a single line of code using the


keyword whereas in Java, you have to define the function using the

public static void


Kotlin also has a safer null handling mechanism compared to Java. It prevents null pointer exceptions by requiring you to explicitly check whether a nullable type variable is null or not. This feature is absent in Java, which requires you to add null checks manually.

Despite these differences, both Kotlin and Java can be used to build robust software applications and are widely used in the industry.

Types of Constructors in Kotlin

In Kotlin, there are mainly two types of constructors:

1. Primary Constructor 2. Secondary Constructor

1. Primary Constructor: Primary constructor is defined in the class header and it initializes the class properties. It is declared using the class name followed by the constructor keyword. Primary constructor does not contain any code block.


class Person(val name: String, var age: Int) {

In the above example, "name" and "age" are the properties of the Person class that are being initialized in the primary constructor using the "val" and "var" keywords respectively.

2. Secondary Constructor: Secondary constructors are used to initialize additional properties or to perform some extra computations on the existing properties of the class. It is declared using the "constructor" keyword. A class can have one or more secondary constructors.


class Person {
  var name: String = ""
  var age: Int = 0

  constructor(name: String, age: Int) {
    this.name = name
    this.age = age

  constructor(name: String) {
    this.name = name
    this.age = 18

In the above example, there are two secondary constructors. The first constructor initializes the "name" and "age" properties of the Person class with the values passed as arguments. The second constructor initializes only the "name" property and sets the "age" property to 18 by default.

Note: If a class has a primary constructor, then each secondary constructor must delegate to the primary constructor using the "this" keyword.

Methods to Iterate Over Data Structures in Kotlin

In Kotlin, there are various methods to iterate over any data structure.

One of the most common methods is the for loop. Here's an example:

val numbers = listOf(1, 2, 3, 4, 5)
for (number in numbers) {

Another method is the forEach loop. It allows you to specify an action to be performed on each element of the data structure. Here's an example:

val names = listOf("Alice", "Bob", "Charlie", "David")
names.forEach { name ->
    println("Hello, $name!")

You can also use the while loop for iteration. Here's an example:

val animals = setOf("cat", "dog", "rabbit", "hamster")
val iterator = animals.iterator()
while (iterator.hasNext()) {
    val animal = iterator.next()

The map function is another way to iterate over any data structure and transform its values. Here's an example:

val numbers = listOf(1, 2, 3, 4, 5)
val squares = numbers.map { number ->
    number * number

Lastly, the filter function allows you to select elements from a data structure based on a condition. Here's an example:

val numbers = listOf(1, 2, 3, 4, 5)
val evenNumbers = numbers.filter { number ->
    number % 2 == 0

These are just a few examples of the different methods available for iterating over data structures in Kotlin. Depending on your use case, one method may be more appropriate than others.

Concatenating Two Strings in Kotlin

In Kotlin, you can concatenate two strings by simply using the plus operator (+). Here's an example:

val string1 = "Hello"
val string2 = "world"
val result = string1 + " " + string2
println(result) // Output: Hello world

Alternatively, you can also use string templates to concatenate strings in Kotlin. Here's how:

val string1 = "Hello"
val string2 = "world"
val result = "$string1 $string2"
println(result) // Output: Hello world

Both methods produce the same output, so you can choose one based on your preference and code readability.

Function Extension in Kotlin

Function extension is a feature in Kotlin that allows adding new functions to existing classes, without modifying them. These functions can be called as if they were a part of the original class.

For instance, if we have a class `Person` that has basic properties such as `name`, `age`, and `gender`, we can use function extension to add new functionality to this class. We can define a new function `isAdult()` that checks whether a person is an adult or not, based on their age. This function can be called on any instance of the `Person` class, as if it were a part of the class definition:

fun Person.isAdult(): Boolean {
    return age >= 18

val john = Person("John Doe", 25, "Male")
val isJohnAdult = john.isAdult()

Here, we have extended the `Person` class with the `isAdult()` function, which takes no arguments and returns a Boolean value. We can call this function on an instance of the `Person` class, such as `john`, to determine whether the person is an adult or not.

Function extension is a powerful feature of Kotlin that allows developers to write more concise and readable code, by adding new functionality to existing classes without compromising the original implementation.

Understanding Companion Objects in Kotlin

In Kotlin, a class can have a companion object, which is an object that's tied to the class, rather than to instances of the class. The companion object can have properties and functions just like a regular object, but it's created using the 'companion' keyword.

One common use of companion objects is to define factory methods for a class. These methods can use private constructors to control the creation of objects, and they can create and return instances of the class using different parameters.

Another use of companion objects is to define constant values or utility functions that are closely related to the class. These values and functions can be accessed using the class name as a qualifier, rather than an instance of the class.

For example, consider the following code snippet:

class MyClass {
    companion object {
        const val MY_CONSTANT = "myConstant"
        fun myFunction() {
            // function body

In this example, we define a companion object for the class 'MyClass'. The companion object has a constant value 'MY_CONSTANT' and a function 'myFunction()'. These can be accessed using the class name as a qualifier, like this:

val myConstantValue = MyClass.MY_CONSTANT

Overall, companion objects are a powerful feature in Kotlin that allow you to define static members and factory methods for a class, as well as organize related code more effectively.

Difference between Open and Public keywords in Kotlin

In Kotlin, the keywords "open" and "public" have different meanings.

"Public" is an access modifier that allows a class, function, or property to be accessed from anywhere in the codebase. If a class, function, or property is declared as public, it can be accessed by any other piece of code that can see it.

"On the other hand, "open" is a keyword used to indicate that a class or function can be extended or overridden by another class. When a class or function is declared as open, it can be inherited from or overridden by a subclass or child class.

In summary, "public" is used for access control, while "open" is used for inheritance and extension.

Explanation of the "when" Keyword in Kotlin

The "when" keyword in Kotlin is a control flow statement that allows developers to evaluate a certain expression and perform different actions based on various conditions. It is similar to the switch statement in Java or C.

The syntax of the "when" statement is as follows:

when (expression) { condition1 -> action1 condition2 -> action2 condition3 -> action3 else -> defaultAction }

The "when" statement evaluates the expression and then checks each condition in sequence. If a condition is true, the corresponding action is executed. If none of the conditions are true, the default action is executed.

The conditions in a "when" statement can be any type of expression, including constants, variables, and functions. Additionally, multiple conditions can be combined using commas or ranges.

For example:

val score = 75 when (score) { in 90..100 -> println("A") in 80..89 -> println("B") in 70..79 -> println("C") else -> println("Fail") }

In this example, the "when" statement is used to determine the letter grade of a student based on their score. If the score falls within a certain range, the corresponding grade is printed to the console. If the score is outside of all ranges, "Fail" is printed.

Overall, the "when" statement is a useful tool for writing readable, concise code in Kotlin.

Advantages of Kotlin over Java

Kotlin has numerous advantages over Java, some of which are:

1. Concise code: Kotlin is a more concise programming language than Java, which means less code is required to perform the same task.

2. Null safety: Kotlin has a strong type system that eliminates the risk of null pointer exceptions at runtime.

3. Interoperability: Kotlin is fully interoperable with Java, which means that Kotlin code can be used alongside Java code in the same project.

4. Functional programming: Kotlin has features that support functional programming, which makes it easier to write concise and efficient code.

5. Extension functions: Kotlin allows adding methods to any class, even if the class is not owned by the developer.

6. Coroutines: Kotlin supports coroutines, which are used to perform long-running tasks without blocking the main thread.

Overall, Kotlin is a more efficient, reliable, and future-proof language than Java.

Kotlin Interview Questions for Experienced

Question 15:

When using Kotlin, which is better to use -

val MutableList


var ImmutableList



It depends on the requirement. If the list needs to be modified, then

val MutableList

should be used as it is a read/write list. On the other hand, if the list cannot be modified, then

var ImmutableList

should be used as it is a read-only list.

Understanding lateinit in Kotlin

In Kotlin, lateinit is used to define a non-null variable without initializing it during the declaration. It is used when the initialization value of a variable cannot be set in the constructor, but will be assigned later on.

Lateinit variables must be mutable and cannot be of primitive data types. This means that they can only be of class types.

Lateinit can be used in scenarios where the initialization of a variable is costly, and it is not required to initialize it immediately at the time of initialization. This can help to optimize the performance of the app.

It is important to note that accessing a lateinit variable before it has been initialized will result in a runtime exception. Therefore, it is crucial to initialize a lateinit variable before accessing it.

Explanation of Lazy Initialization in Kotlin

Lazy Initialization is a design pattern that allows the delayed loading of objects until they are actually needed. In Kotlin, this can be achieved through the use of the `by lazy` keyword.

Here's an example:

val myString: String by lazy {
    // some computation to obtain the value
    "Hello, World!"

In this example, `myString` is lazily initialized with the value "Hello, World!" when it is first accessed. The code inside the curly braces is only executed when `myString` is accessed for the first time.

Lazy Initialization can be useful for optimizing program performance since it avoids unnecessary computation and memory usage. However, it should be used judiciously, as overuse can lead to decreased readability and maintainability of code.

Differentiating between lateinit and lazy initialization

In Kotlin, lateinit and lazy initialization are two ways to initialize variables.

lateinit is used when you need to create a non-null object but you don’t have the value at the time of initialization. It tells the compiler not to initialize the value at the time of creation, but you are required to assign a value before using it. Lateinit initialization can only be done for mutable types and not for nullable and primitive types.

For example:

lateinit var myString: String

fun initializeString() {
    myString = "Initialized"

lazy initialization, on the other hand, is used when you don’t want to initialize a variable until you need it. In comparison to lateinit, you don’t have to initialize the value before using it, the value is assigned when first accessed. For this, it is only used for val types (immutable).

For example:

val myString: String by lazy { 
    println("String initialization")

In case you should use lateinit when you need to initialize mutable properties in the class not within a constructor and lazy initialization when initialization is lazy, and you want to access it only when needed, this helps you obtain the value without calculating it unnecessarily.

Understanding Coroutines in Kotlin

Coroutines are an essential feature in Kotlin that allow developers to write asynchronous, non-blocking code without requiring them to deal with threads explicitly. They can be seen as lightweight threads that can be launched, paused, and resumed, allowing for efficient management of system resources.

In practical terms, using coroutines can result in cleaner code, better task organization, and reduced code complexity, which ultimately leads to fewer bugs and a more maintainable codebase. They also enable more straightforward and intuitive handling of background work, such as network requests or database operations.

Overall, coroutines are a powerful tool that can help developers write high-performing, scalable applications with minimal overhead.

Scope Functions in Kotlin

In Kotlin, scope functions are higher-order extension functions that are used to define a scope and execute code within that scope. There are five different types of scope functions available in Kotlin: let, run, with, apply, and also.

1. let - This function is used when you want to perform an operation on a nullable object. It takes the object it is called on as its argument and returns the result of the lambda expression.

2. run - This function is used when you want to perform an operation on a non-nullable object and return the result of the lambda expression. It can also be used to initialize variables.

3. with - This function is similar to run, but it is used to perform operations on an object without the need to return a result from the lambda expression.

4. apply - This function is used when you want to configure an object. It takes the object it is called on as its argument and returns the same object.

5. also - This function is similar to let, but it is used for performing side-effects and does not return a result.

Scope functions are often used to improve the readability and maintainability of code in Kotlin, as they provide a concise way to execute code within a specific scope.

Understanding the Suspend Function in Kotlin

A suspend function is a Kotlin feature that allows the execution of long-running operations without blocking the execution thread. When a function is declared as "suspend," it indicates that it can be paused and resumed at a later stage, allowing other functions to be executed.

The suspend function provides an interface between the coroutine and the lower-level code that executes the operation. It is a way of making the long-running operation more efficient by allowing the thread that executes the operation to be released and used by other code.

To use the suspend function, it is necessary to implement a coroutine, which is a lightweight thread that can be used to perform background tasks. The main advantage of using suspend functions with coroutines is that it can simplify the code and make it easier to write asynchronous code that is easy to read and maintain.

Overall, the suspend function is an essential part of the Kotlin language that allows developers to write more efficient and high-performing code.

Understanding Sealed Classes in Kotlin

In Kotlin, a sealed class is a special type of class that can only be inherited by a fixed set of classes declared within it. It provides a way to restrict the implementation of a class hierarchy and ensure that all possible variations of a type are accounted for.

Sealed classes are often used to model restricted class hierarchies: a perfect use case is when we want to define a type that can take on only certain values and disallow any other value.

A sealed class is declared using the "sealed" modifier, and all subclasses must be declared within the same file where the sealed class is declared. This allows the compiler to know all possible subclasses and warn developers if they forget to handle a specific case in a when-expression, for example.

Here is an example of a sealed class in Kotlin:

sealed class Result<out T>

class Success<out T>(val value: T) : Result<T>()

class Failure(val error: Throwable) : Result<Nothing>()

In this example, the "Result" class is sealed and has two subclasses: "Success" and "Failure". The "Success" class takes a generic type "T" and stores a value of that type. The "Failure" class stores an instance of a Throwable.

By using a sealed class, we can ensure that all possible outcomes of a function that returns a Result type are accounted for in a when-expression, improving the safety and readability of our code.

Understanding Backing Fields in Kotlin

In Kotlin, a backing field is a field that is automatically generated by the compiler when a property is declared. It is used to store the actual data of the property.

For example, consider the following code snippet:

var name: String = "John"
    get() = field
    set(value) {
        field = value.capitalize()

In this code, the `name` property has a backing field called `field`, which is used to store the actual value of the property. The `get()` and `set()` methods are called whenever the property is accessed or modified, respectively.

Backing fields are hidden by default, but can be accessed explicitly using the `field` keyword. They are useful for adding custom behavior to properties without having to manually implement getters and setters.

Differentiating Between Launch/Join and Async/Await in Kotlin

In Kotlin, both Launch/Join and Async/Await are used for concurrency and parallelism. However, there are significant differences between them.


- Launch creates a new coroutine and runs it asynchronously. - Join, on the other hand, waits for the launched coroutine to complete before moving on to the next line of code. - Launch/Join are useful for running a coroutine separately from the main thread.

Here's an example:

fun main() {
    GlobalScope.launch {

In this example, the coroutine launched by GlobalScope.run() prints "Hello" and "Kotlin" after a delay of one second each. The join() function is called to wait for the coroutine to complete before printing "World".


- Async creates a new coroutine and returns a Deferred object. - Await waits for the Deferred object to complete and returns the result. - Async/Await are useful for running multiple coroutines concurrently and then waiting for their results.

Here's an example:

fun main() {
    val deferred1 = GlobalScope.async {
    val deferred2 = GlobalScope.async {
    GlobalScope.launch {
        val combined = "${deferred1.await()} ${deferred2.await()}"

In this example, two coroutines are launched using GlobalScope.async(), with each coroutine returning a String after a delay of one second. The await() function is called on both Deferred objects to wait for the results. The two Strings are combined into a single String and then printed. Again, the join() function is used to wait for the launched coroutine to complete before printing "World".

To summarize, Launch/Join is useful for running independent coroutines sequentially whereas Async/Await is useful for running multiple coroutines concurrently and then waiting for their results.

What are some of the disadvantages of Kotlin?

Kotlin is a great programming language, but it also has some drawbacks. One disadvantage is that its compilation time is slower compared to some other programming languages. Another downside is the lack of availability of developers who have expertise in Kotlin, making it challenging to find qualified professionals to hire. Additionally, there may also be compatibility issues with some older libraries or frameworks that are still written in Java.

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