Top 25 Kotlin Interview Questions And Answers in 2024

Editorial Team

Kotlin Interview Questions And Answers 

Kotlin is a modern programming language that enjoys popularity thanks to its solid back-end development features. This cross-platform general-purpose programming language fully interoperates with Java. It first appeared in 2011 and supports nine paradigms, i.e., concurrent, object-oriented, functional, declarative, block-structured, reflective, generic, imperative, and multi-paradigm.

We may have something for anyone skilled in Kotlin looking for a job as our article covers some of the common Kotlin interview questions to expect in your job hunt. We have included responses to help you come up with the best answers if any of these questions come up in your interview. Let’s get started. 

1. Why Do You Prefer Using Kotlin In Your Programming Tasks? 

Kotlin has several features that make it my favorite programming language. It is a good choice for software development thanks to its code lines that can be reduced significantly, giving you compact code. It can also support several extension functions without code modifications making the existing code more appealing. Lastly, Kotlin is fully interoperable with Java, allowing Java code to use Kotlin code and vice versa. The interoperability also makes it possible to use Java-based applications within Kot lin’s environment. 

2. Explain The Different Data Types In Kotlin 

Kotlin has both primitive and reference data types, all contained as objects. They include Boolean data type, similar in both Kotlin and Java and represents true or false; character data type which represents small and capital letters from a-z, numerals from 1-9, and a myriad of symbols; array data type, which represents arrays; string data type represented by the type String with characters enclosed by double quotations; floating data type divided into float and double data types, and lastly number data type divided into six sub-units. 

3. Do You Know The Different Types Of Constructors Found In Kotlin? 

Yes. Kotlin has primary and secondary constructors which serve different purposes. The primary constructor succeeds the class name and is often initialized in the class header. It doesn’t need any parameters and can be declared using the ‘constructor’ keyword, which can also be omitted if annotations or access modifiers are unavailable. On the other hand, the secondary constructor is preceded by the constructor keyword and comes in handy in adding logic to the class and initializing variables. 

4. Differentiate Between The Two Types Of Keywords In Kotlin

Kotlin comes with open and public keywords, which have different meanings and serve distinct functions. The first and most commonly used keyword is ‘open,’ translating into ‘open for expansion.’ This type of keyword denotes an open method that can be overridden, albeit not by default. The public type of keywords is used in the absence of a visibility modifier. It denotes that a user can access all the declarations from any point inside the program. 

5. Do You Think That Kotin Towers Above Java In Some Aspects, Especially Since You Have Used Both? 

Kotlin has a number of features and capabilities that Java doesn’t enjoy. For example, Kotin supports extension functions for clearer and cleaner code, while Java does not. Kotlin is also more compact and expressive than Java, leaving relatively less room for errors. Other reasons include its null safety feature, support for a common codebase, and the fact that you don’t have to create getters and setters for every object like in Java. 

6. Define Lazy Initialization 

Lazy Initialization is a function that only initializes an object during its usage. It is handy for classes with objects capable of delaying the class creation process due to time-consuming initialization. It contributes to a faster and more efficient code as it prevents the initialization of an object that is not used throughout the process. 

7. What Do You Understand By A Suspend Function In Kotlin? 

Kotlin has suspend functions that can be started, paused, and later resumed. However, they can only be invoked from a coroutine or a different suspend function. A code that calls a suspending function will easily pause execution and restart later. It’s also important to mention that even though suspending functions can easily call ordinary functions, it takes another suspending function to prevent this execution if the need arises. 

8. What Is The Difference Between Launch/Join And Async/ Await Commands In Kotlin? 

In the launch/ join pair, the launch command starts and stops a coroutine as if a new thread has started. The other command, join, gives the launched coroutine time before propagating its exception. In the async/ await command, the async keyword initiates a coroutine tasked to compute a result. The await keyword, represented by an instance of Deferred, must be used on the result. Any uncaught exceptions are not transmitted and cannot get executed until they are processed. 

9. From Experience, What Are Some Of The Shortcomings Of Kotlin? 

Despite its advantages, Kotlin also has several shortcomings capable of negatively impacting user experience. First, Kotlin lacks checked exceptions, which makes it relatively difficult to determine if a code is stable. Also, being a relatively younger language, Kotlin needs more learning resources, which is a huge disadvantage given the number of developers using it for their software development tasks. Other disadvantages include less visibility, given that functions such as getters, tostring, and setters are automatically added for you; varying compilation speeds, which can inhibit its ability to achieve clean builds; and the presence of keywords with non-obvious meanings. 

10. What Made You Switch From Java To Kotlin? 

Even though Java is one of the most powerful programming languages I have used in my career, I was drawn to Kotlin’s compactness and unique features. It is easier to write simpler and cleaner codes with Kotlin as compared to Java, which is a huge selling point. Kotlin is also tool-friendly, allowing developers to build programs using the command line and Java IDE. Other reasons include the language’s support for different extension functions and properties without any code modification requirement and its conciseness which eliminates the need of writing extra codes. 

11. Walk Us Through The Different Variables In Kotlin And How You Would Declare Them? 

Kotlin has two types of variables, immutable and mutable. The immutable variables, commonly known as read-only variables, are declared via the val keyword. Their values don’t have to be known during compile-time and stay permanent after declaration. Mutable variables are flexible since their values can be changed. They are declared using the keyword ‘var’. It’s important to declare all the Kotlin variables before use to avoid a syntax error. The type of variable declared also determines the type of data that goes into the memory address. 

12. What Are Data Classes? Can You Tell The Functions Automatically Derived By The Data Classes Compiler? 

A data class is a class that contains data and offers typical functions. For data class consistency, it is important to have at least one perimeter for the primary constructor, which can only be val or var, and to use data classes for interface implementation only. A stable data class will not support open, sealed, abstract, or inner data classes. 

The functions automatically derived by the data classes compiler include toString(), which returns a string with all the data class’s parameters; equals(), which brings back True if two objects have similar contents; copy()which duplicates and objects and changes select characteristics leaving the rest untouched and hashCode() uses to derive an object’s hashCode value. 

13. What Do You Understand By The Null Safety Feature In Kotlin? 

Kotlin has a null safety feature in its type system that removes null references from a particular code. This helps prevent a system crash or application failure, which occurs when a program throws NullPointerExceptions as it runs. Kotlin can also differentiate between nullable references and non-null references. I should also add that assigning null to a string variable will result in a compiler error. 

14. Mention And Explain The Types Of Operators Provided By Kotlin In A Null Encounter

Kotlin provides three main operators, i.e., safe call ( ?.), Elvis (?:), and Not Null Assertion (!!) operators, to dictate the best course of action in the event of a null encounter. The safe call operator comes in handy for trivial null comparisons with an exhausting number of nested if-else expressions by allowing null check and method call performances using a single expression. The Elvis operator, the second option provided, returns a non-null/ default value for null variables. Lastly, the Not Null Value changes a null value into a non-null type. It is used to ask for a NullPointerException. 

15. What Are The Main Differences Between Kotlin And Java? 

Even though Kotlin and Java are interoperable, they slightly differ in features. First, all Kotlin variables are non-nullable by default, making it impossible to assign null values to them, while Java allows developers to assign null to variables. Java only throws the null pointer exception, which can still be managed,  when an object reference is accessed with a null value. Also, Kotlin supports functional programming, given that it is a procedural and functional programming language with features such as lazy evaluation, while Java only allows functional programming for Java 8 and above users. Other differences include the checked exceptions feature available in Java but not Kotlin, a better smart casting ability with Kotlin as compared to Java, and the mandatory variables declaration requirement in Java which does not apply to Kotlin. 

16. How Do You Normally Iterate Over Kotlin Data Structures? 

Data structures in Kotlin can be iterated using three methods. The first one, for loop, scans any data structure supplying an iterator. It is unique and doesn’t share a similar working mechanism as the for loop function in other programming languages. The second method is While Loop, which has a code block and a condition that must first be assessed before the code block executes. The last method is the Do While Loop, where the condition assessment only occurs after the block statements execute. 

17. Why Do Most Developers Prefer Immutable Lists? Are There Any Disadvantages? 

Immutable lists are only used for view-only models. They are mostly preferred because of their functional programming support, evident in collection methods such as reduce, filter and map. They cannot be altered, meaning that the object’s value stays permanent right from the definition. Lastly, they come in handy in multi-threaded systems with no write access requirements. Despite the benefits, using immutable lists also comes with some disadvantages. It becomes tedious to add/remove simple pieces since the entire collection must be copied, and the code gets more difficult when single lines are frequently altered. 

18. What Do You Understand By Lateinit In Kotlin? 

Lateint is a common keyword and abbreviation in Kotlin that fully means late initialization. It is used to postpone the initialization of a constructor. However, the keyword does not work for primitive attributes such as Long and Int. It’s also impossible to use Val because of the late initialization of the constructor. Accessing a Lateinit property before initialization reveals a special exception that identifies the property and points out that it still needs to be initialized. 

19. Differentiate Between Lateinit And Lazy Initialization 

There are five main differences Between Lateinit and Lazy Initialization. First, Lateint seeks to postpone the initialization, while lazy Initialization initializes an object later when it is in use. Unlike lateint, it also maintains a single copy of the object during the entire program. Second, Lateint allows object initialization from anywhere in the program, while in lazy initialization, only the initializer lambda has the ability. Third, lateint supports multiple initializations, whereas the latter only supports one initialization. Fourth, Lateint is not thread-safe, while in lazy initialization, thread-safety kicks in by default, meaning that the user does not have to ensure correct initialization. Lastly, lateint works with var, while lazy Initialization works with Val. 

20. How Do You Normally Decide When To Use Lateinit Or Lazy Initialization? 

I normally consider three factors when deciding whether to use Lateinit or Lazy Initialization for property initialization. They include whether the property is mutable or immutable, where the properties are set, and the number of times they should be initialized. Lateinit comes in handy when the properties are mutable or have subjects that can change at a later period and when they are set externally. Lazy Initialization applies when a property is more internally set, meant to be initialized once, and can be accessed by everyone. 

21. Define Scope Functions In Kotlin 

Kotlin has several functions that help code execution within an object’s context. They generate a temporary scope when a lambda expression is used to call them on an object, hence the term scope functions. They are also unique, given that their objects can be accessed without being identified. These functions contribute to Kotlin’s superiority as a programming language by making the code more concise, legible, and clear. 

22. Explain The Different Types Of Scope Functions Found In Kotlin 

There are five main scope functions in Kotlin, i.e., let:-, apply:-, with:-, also:-, and run:-. Let:- is mostly used to initiate null safety calls in collaboration with the safe call operator (?.). It returns a lambda result and has ‘it’ as its context object. Apply is used to initialize receiver object members. Its return value is its context object ‘this.’ The third scope function is ‘with,’ whose return value is a lambda result. It is best used to call functions in context objects without giving the lambda result. The fourth type is also used for any additional operations after the initialization of object members. The last scope function is run:-, used to make null safety calls and an array of calculations. It is also used where the object lambda requires value initialization and computation. 

23. Tell Us More About The Sealed Classes In Kotlin 

Kotlin provides a unique class known as sealed classes, consisting of a series of subclasses that comply with class hierarchies. Sealed classes ensure type safety by directing the compiler to validate different types during compilation and throw an exception whenever a variable is paired with a wrong type. 

24. Which Improvements Would You Like To See In The Future When It Comes To Kotlin? 

Even though Kotlin is a great programming language as it is, there is still room for improvement. First, it should be more visible to allow users to trace a program’s logic like in its counterpart Java, which will come in handy in processes such as bug hunting. It should also work on its compilation speeds, which vary depending on the operation. Lastly, Kotlin should include checked exceptions that help programmers determine whether they have stable code. 

25. Which Programming Languages Are You Proficient In Apart From Kotlin? 

I am proficient in 7 programming languages owing to my love for coding and software development. I learned Python at the age of 17 and Java, which became my favorite programming language, a while later and then heard of Kotlin. I am also proficient in C#, Swift, PHP, and C++, which makes me confident that I can handle any software development task thrown my way. 


These are a few good recommendations to prepare you for your upcoming Kotlin interview. Remember to brainstorm only the best responses to increase your chances of landing the job. Additionally, work on giving the interviewer an excellent first impression, and you will be good to go.