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20 Kotlin interview questions for junior and senior developers

20 Kotlin interview questions for junior and senior developers

Tabnine Team /
5 minutes /
November 21, 2019

Kotlin Interview Questions

Android boasts the largest smartphone OS market share, as it currently sits at 76%, while predictions say that it will grow in the future. 

This means that building an Android app can be a successful and lucrative business idea. 

But, for translating this idea into a reality, you need a well-versed and skilled Kotlin developer or a dev team. Kotlin is a general-purpose programming language and as of May 2019, it’s a preferred choice of Android app developers.

Just like Angular.JS is one of the most powerful and useful front-end frameworks for developing web apps – here’s a list of Angular interview questions in case you need someone to help you create amazing UI, the same can be said for Kotlin when it comes to building Android apps. 

However, business owners and HR professionals who are in charge of the screening, interviewing, and hiring process are rarely development experts. This means they aren’t familiar with the nitty-gritty of development from a technical perspective, which can result in their failure to focus on a particular set of skills and characteristics vital for a good Kotlin developer. 

This guide will help you navigate your interviews and identify your ideal hire, and if you need a break for interview questions there are also a few job interview memes to keep you entertained, we aim to please, after all.

job interview questions funny

Interview Questions and Answers for Junior Kotlin Developers

These couple of questions will help you pick a junior developer who’s the right fit for your project. Given that they would work under the supervision of senior Kotlin developers and execute the tasks given by them, it’s essential to understand that your new hire doesn’t have to be an expert in the field but someone with the potential to learn and grow. 

1: What is Kotlin?

A: Kotlin is a statically-typed language that runs on the Java Virtual Machine. It can seamlessly interoperate with Java and is considered an alternative to the standard Java compiler. 

2. Is Kotlin better than Java? 

A: It’s simpler and much cleaner than Java, as it’s more concise and uses fewer lines of code in order to solve the same problems. Besides being generally more efficient and effective, Kotlin is safer in the sense that it prevents certain common coding errors. As a result, there are fewer app crashes and system failures. Kotlin comes with some features that aren’t available in Java, such as smart casts, operator overloading, data classes, null safety, coroutines, to name just a few. 

3. What are extension functions in Kotlin?

A: It’s a mechanism that allows extending a class without the need to inherit from the class or use Decorator or other design patterns. 

4. What is null safety? 

A: This is a feature that prevents a null reference exception, the equivalent of Java’s NullPointerException in real time. 

kotlin job interview questions meme

5. Is there a ternary operator like in Java? 

A: No, there isn’t, but what a ternary operator does can be achieved through an if-else expression. 

6. What’s the difference between val and var in Kotlin?

A: Both are used for declaring a variable. However, while val is a final variable that is immutable and can be initialized only once, var is a general variable that can be assigned multiple times. 

7. What are the basic data types in Kotlin? 

A: Numbers, characters, Booleans, arrays, and strings. 

8. How many constructors are there in Kotlin? 

A: A constructor’s main role is to initialize the properties of a class. In Kotlin, there can be a primary constructor and one or more secondary constructors. 

9. What are the extension methods that Kotlin provides to the Java.io file list? 


  • bufferedReader():read contents of a file to BufferedReader
  • readBytes() : read contents of a file to ByteArray
  • readText(): read contents of a file to a single String
  • forEachLine() : read a file line by line in Kotlin
  • readLines(): read lines in a file to List

10. What are the structural expressions in Kotlin? 

A: There are three structural expressions:

  • Return: helps to return from the closest function or an anonymous function. 
  • Break:  helps to terminate the closest enclosing loop. 
  • Continue: helps proceed to the next step of the closest enclosing loop. 

Interview Questions and Answers for Senior Kotlin Developers 

If you need to hire a more experienced Kotlin developer, here are some questions that can help you pick the one that will be able to run your project properly and monitor junior developers. 

1. What is the difference between const and val? 

A: Const is a keyword used to define compile-time constants, while val is used to define runtime constants. 

2. Is it possible to migrate code from Java to Kotlin? 

A: Yes, it is. IntelliJ IDEA is a tool that can help you do that. 

3. What are data classes in Kotlin?

A data class is similar to a regular class with certain additional functionalities. For example, in order to store data items in Java, it’s necessary to create a class by setting the variables. In Kotlin, all it takes is adding keyword data to the class, and the compiler will automatically generate a default getter and setter. So, basically, there’s no need to write or generate the extensive boilerplate code. 

4. What modifiers are available in Kotlin?

A: Kotlin offers four visibility modifiers:

  • Private: visible inside this class and all its members only.
  • Protected: the same as private, but visible in subclasses too.
  • Internal: visible inside the same module 
  • Public: by default, all the declarations are visible to everyone. 

5. What is a string in Kotlin? 

A:  It’s a basic data type. Strings are immutable and Kotlin has two types of string literals: 

  • Raw strings, which are delimited by a triple quote and can contain newlines, arbitrary text, and any other character.   
  • Escaped strings, which handle special characters by escaping them. 

6. What is the Init Block?  

A: It’s the initializer block and the code inside of it is executed when the primary constructor is instantiated. 

7. Is new a keyword in Kotlin? 

A: No, it’s not and it doesn’t have to be used to instantiate a class. 

8. Are there primitives in Kotlin? 

In a nutshell, there are no out-of-the-box primitives in Kotlin – they can’t be created at a code level. Given that Kotlin has been designed to cooperate with Java and the JVM seamlessly, certain types like int or float can act like primitives in certain cases. 

9. Is there an equivalent for switch in Kotlin?

A: Yes, there is. The When keyword is used instead. 

10. What are deconstructing declarations? 

A: Kotlin comes with a functionality that allows developers to assign multiple values to variables. This kind of syntax allows for creating multiple variables that can be used independently at once.

These 20 questions can help you sift through a large number of Kotlin dev applicants and find the one who’ll help you build amazing Android apps.