Top 20 Kotlin Interview Questions and Answers for Java developers in 2023

Hello guys, if you are preparing for Kotlin Developer Job interview and looking for frequently asked Kotlin interview questions then you have come to the right place. In the past, I have shared Core Java questions, Scala interview questions as well as Groovy Questions and in this article, I am going to cover common Kotlin questions from interviews. In last a couple of years Kotlin has emerged from the shadow of Java and demand for Kotlin is increasing day by day. Google's decision to accept Kotlin as the official programming language for Android development triggered this rise and now Kotlin is also viewed as preferred backend development language, a shock for Java and Scala but its true. 

I have attended a couple of Kotlin interviews in past and the level of Kotlin question is still not that deep as in case of Java as there are not many Kotlin expert in the market yet. This is a great thing to consider as demand is high and supply is low which also means this is probably the best time to learn Kotlin and get that Kotlin developer job with the raise. 

If you don't like Kotlin or struggle with Kotlin development you can always come back as Java developer and with Kotlin knowledge in your belt  you can even command higher salary than what you are getting now. 

Apart form Android app development, that's another reason I have been suggesting Java developer to learn Kotlin in 2023. Now coming back to questions, they are very simple and just test your basic Kotlin knowledge. If you know Kotlin or used Kotlin in past you can easily answer these questions but if you struggle then I suggest you to go through these best Kotlin online courses to learn and revise essential Kotlin concepts.  

20 Kotlin Interview Questions with Answers for beginners [2023]

If you want that Kotlin Developer job you always wanted then you have to be equipped with knowledge as far as Kotlin programming language is concerned. This knowledge will help you do well when you face the interview panel on the day for your Kotlin interview . 

You can only claim you have knowledge if you can satisfactorily answer questions regarding Kotlin. These 20 Kotlin interview questions will give you the much needed revision and practice you need before going for your real Kotlin Interview. 

1. What is Kotlin?
Answer: Kotlin is a statically-typed programming language which runs on the JVM. It can be compiled either using Java source code or LLVM compiler.

2. How many constructors are available in Kotlin?
Answer: Two types of constructors available in Kotlin are:

  • Primary constructor
  • Secondary constructor
  • Who is the developer of Kotlin?
  • Answer: Kotlin was developed by JetBrains.

3. What are the most important benefits of using Kotlin?

  • Kotlin language is easy to learn as its syntax is similar to Java.
  • Kotlin is a functional language and based on JVM. So, it removes lots of boiler plate
  • It is an expressive language which makes code readable and understandable.

4. What are the basic data types used in Kotlin?
Answer: Here is the list of basic data types you will find in Kotlin programming language:

  • Strings
  • Numbers
  • Arrays
  • Booleans
  • Characteristics
Here is a nice diagram which shows the basic data types used in Kotlin programming language:

Kotlin data types

5. Which type of programming does Kotlin support?
Answer: Kotlin supports only two types of programming which are as follows:

  • Procedural programming
  • Object-oriented programming

6. What are the structural expressions in Kotlin?
Answer: There are three Structural expressions in Kotlin and are as follows:

  • Return: It returns from the nearest enclosing function or anonymous function by default.
  • Break: This expression terminates the closest enclosing loop.
  • Continue: This expression proceeds you to the next closest enclosing loop.

7. What are the modifiers available in Kotlin?

  • Access modifier in Kotlin provides the developer to customize the declarations as per the requirements. Kotlin provides four modifiers. They are:
  • Private: This makes the declaration visible only inside the file containing a declaration.
  • Public: It is by default, which means that the declarations will be visible everywhere.
  • Internal: This makes the declaration visible everywhere in the same modules.
  • Protected: This keeps the declaration protected and is not available for top-level declarations.

8. What is null safety in Kotlin, and why is it important?
Answer: Null safety is a feature of Kotlin that ensures that variables cannot be assigned a null value. This is important because it helps avoid NullPointerException errors, which can be difficult to debug.

9. What are some of the basic syntax rules for Kotlin?
Answer: Some of the basic syntax rules for Kotlin include the use of semicolons to terminate statements, the use of curly braces to delimit blocks of code, and the use of the val keyword to declare immutable variables.

Top 20 Kotlin Interview Questions with Answers for Beginners and Experienced

10. What is a lambda expression in Kotlin?
Answer: A lambda expression is an anonymous function that can concisely represent a function with a single parameter. Lambda expressions are often used in conjunction with higher-order functions, such as map and filter.

11. How do you declare a default argument in Kotlin?
Answer: To declare a default argument in Kotlin, use the default keyword when defining a function parameter. This will specify a default value for that argument, which will be used if no actual value is supplied when calling the function.

12. Can Kotline offer any additional functionality for standard Java packages or standard Java classes?
Answer: Kotlin programs can run on standard JVM like any another compiled Java code. It allows JVM to compile any program to byte-code. It is accessible using Java Virtual Machine. Therefore, Kotlin is almost similar to Java. Moreover, Kotlin applications can be built with parts of Java code.

13. What is the difference between open and public keywords in Kotlin?
Answer: The keyword “open” refers to the term “open for expansion”. The open annotation on a class is the polar opposite of the final annotation in Java: it allows others to inherit from it. By default, a class cannot be inherited in Kotlin. 

In Kotlin, an open method signifies that it can be overridden, whereas it cannot be by default. Instead, any methods in Java can be overridden by default.  Also, all the classes are public by default. If no visibility modifier is specified, public is used by default, which means our declarations will be accessible everywhere inside the program.

14. What are the advantages of Kotlin over Java?
Answer: Here are key advantages of Kotlin over Java:

  • Data class: In Java, you must create getters and setters for each object, as well as properly write hashCode (or allow the IDE to build it for you, which you must do every time you update the class), toString, and equals. Alternatively, you could utilize lombok, but that has its own set of issues. In Kotlin, data classes take care of everything.
  • Patterns of getter and setter: In Java, for each variable, you use it for, rewrite the getter and setter methods. You don’t have to write getter and setter in kotlin, and if you must, custom getter and setter take a lot less typing. There are additional delegates for identical getters and setters.
  • Extension Functions: In Java, there is no support for extension functions. Kotlin on the other hand provides support for extension functions which makes the code clearer and cleaner.
  • Support for one common codebase: You may extract one common codebase that will target all of them at the same time using the Kotlin Multi-Platform framework.
  • Support for Null Safety: Kotlin has built-in null safety support, which is a lifesaver, especially on Android, which is full of old Java-style APIs.
  • Less prone to errors: There is less space for error because it is more concise and expressive than Java.
Here is a nice diagram which illustrate the difference between Kotlin and Java, you can see what you can achieve in just 3 lines of code in Kotlin which normally takes 20 lines in Java:

What are the advantages of Kotlin over Java?

15. What is the difference between a data class and a regular class in Kotlin?
Answer: A data class is a class intended to hold data. A regular class is a class that can perform arbitrary operations. Data classes are typically simpler and more efficient than regular classes.

16. How do you declare a function in Kotlin?
Answer: To declare a function in Kotlin, use the fun keyword followed by the name of the function and the parameters that it accepts. You can then define the function body within curly braces.

17. What is the difference between a suspend function and a regular function?
Answer: A suspend function can be suspended, meaning that you can pause its execution and resume it at a later time. A regular function cannot be suspended and will always execute to completion.

18. What are coroutines in Kotlin?
Answer: Coroutines is one of the amazing Kotlin features that allow for concurrent or parallel execution of code. Coroutines can improve application performance by taking advantage of multiple cores in a processor. 

Here is a nice diagram which shows how you can use multiple coroutine inside each thread in a Kotlin application:

What are coroutines in Kotlin?

19. How do you cancel a coroutine in Kotlin?
Answer: Call the cancel function on the CoroutineScope object to cancel a coroutine in Kotlin. This will cancel the coroutine and free up any resources.

20. What is type inference in Kotlin?
Answer: Type inference is the process of automatically determining the type of a variable or expression based on its value. In Kotlin, type inference determines the type of variables when they are first declared and the return type of functions.

That's all about the frequently asked Kotlin Interview Questions and answers for beginners and experienced Java developers. You must now be more than ready to handle the Kotlin interview that is awaiting you. 

You are at a better position as far as passing your interview is concerned. This is because you have found the right questions with answers that you will face during the interview. Just make sure you master everything and you will smile all day.

I have purposefully not included more in-depth Kotlin question in this article but if you guys would like, I can do that, just let me know your experience in comments. You can also share any Kotlin question you may have or something which was asked to you and you couldn't answer during interview.
Other Interview Question Articles You may like to explore
Thanks for reading this article so far. All the best for your Kotlin Developer interviews and if you have any questions which don't know answer or any doubt feel free to ask in comments. 

P. S. - If you are new to Kotlin and want to learn Kotlin in depth, then I highly recommend you check out these best Kotlin courses from Udemy, which are both comprehensive and very engaging. You can use this course to learn Kotlin for server-side Java development and mobile app development with the Android platform.

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