CodeSubmit Interview Series

The 8 Best Kotlin Interview Questions in 2023

Looking for your next Kotlin developer? Then, you need the right interview questions. Here are the top Kotlin interview questions. Read on!

Interviewing Kotlin developers 

Kotlin is a statically typed, general-purpose, cross-platform high-level programming language with type interference. Kotlin was developed for the Java Virtual Machine and Android to be used to create Android apps, server-side apps, and other applications. The focus is on safety, clarity, interoperability, and tooling support, and can be used in all projects that use Java

But how do you identify the best Kotlin developers? 

Start with these interview questions: 

Beginner Kotlin interview questions

What are the top junior Kotlin developer questions? Here are some of the top questions to ask at your next interview. 

1. What are the main benefits of Kotlin? 


Kotlin comes with a number of benefits, including: 

  • Simple: Kotlin is simple to learn and use. Compiling code is simple, which improves overall performance. 

  • Concise: Kotlin is concise as it reduces code writing. 

  • Compact: Kotlin code lines can be reduced by 40% compared to Java, as an OOPs-based language. 

  • Open source: Kotlin is open source for Android and it uses the Java Virtual Machine to combine the benefits of OOPs and functional programming. 

  • Extensions: Kotlin supports plenty of extension functions and properties without modifying the code. It can extend the functionality of classes without impacting the code. 

  • Null safety language: Kotlin aims to eliminate the NullPointerException from the code. 

  • Full Java interoperability: Kotlin and Java work together and you can use Kotlin in a Java project. In the same way, Java applications can be used with Kotlin’s environment. 

  • Compilation time: Kotlin performs faster than Java thanks to its fast compilation time. 

  • Smart cast: Kotlin uses the smart cast technique, meaning that the cost of an application can be reduced while speed and performance are improved. 

2. What are the main disadvantages of Kotlin? 


One of the main disadvantages of Kotlin is the non-obviousness of the meaning of keywords, as well as the inconsistency of keywords (for instance, function is “fun” but interface is “interface”). 

Another disadvantage is that Kotlin doesn’t have checked exceptions to ensure that the code is robust. Kotlin also hides a lot without offering a way to track things, something that is important for finding bugs. 

Other features that Java has but Kotlin does not include checked exceptions, static members, non-private fields, wildcard types, and primitive types that are not classes. 

3. Explain the difference between “open” and “public.”


Open means “open for expansion” and in a class, it’s the opposite of Java’s final. Others can inherit from the class.

By default, however, a class can’t be inherited in Kotlin.  An open method can’t be overridden by default, which it can in Java; only if it’s open. 

However, public is used by default if no visibility modifier has been specified. Your declarations will be visible everywhere. 

Advanced Kotlin interview questions

What are the top junior Kotlin developer questions? Here are some of the top questions to ask at your next interview. 

4. Is it better to use val mutableList or var immutableList in Kotlin? 


Mutable and immutable lists improve design clarity as they push the developer to think about and clarify the collection’s purpose. Mutable lists are used if the collection will alter as part of the design and immutable lists if the model is meant to be viewed. 

Val and var, again, have specific purposes. They specify how a variable’s value or reference should be handled. Var is used when the value or reference can be altered at any moment and val when the variable’s value or reference can be assigned once and can’t be modified later on. 

Often, immutable lists are preferred because they promote functional programming where state is passed to the next function. This way, a new state is constructed by being based on it, instead of being altered. 

It’s also often easier to understand and debug software. Immutable resources don’t induce race conditions because no write access is required in multi-threaded systems. 

But there are downsides, too. Copying large collections to add or remove a single piece is expensive in immutable lists. Code can also become more difficult when you need to alter single fields frequently.

5. Define coroutines in Kotlin. 


Kotlin doesn’t have async and await keywords. Kotlinx.coroutines is a rich library for coroutines with high-level coroutine-enabled primitives. Kotlin coroutines offer an API to write asynchronous code sequentially. They are similar to lightweight threads because they don’t allocate new threads when creating them. 

The coroutines can be suspended and resumed during execution. You can have a long-running task being executed one by one. 

6. Explain when string interpolation can be performed in Kotlin. 


String interpolation can be used when you want to use a variable or perform an operation inside a string. For string interpolation, the $ sign can be used to use a variable in the string or to perform an operation in between the {} sign. 

7. Explain the difference between lateinit and lazy in Kotlin. 


The main differences between lateinit and lazy in Kotlin are that lazy can only be used for val properties and lateinit can only be applied to var as it can’t be compiled to a final field. Immutability isn’t guaranteed. If you want the property to be initialized from the outside without knowing how before, you have to use lateinit. 

8. How is using a Sealed Class better than Enum? 


Sealed classes have different types of subclasses and contain the state. Subclasses that extend Sealed classes should be nested classes or declared in the same file as the Sealed class. 

Over to you!

There you have it! Now you know what the top Kotlin interview questions are. 

However, interviews are one part of the process. You also need to assess developers and their skills.

How? CodeSubmit offers industry-leading assessment tests to combine with your interview process. We cover 70+ languages and take-home and CodePair assignments. 

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