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Kotlin, a programming language created by JetBrains, has quickly gained popularity, especially among the native Android development crowd. Its creation significantly improved language design, mainly because of its full interoperability with Java and the JVM platform at large.

This compatibility is fundamental, as it seamlessly allows Kotlin to merge with existing codebases, libraries, and frameworks. In doing so, it provides a modern alternative without abandoning the established ecosystem.

What truly sets Kotlin apart are the innovative features it introduced, significantly differentiating it from Java and other languages. These features can be categorized into several key areas:

1. Class-related enhancements: Kotlin brought a slate of improvements around object-oriented programming, notably sealed classes, data classes, and companion objects
2. Null safety: one of the most loudly praised features, commonly referred to as the cure for a billion-dollar mistake
3. Syntactic sugars: modern language features that enhance readability and reduce verbosity. For example, extension functions, lambda expressions, range expressions, default and named arguments, top-level functions, etc.
4. Coroutines: introduced later in its evolution, coroutines have been a game changer in asynchronous programming in Kotlin, providing a simpler, more efficient alternative to traditional concurrency and threading models found in Java

Kotlin's rise to fame was significantly boosted by Google's decision to endorse it as a first-class language for native Android development. This endorsement not only validated Kotlin's strengths but also opened the gates for widespread adoption in mobile app development.

Beyond Android, Kotlin has proven its versatility and strength in backend development, notably with frameworks like Spring Boot and, most famously, Ktor.

Additionally, Kotlin powers Compose for Desktop and the innovative Kotlin Multiplatform, which allows developers to write cross-platform code that is interoperable with existing platforms.

Essential skills for Kotlin developers

Narrowing down essential technical skills typically expected from an experienced developer is a rather difficult task due to the complexity of the Kotlin programming language and its adaptability to any task. After all, Kotlin and Java aim to be languages that can solve any task.

You should note that suggestions provided in our hiring guide may vary depending on the role a Kotlin developer would undertake.

Proficiency in Kotlin syntax and fundamentals

A Kotlin developer must deeply understand the language's syntax and core concepts. This includes mastery over Kotlin's unique features, such as null safety, extension functions, and coroutines, along with a solid grasp of its type system and control structures. This proficiency is fundamental, as it forms the bedrock upon which more complex applications and systems are built.

Experience with Java and migrating existing applications

Given Kotlin's interoperability with Java, experience in Java is indispensable for a Kotlin developer. This skill is particularly vital when working on projects that require integrating Kotlin into existing Java codebases or transitioning applications from Java to Kotlin. A developer skilled in both languages can leverage the strengths of each, ensuring seamless integration and efficient migration, capitalizing on Kotlin's modern features while maintaining the robustness of Java's tried-and-tested libraries and frameworks.

Understanding functional and reactive programming

Kotlin's support for functional programming paradigms opens a realm of possibilities for writing cleaner, more concise code. A proficient Kotlin developer should be comfortable with concepts like higher-order functions, lambdas, and immutability. Moreover, knowledge of Kotlin's Flow API is crucial in the context of reactive programming.

This understanding allows developers to write more efficient, scalable, and responsive applications, particularly in scenarios involving real-time data processing, network operations, or any context where asynchronous programming is pivotal.

Understanding and implementing best practices

While technical know-how is essential, the ability to write high-quality, clean, and maintainable code often distinguishes a great developer. This involves adhering to best practices such as writing readable and self-explanatory code, effectively utilizing design patterns, and maintaining a strong focus on testing and debugging.

A developer's commitment to code quality and best practices improves the immediate output and ensures the long-term sustainability and scalability of the software they develop. Skills like refactoring, understanding architectural principles, and staying updated with the latest coding standards and practices in Kotlin contribute significantly to the overall quality of the projects they contribute to.

Nice-to-have skills

This section contains some skills that you might want to look for in a candidate. They are made to be as generalized as possible because different businesses have different needs, and Kotlin is a programming language used across many domains.

Familiarity with popular libraries and frameworks

A well-rounded Kotlin developer often benefits from familiarity with various popular frameworks and libraries. This knowledge extends beyond the core language, encompassing tools that enhance productivity and solve specific problems.

Such familiarity streamlines development processes and opens doors to a broader range of project types and technical challenges.

Experience with native Android development

While Kotlin's applicability spans various platforms, its role in Android development is particularly notable. Experience in native Android development, including an understanding of Android SDK, Jetpack components, and UI design principles, is a valuable asset. This skill is especially beneficial given Kotlin's status as a preferred language for Android.

Soft skills

As for any experienced developer, look for soft skills in your candidate. This includes attentiveness and willingness to understand business needs. Good Kotlin developers are not only experts in the Elvis operator but also in communication skills.

Interview questions to ask a potential candidate

Although Proxify already vets developers for their skills, here are some questions and respective answers, we would suggest asking to evaluate a potential candidate for the role of a Kotlin developer.

1. Explain the difference between "val" and "var" in Kotlin.

Expected answer: In Kotlin, val declares a read-only (immutable) property or local variable, meaning its value cannot be changed once assigned. On the other hand, var declares a mutable property or local variable, allowing its value to be changed.

2. How does Kotlin handle null safety, and what exactly are nullable and non-nullable types?

Expected answer: Kotlin introduces null safety by distinguishing between nullable and non-nullable types. A non-nullable type cannot hold a null value, preventing null pointer exceptions. Nullable types, declared with a question mark, can hold a null. Kotlin provides safe calls (?.), the Elvis operator (?:), and the !! operator to handle nullable types safely and explicitly.

3. In as much detail as possible, explain what coroutines are, and how they differ from threads.

Expected answer: Coroutines are a Kotlin feature for managing asynchronous programming and concurrency. They differ from threads in being lightweight, not bound to any specific thread, and can suspend and resume execution without blocking the thread. Coroutines allow for efficient use of resources, as many coroutines can run on a few threads and facilitate asynchronous operations like network IO or database transactions without complex callback structures.

4. How does Kotlin implement delegation, and what is the "by" keyword used for?

Expected answer: Kotlin implements delegation through the by keyword, allowing a class to delegate the implementation of an interface to another object. It's a form of composition over inheritance, promoting code reuse and flexibility. The by keyword automatically forwards calls to the delegate object, reducing boilerplate code.

5. What are reified type parameters, and how are they an improvement over Java's generics system?

Expected answer: Reified type parameters in Kotlin allow for keeping the type information at runtime, unlike Java's generics which undergo type erasure. This feature is enabled through inline functions. Reified types enable operations like is T checks or T::class within the function body, providing more flexibility and power than Java's generics.

6. Explain covariance, variance, and contravariance.

Expected answer: Variance in Kotlin relates to how subtyping between different types works with generic types. Covariance (out) allows a type parameter to be a subtype of another, supporting read-only operations. Contravariance (in) allows a type parameter to be a supertype of another, suitable for write-only operations. Invariance doesn’t allow any subtyping with generics.

7. What is late initialization?

Expected answer: Late initialization in Kotlin refers to the lateinit keyword, which allows the declaration of non-nullable properties without immediate initialization. It's useful when a property cannot be initialized during object construction but is guaranteed to be initialized before use. This avoids unnecessary null checks or temporary null assignments.

8. Explain the differences between various scope functions.

Expected answer: Kotlin's scope functions (let, run, with, apply, also) each serves different purposes. let and also return the result of the lambda expression, let with it as the context and also with this. run, with, and apply return the context object, with run and with using this as the context and apply returning the context object itself after execution of the block.

9. What is a companion object, and how does it compare to Java's static methods?

Expected answer: A companion object in Kotlin is a singleton object within a class that allows members to be accessed in a static-like manner, similar to Java's static methods. However, unlike Java's static methods, companion objects are actual objects, allowing them to implement interfaces and have a state.

10. What is the purpose of an inline function?

Expected answer: Inline functions in Kotlin are functions the compiler will 'inline' at the call site, meaning the function's body is copied directly into the calling code. This reduces the overhead of a function call and enables features like reified type parameters. It's handy for higher-order functions to avoid the overhead of object allocation for function parameters.

11. Explain what Flows are.

Expected answer: Flows in Kotlin are a type of cold stream used for representing values that are asynchronously computed over time. They are part of Kotlin's coroutines and are designed to handle data streams that can be emitted sequentially. Unlike sequences, which are computed synchronously, Flows are inherently asynchronous and suitable for reactive programming models.

12. Bonus: talk about your experience in a domain where you used Kotlin.

Expected answer: This answer will vary based on personal experience. An example could be using Kotlin for Android development, highlighting the language's concise syntax, null safety, and coroutine support, simplifying the development process and improving app performance and reliability.

Industries and applications

Kotlin has emerged as a dynamic and versatile language in the software development industry, finding applications in various domains. Its impact on businesses and the industry at large is significant, driven by its robust features and growing adoption.

One of the primary areas where Kotlin has significantly impacted is Android app development. Over 95% of the top 1000 Android apps use Kotlin. Its adoption in Android development is driven by its modern and expressive language features, seamless interoperability with Java, concise syntax, and robust null safety mechanisms​​​​. Android Studio, the integrated development environment for Android, provides more and more Kotlin-first features.

Kotlin's use in backend development has been gaining traction, particularly with frameworks like Ktor. Ktor, a lightweight framework for building asynchronous servers and clients in connected systems, is entirely written in Kotlin. This framework stands out for its simplicity, flexibility, and ability to create highly scalable and performant backends.

In the realm of data science, Kotlin is gradually carving out a niche for itself. Although languages like Python and R predominantly dominate this space, Kotlin brings unique advantages, especially in terms of performance and type safety. Kotlin's interoperability with Java allows it to leverage the vast ecosystem of Java libraries, which is crucial for data processing and analytics. Additionally, the introduction of libraries such as Krangl – a Kotlin library for data wrangling – extends Kotlin's capabilities in data science.

The impact of Kotlin on businesses is multifaceted. It enhances developer productivity and satisfaction, which translates to faster development cycles, reduced costs, and improved app quality. The language's design focuses on safety, clarity, and tooling support, making it an attractive option for businesses aiming to modernize their tech stack or develop robust applications.

Summary

Hiring a Kotlin developer today encompasses much more than merely focusing on a single domain. Kotlin's adaptability and the breadth of its application mean that these professionals often possess expertise across various domains. Whether it's native Android development, where Kotlin has become a dominant force, backend development with frameworks like Ktor, or even in cross-platform technologies where native plugin integration is crucial, Kotlin developers bring a wealth of knowledge and versatility. This versatility is particularly valuable in an industry that increasingly values cross-functional and multi-domain expertise.

Kotlin's maturity as a programming language is evident in its significant evolution over the past few years. Continually evolving, Kotlin has solidified its position in the software development landscape and influenced the development of other programming languages. A notable example of this influence is seen in the recent changes to Java, where Kotlin's innovations inspire some features. This aspect highlights Kotlin's role not just as a tool for development but also as a catalyst for broader advancements in programming languages. Its ongoing development promises to keep it at the forefront of technology, making it an increasingly attractive choice for businesses and developers.

Kotlin represents a blend of maturity, innovation, and versatility. For businesses, this translates to a language that can adapt to various needs and drive technological advancement. For developers, it offers a landscape of opportunities to work in diverse domains while staying at the cutting edge of programming language evolution.