Hire the Top 3% of Freelance Kotlin Developers

Kotlin’s Rapid Growth Creates High Developer Demand

Released in 2016, Kotlin is very young but also one of the fastest-growing programming languages. Within a few years of its release, it became Google’s recommended language for Android development, and the Android ecosystem has continued to become increasingly Kotlin-first. Kotlin is ranked as developers’ favorite language (relative to the number of language users) as of 2022. The object-oriented language is a clear choice for Android apps and has proven worthy of many other types of development.

Since Kotlin is a newer, fast-growing, and versatile language, finding the best Kotlin developers for hire is no simple task: It involves a deep understanding of the role’s responsibilities, the necessary level of expertise, and the required technical and complementary skills. This guide will help you streamline the screening process and determine how to hire Kotlin app developers whose abilities best match your business needs.

What attributes distinguish quality Kotlin Developers from others?

First and foremost, a strong Kotlin developer should have excellent coding skills, in-depth knowledge of programming concepts, and experience using standard libraries. Every quality Kotlin developer possesses a blend of technical, theoretical, and soft skills:

Technical Skills

• Kotlin – Candidates should have a deep understanding of the Kotlin language and its unique features (e.g., null safety, lambda expressions, extension functions).

• Java – As Kotlin is fully interoperable with Java, Kotlin developers should have a good grasp of Java and its ecosystem; many projects require rewriting existing Java code into Kotlin.

• Framework and libraries – Engineers can work more efficiently in Kotlin through their existing knowledge of functional libraries and frameworks (e.g., Spring Boot and Ktor). For successful testing, they must also understand frameworks like JUnit and Mockito.

Theoretical Knowledge

• Object-oriented and functional programming – Kotlin is an object-oriented language that supports functional programming; knowledge of these concepts and their principles will help developers code effectively.

• Design patterns – Candidates should have experience with a broad sampling of design patterns, including some that are Android-specific.

• Agile development – As Agile is the most widely used development methodology, developers should be comfortable with Agile and related methods (e.g., Scrum or Kanban).

• Code quality and best practices – Candidates should be familiar with best practices relevant to Kotlin or associated technologies (e.g., Android best practices) and be prepared to write clean and testable code that performs and scales.

Soft Skills

• Problem-solving – Strong analytical and problem-solving skills are crucial for any software development role, and candidates should demonstrate the ability to think algorithmically and tackle complex challenges.

• Communication – All engineers must communicate well to explain complex software solutions to technical and nontechnical audiences.

• Time management – Candidates should speak to past Kotlin or engineering projects where they employed time management skills to meet tight deadlines.

These standard requirements apply to every Kotlin engineer. You’ll want to vet candidates for additional skills related to your project and application type.

How can you identify the ideal Kotlin Developer for you?

Hiring the best candidate requires identifying developer skills specific to your business needs. For instance, as Kotlin is commonly used in Android development, suppose you’re hiring experts to build Android apps. In that case, candidates will need substantial experience with the Android SDK, various Android APIs and architectural patterns, Jetpack libraries (e.g., Jetpack Compose), and Android Studio.

You may also want to consider your budget, time constraints, and available training and onboarding resources. These factors may be combined into a clear problem statement addressing the following points:

1. Identify your business needs.
   1. Are you hiring for a full- or part-time role?
   2. Are you looking to hire a dedicated Kotlin developer?
   3. How long will the engagement last? Do you plan to staff a temporary project or build a long-term working relationship with candidates?
   4. What is your budget for this position?
   5. Are you hiring for an existing project or starting from scratch?
2. Define the technologies and skills needed for the specific role.
   1. Are you looking for a Kotlin developer to work on mobile projects, web projects, or both? What additional skills are required for the role?
   2. What tools and platforms are used in your business? Does the candidate have experience with the technology stack?
3. Consider your existing resources.
   1. Will additional onboarding time be required for this candidate based on their experience and expertise?
   2. Do you have internal resources to help onboard and train new candidates?

At this stage, you may also consider whether you plan to hire remote Kotlin developers or if you require on-site or hybrid engineers; considering remote workers will widen the applicant pool. Your problem statement will also inform whether you need a junior or senior Kotlin developer.

Differentiating between junior, mid-level, and senior Kotlin developers

To identify whether your needs are best suited for a junior, mid-level, or senior Kotlin developer, it is essential to first understand the differences among the types. The distinguishing factors are not limited to plain years of programming experience or technical skills (though these are both important to assess):

In general, if you are starting a project from scratch, you should opt for a senior developer. Depending on the complexity of existing projects, mid-level or junior developers can be considered. However, junior candidates may require some training and support to learn the project’s technologies.

Identifying complementary skill requirements

Depending on the project, Kotlin developers may be required to work with additional technologies—and possess additional proficiencies. The following languages are especially relevant for typical Kotlin projects:

• SQL – Databases are used in almost every project, whether Android or web development. Therefore, basic database skills are a requirement for any developer, especially knowledge of SQL and SQL databases (due to their popularity).
• C or C++ – Certain complex Android projects (e.g., games) might have libraries written in C or C++. In these cases, developers should have a solid background in C or C++ (or at least basic knowledge of them and the drive to fully learn these languages).

Hiring Kotlin vs. Java developers

Kotlin and Java have a few differences that will influence your hiring decision when evaluating their developers. Java is mature (released in 1995), while Kotlin is more modern (released in 2016). Whereas Java code is more verbose, Kotlin offers concise syntax; this significantly reduces the amount of required boilerplate code, making it more readable and less error-prone. Kotlin boasts other advantages, like its feature benefits (e.g., null safety, extension functions, static typing, smart casting) and compatibility with functional programming.

If you are building an Android app, you’ll want to search for Kotlin developers for hire due to these advantages and Kotlin’s status as the recommended Android language. However, recall that Kotlin will likely not be the first language of senior Kotlin developers—many of these experts have previously held the title of “Java engineer,” and the developer pools may overlap significantly (at least in the case of Android). For other projects, it may be best to examine the use cases for Kotlin versus Java programming in greater detail before deciding.

How to Write a Kotlin Job Description for Your Project

With an understanding of the skill requirements needed for this role, your next step is to post a job description to attract the right candidates. Be as descriptive as possible, providing information about the scope of the project, your target platform(s)—web, Android, or both—and the goals you are trying to achieve. Depending on the seniority level you seek, you may be more or less specific.

After conducting a Kotlin coding assessment, filter the applicants based on how they align with your requirements. While making your selection, consider the extent to which their expertise matches your business domain and the necessary technical skills. Additionally, test the candidate’s skills with a Kotlin coding exercise.

What are the most important Kotlin Developer interview questions?

A well-rounded Kotlin interview includes technical Kotlin questions and questions assessing a candidate’s fit with the team. While the company culture and team environment will vary project to project, foundational technical questions about Kotlin will apply universally to interviews.

The questions below provide suggested topics, but they are not exhaustive and should be supplemented with additional questions on Kotlin, Android, or other relevant topics:

What is the difference between fold and reduce?

Both fold and reduce are aggregate functions; they apply an operation to the elements of a collection and return the accumulated result. The difference is that fold uses an initial input value as the accumulated value on the method’s first step, while reduce uses the first and the second elements of the collection as operation arguments. When working exclusively with nonempty collections and combining all elements into a single result of the same type, reduce is helpful. If a developer wants to provide an initial value or change the result type, then fold is the right choice.

Consider it a red flag if a candidate can’t describe at least one of these functions.

What are the recommended approaches for creating constants?

A candidate should mention that in order to declare constants in Kotlin, a developer can:

• Declare the constants in a class file.
• Create a dedicated file where all constants can be stored.

Addtionally, a candidate should mention that creating a dedicated file is often seen as the best practice, as it supports collaboration and makes it easier to maintain code. However, this approach instantiates a new object under the hood when the bytecode is generated. Also, they shoulde note that Kotlin constants can hold only primitive types.

What is a sealed class, and what is it used for?

Sealed classes are closed classes that follow constrained or bounded class hierarchies. Direct subclasses are defined, and no other subclasses may exist outside the module within which the sealed class is created (e.g., in third-party client code). Sealed class constructors are private by default, and instances of sealed classes cannot be created since they are abstract.

Sealed classes are commonly used when a value can have one of the types of a limited set but cannot have any other type.

What is a data class?

A Kotlin data class is a simple class that exists to hold data. It has standard functionality and utility functions (equals, hashCode, toString, componentN, and copy).

There are specific rules for creating a data class in Kotlin:

• The primary constructor needs to have at least one parameter.
• All primary constructor parameters must be val or var.
• A data class cannot be an abstract, open, sealed, or inner class.

How is null safety achieved in Kotlin?

Kotlin is designed to ensure null safety and offers multiple methods of doing so. Its regular variables are non-nullable (e.g., a String cannot hold null), and developers must use nullable variables (e.g., String?) to hold null values. Moreover, Kotlin safe calls (?.) offer a simple way to access properties or perform operations for non-null values only.

How is Kotlin-Java interoperability possible? What makes it work?

Kotlin makes full interoperability with Java possible because it produces Java-compatible bytecode that can run on the Java Virtual Machine (JVM). This means that the Kotlin compiler can target the JVM and thus can interact smoothly with Java bytecode.

What’s the difference between val and var declarations?

Kotlin’s val and var keywords are both used to declare variables. However, val is used for variables whose values don’t change, while a variable declared with var can have its value changed.

Does Kotlin support multiple inheritance? That is, can you extend multiple classes?

Kotlin doesn’t support multiple inheritance; instead, it opts for composition, which is an object-oriented strategy and makes code more reusable and maintainable. Developers who have experience with Kotlin and its architecture should be able to speak to the benefits of composition and how it is implemented.

Why do companies hire Kotlin Developers?

As Kotlin continues to gain popularity, the demand for skilled Kotlin developers increases. It is the primary programming language for Android apps. Whether you are creating an Android app from scratch or updating existing apps (i.e., legacy Java code), Kotlin experts should be your first choice.

Moreover, Kotlin is not limited to Android or the mobile domain. Combining the best bits of imperative, functional, and object-oriented programming, Kotlin is type-safe with modern syntax and therefore can be used for various other purposes, such as web front-end development, server-side applications, and data science.

For your Android and web application development needs, hiring the right Kotlin expert with skills that complement your project will add value to your business. With the tips in this guide, you are better prepared for every step of the hiring process.

The technical content presented in this article was reviewed by Dario Milicic.