Toptal is a marketplace for top Kotlin developers, engineers, programmers, coders, architects, and consultants. Top companies and startups can hire Toptal dedicated (full-time), hourly, or part-time Kotlin freelancers for their mission-critical software projects.
Tharindu is a senior software engineer with over 10 years of experience in software development. He specializes in mobile, web, and back-end platforms and is an expert in platform development, enterprise architecture, Agile methodologies, and web-based applications. Tharindu has successfully built a corporate mobile banking application for one of the largest banks in Asia. He is a team player with an innovative mindset and possesses proficient problem-solving and analytical skills.
Asma is a software engineer with five years of experience specializing in web and mobile app development. She's worked on multiple critical and sensitive governmental projects and delivered high-quality apps with 225,000+ users. You can rely on her attention to detail and her ability to stretch and work outside her comfort zone, whatever the project. Asma is passionate about her field and enjoys learning cutting-edge technologies and leveraging them to get the best product possible.
Anthony is a results-oriented mobile software engineer with over 14 years of experience and a record of constructing and spearheading robust teams to create exemplary cross-platform iOS and Android mobile applications. Skilled in modern architectures like Kotlin Multiplatform, he excels at streamlining development processes and delivering high-caliber solutions. Anthony aspires to apply his profound technical prowess and leadership acumen to foster innovation at a progressive organization.
Chris is an experienced Android developer that has delivered quality applications across multiple industries. He has nearly 15 years of experience and holds a Master of Science degree in Computer Science from New York University. He primarily uses Kotlin, Android SDK, JUnit, RxJava, and MVVM.
Denis is a software developer with 12+ years of experience in Android and iOS mobile native development and architecture. He is an expert in Kotlin, Java, Swift, Objective-C, .NET, and C++. With solid technical and communication skills, Denis easily solves technical leadership challenges and provides guidance to development teams.
Roman is a senior Android developer, proficient in Kotlin, Java, Android, and modern app dev frameworks. He is experienced in building large-scale mobile applications with modular architecture with millions of users and more than 100,000 lines of code, automating build, testing, and release processes. Roman always communicates clearly and transparently, and he can jump in and be productive from day one of the project.
Nikita is a skilled software engineer based in Amsterdam. His expertise spans various technologies from mobile (Kotlin, Android, React Native, and Flutter) to server-side, leveraging Scala and PHP/Hack. He can also lead through mentoring, optimizing codebases, and enhancing user experiences across different projects. With a bachelor's in computer science and a commitment to continuous improvement, Nikita is certainly a valuable asset to any tech team.
Nodar is an experienced Android developer with over six years of working experience. He specializes in native mobile development using Android SDK, Kotlin, and Java. He enjoys developing and delivering high-quality Android apps with industry best practices and the latest technologies. Nodar has a strong background in maths, which gives him the ability to solve complex technical problems and implement optimal solutions.
Arslan is a professional Android developer with over six years of experience designing, developing, testing, and maintaining cutting-edge apps. He is proficient in Kotlin, Coroutines, Jetpack Compose, Material Design, Retrofit, and Dagger. He likes to write clean and optimized code, utilizing emerging technologies and best software design practices. Arslan strongly believes in continuous improvement and strives for excellence by constantly trying new things and developing new skills.
Siyavushkhon is an innovative and motivated mobile developer. He's worked as a native Android developer for four years using Java and Kotlin programming languages. Over the last two years, he's been contributing as a full-stack mobile developer on the Flutter framework. So far, he's delivered solutions in many different fields, such as media streaming portals, fintech apps, and AI systems on end devices for face identification tasks.
Rodrigo is a software engineer with over a decade of experience developing applications for various industries and working with teams around the globe. He has a computer science degree from Buenos Aires University and specializes in Android, Kotlin, Java, PHP, APIs, Bash, and databases. Rodrigo is a focused, organized, and productive developer who has led many teams, managed entire full-stack development projects, and excelled as a collaborative individual contributor.
Kotlin developers use object-oriented and functional programming concepts to build Android apps, web front ends, server-side software, and more. This hiring guide reveals the technical, theoretical, soft, and complementary skills required for Kotlin development for a range of use cases.
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How to Hire Kotlin Developers Through Toptal
1
Talk to One of Our Client Advisors
A Toptal client advisor will work with you to understand your goals, technical needs, and team dynamics.
2
Work With Hand-selected Talent
Within days, we'll introduce you to the right Kotlin developer for your project. Average time to match is under 24 hours.
3
The Right Fit, Guaranteed
Work with your new Kotlin developer for a trial period (pay only if satisfied), ensuring they're the right fit before starting the engagement.
Find Experts With Related Skills
Access a vast pool of skilled developers in our talent network and hire the top 3% within just 48 hours.
To hire the right Kotlin developer, it’s important to evaluate a candidate’s experience, technical skills, and communication skills. You’ll also want to consider the fit with your particular industry, company, and project. Toptal’s rigorous screening process ensures that every member of our network has excellent experience and skills, and our team will match you with the perfect Kotlin developers for your project.
What do I need to look for when I choose a Kotlin developer online?
When hiring a Kotlin developer online, focus on a candidate’s theoretical and technical expertise. Look for knowledge of programming languages (e.g., Kotlin and Java), Agile software development, related technologies (e.g., SQL and C/C++), and general computer science concepts (e.g., algorithms). Additional skills, such as knowledge of project-specific frameworks and libraries, may be needed depending on your business requirements. Candidates should also be evaluated on soft skills like communication and problem-solving. Finally, it is essential to focus on the candidate’s potential to fit your company’s culture and vision.
How do I choose between two quality Kotlin development candidates?
When choosing between two promising candidates with the core technical skills for Kotlin development, it is important to focus on their experiences and skills related to your project requirements. Identify whether your needs are best suited for a junior, mid-level, or senior developer and evaluate how well the candidates meet the desired developer profile. (For senior roles, a proven track record of technical excellence and the ability to take ownership of projects and make decisions is expected—your chosen candidate should possess these attributes.) You should also consider which candidate fits best with your budget, existing team, and available onboarding resources.
Are Kotlin developers in demand?
Kotlin is growing rapidly, and with its popularity comes a high demand for Kotlin developers. It is already the primary (and Google-recommended) language for Android development. The language boasts many benefits, mixing the best parts of imperative, object-oriented, and functional programming. As a result, it is used in a variety of non-Android projects too, including web development and server applications.
How quickly can you hire with Toptal?
Typically, you can hire a Kotlin developer with Toptal in about 48 hours. For larger teams of talent or Managed Delivery, timelines may vary. Our talent matchers are highly skilled in the same fields they’re matching in—they’re not recruiters or HR reps. They’ll work with you to understand your goals, technical needs, and team dynamics, and match you with ideal candidates from our vetted global talent network.
Once you select your Kotlin developer, you’ll have a no-risk trial period to ensure they’re the perfect fit. Our matching process has a 98% trial-to-hire rate, so you can rest assured that you’re getting the best fit every time.
Should I use Kotlin instead of Java for Android app development?
When it comes to Android mobile app development, Java had been the go-to language for a long time. However, since its release in 2016, Kotlin has become the front-runner as a newer and faster alternative, and is now the language Google recommends for native Android app development. More than 95% of the top 1,000 Android apps include Kotlin code, and more than 60% of Android professionals use Kotlin.
How are Toptal Kotlin developers different?
At Toptal, we thoroughly screen our Kotlin developers to ensure we only match you with the highest caliber of talent. Of the more than 200,000 people who apply to join the Toptal network each year, fewer than 3% make the cut.
In addition to screening for industry-leading expertise, we also assess candidates’ language and interpersonal skills to ensure that you have a smooth working relationship.
When you hire with Toptal, you’ll always work with world-class, custom-matched Kotlin developers ready to help you achieve your goals.
Can you hire Kotlin developers on an hourly basis or for project-based tasks?
You can hire Kotlin developers on an hourly, part-time, or full-time basis. Toptal can also manage the entire project from end-to-end with our Managed Delivery offering. Whether you hire an expert for a full- or part-time position, you’ll have the control and flexibility to scale your team up or down as your needs evolve. Our Kotlin developers can fully integrate into your existing team for a seamless working experience.
What is the no-risk trial period for Toptal Kotlin developers?
We make sure that each engagement between you and your Kotlin developer begins with a trial period of up to two weeks. This means that you have time to confirm the engagement will be successful. If you’re completely satisfied with the results, we’ll bill you for the time and continue the engagement for as long as you’d like. If you’re not completely satisfied, you won’t be billed. From there, we can either part ways, or we can provide you with another expert who may be a better fit and with whom we will begin a second, no-risk trial.
Khachatur is an Android developer who has contributed to more than 10 apps published on the Google Play store, including the Zedge app, which has ~1 million daily active users. He has a master’s degree in software engineering from the University of Tartu.
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:
Identify your business needs.
Are you hiring for a full- or part-time role?
Are you looking to hire a dedicated Kotlin developer?
How long will the engagement last? Do you plan to staff a temporary project or build a long-term working relationship with candidates?
What is your budget for this position?
Are you hiring for an existing project or starting from scratch?
Define the technologies and skills needed for the specific role.
Are you looking for a Kotlin developer to work on mobile projects, web projects, or both? What additional skills are required for the role?
What tools and platforms are used in your business? Does the candidate have experience with the technology stack?
Consider your existing resources.
Will additional onboarding time be required for this candidate based on their experience and expertise?
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):
Junior developers
These engineers are relatively inexperienced and need support from mid-level or senior developers, including extra resources and guidance on project direction. They focus on writing working Kotlin code but may not ensure its maintainability and scalability.
Mid-level developers
These professionals should be expected to write working, clean, and scalable code, and work independently to complete tasks. They have a proven track record of delivering software solutions to production.
Senior developers
These experts possess a greater breadth and depth of skills and experiences than junior and mid-level developers; they can find simple solutions to complicated problems. In addition, they can coach others, take ownership of projects, and drive important decisions. Since Kotlin is a relatively young language, it is usually not a senior developer’s first programming language.
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.