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.
Daniel is a full-stack developer with 15+ years of professional experience in iOS, Android, and server-side development. He developed several apps and published them on the App Store. Daniel's an experienced mentor who can explain technical concepts and support non-technical people in making informed decisions. He'd love to help his clients build apps from scratch and reboot older projects, as he excels at organizing projects, hiring new roles, and managing the development process.
Guillaume has been a software engineer for over two decades which initially began with an extensive and lengthy experience on Unix/Linux systems and C++. He then moved onto the macOS/iOS ecosystem in 2011 because it felt more vibrant and interesting. Along with a wealth of professional experience, Guillaume brings pragmatism and excellent organizational and communication skills.
Bosnia and HerzegovinaToptal Member Since September 11, 2019
Along with an iOS developer certification from the IT Academy in Belgrade, Armin is a professional, experienced QA engineer with a demonstrated history of working in the information technology and services industry. He's particularly skilled in Swift, iOS, Xcode, Android Studio, IntelliJ, NetBeans, Java, and automation. Where Armin shines is in the establishment and optimization of automation processes to make them more effective and efficient.
Ivan is a product-oriented software engineer with broad experience in the development process of apps for Android and iOS eco-systems. He's crafted apps for a variety of clients from small business owners to large mobile network operators and banks. Clean, testable, and efficient codebase is what drives him, as well as a happy and successful client, an indicator of high-quality work.
Tomasz is an Android developer who specializes in creating user-friendly and useful applications with the newest technologies. He enjoys experimenting with different languages and technologies, extending his skill set beyond Android to tasks leveraging React or iOS. Tomasz makes it a personal goal to ensure the client is fully satisfied, and often likens the process of software development to writing a good, interesting novel.
Krzysztof is a software developer with several years of experience—mostly in Java back-end projects, but he's also familiar with front-end and DevOps tech. He's worked in all sorts of industries (finance and tech among others) with notable clients like Sony, ING Bank, Tesco Bank, Payworks (now Visa). Along with mentoring junior developers and promoting Agile methodology, Krzysztof has worked on analysis, architecture design, and test automation.
Miguel describes his job as turning ideas and requirements into production-ready, maintainable, and scalable code that runs within a well-engineered environment. He has collaborated on different projects with many technologies in his years of experience as a developer, architect, and consultant. Regardless of the task at hand or the technology, Miguel embraces software development challenges with professionalism and enthusiasm.
Aleksandar has worked on various software and product development projects for Toptal clients as an Android developer, for clients through his own agency as a developer, project manager, and business owner, and also on his own startup projects as a founder and technical co-founder. He understands software products and their lifecycle from various perspectives and roles. He is an open-minded individual with excellent strategic, analytical, and empathic skills.
Ganga has 17 years of experience developing and architecting applications in the cable and telecommunication sector, of which the last 6+ years is in native Android development. He has worked with companies like Comcast Cable, British Telecomm, Texas Instruments, and DHL. His strengths include focusing on the end user, aligning with the clients' vision and goals, adapting to technology innovations, and his experience in the E2E product lifecycle.
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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What our clients think
Clients Rate Toptal Kotlin Developers4.3 / 5.0on average across 209 reviews as of Dec 8, 2023
Tripcents wouldn't exist without Toptal. Toptal Projects enabled us to rapidly develop our foundation with a product manager, lead developer, and senior designer. In just over 60 days we went from concept to Alpha. The speed, knowledge, expertise, and flexibility is second to none. The Toptal team were as part of tripcents as any in-house team member of tripcents. They contributed and took ownership of the development just like everyone else. We will continue to use Toptal. As a startup, they are our secret weapon.
Brantley Pace, CEO & Co-Founder
I am more than pleased with our experience with Toptal. The professional I got to work with was on the phone with me within a couple of hours. I knew after discussing my project with him that he was the candidate I wanted. I hired him immediately and he wasted no time in getting to my project, even going the extra mile by adding some great design elements that enhanced our overall look.
Paul Fenley, Director
K Dunn & Associates
The developers I was paired with were incredible -- smart, driven, and responsive. It used to be hard to find quality engineers and consultants. Now it isn't.
Ryan Rockefeller, CEO
Toptal understood our project needs immediately. We were matched with an exceptional freelancer from Argentina who, from Day 1, immersed himself in our industry, blended seamlessly with our team, understood our vision, and produced top-notch results. Toptal makes connecting with superior developers and programmers very easy.
Jason Kulik, Co-Founder
As a small company with limited resources we can't afford to make expensive mistakes. Toptal provided us with an experienced programmer who was able to hit the ground running and begin contributing immediately. It has been a great experience and one we'd repeat again in a heartbeat.
Stuart Pocknee , Principal
Site Specific Software Solutions
We used Toptal to hire a developer with extensive Amazon Web Services experience. We interviewed four candidates, one of which turned out to be a great fit for our requirements. The process was quick and effective.
Abner Guzmán Rivera, CTO and Chief Scientist
Sergio was an awesome developer to work with. Top notch, responsive, and got the work done efficiently.
Dennis Baldwin, Chief Technologist and Co-Founder
Working with Marcin is a joy. He is competent, professional, flexible, and extremely quick to understand what is required and how to implement it.
André Fischer, CTO
We needed a expert engineer who could start on our project immediately. Simanas exceeded our expectations with his work. Not having to interview and chase down an expert developer was an excellent time-saver and made everyone feel more comfortable with our choice to switch platforms to utilize a more robust language. Toptal made the process easy and convenient. Toptal is now the first place we look for expert-level help.
Derek Minor, Senior VP of Web Development
Networld Media Group
Toptal's developers and architects have been both very professional and easy to work with. The solution they produced was fairly priced and top quality, reducing our time to launch. Thanks again, Toptal.
Jeremy Wessels, CEO
We had a great experience with Toptal. They paired us with the perfect developer for our application and made the process very easy. It was also easy to extend beyond the initial time frame, and we were able to keep the same contractor throughout our project. We definitely recommend Toptal for finding high quality talent quickly and seamlessly.
Ryan Morrissey, CTO
Applied Business Technologies, LLC
I'm incredibly impressed with Toptal. Our developer communicates with me every day, and is a very powerful coder. He's a true professional and his work is just excellent. 5 stars for Toptal.
Pietro Casoar, CEO
Ronin Play Pty Ltd
Working with Toptal has been a great experience. Prior to using them, I had spent quite some time interviewing other freelancers and wasn't finding what I needed. After engaging with Toptal, they matched me up with the perfect developer in a matter of days. The developer I'm working with not only delivers quality code, but he also makes suggestions on things that I hadn't thought of. It's clear to me that Amaury knows what he is doing. Highly recommended!
George Cheng, CEO
As a Toptal qualified front-end developer, I also run my own consulting practice. When clients come to me for help filling key roles on their team, Toptal is the only place I feel comfortable recommending. Toptal's entire candidate pool is the best of the best. Toptal is the best value for money I've found in nearly half a decade of professional online work.
Ethan Brooks, CTO
Langlotz Patent & Trademark Works, Inc.
In Higgle's early days, we needed the best-in-class developers, at affordable rates, in a timely fashion. Toptal delivered!
Lara Aldag, CEO
Toptal makes finding a candidate extremely easy and gives you peace-of-mind that they have the skills to deliver. I would definitely recommend their services to anyone looking for highly-skilled developers.
Michael Gluckman, Data Manager
Toptal’s ability to rapidly match our project with the best developers was just superb. The developers have become part of our team, and I’m amazed at the level of professional commitment each of them has demonstrated. For those looking to work remotely with the best engineers, look no further than Toptal.
Laurent Alis, Founder
Toptal makes finding qualified engineers a breeze. We needed an experienced ASP.NET MVC architect to guide the development of our start-up app, and Toptal had three great candidates for us in less than a week. After making our selection, the engineer was online immediately and hit the ground running. It was so much faster and easier than having to discover and vet candidates ourselves.
Jeff Kelly, Co-Founder
We needed some short-term work in Scala, and Toptal found us a great developer within 24 hours. This simply would not have been possible via any other platform.
Franco Arda, Co-Founder
Toptal offers a no-compromise solution to businesses undergoing rapid development and scale. Every engineer we've contracted through Toptal has quickly integrated into our team and held their work to the highest standard of quality while maintaining blazing development speed.
Greg Kimball, Co-Founder
How to Hire Kotlin Developers through Toptal
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To hire Kotlin app developers, you should start by clearly defining the role’s requirements, the necessary level of expertise, and the technical skills relevant to your project. Next, filter the candidates based on these requirements, taking into account complementary skills and the business domain; for example, many Kotlin engineers must have a strong Android background for mobile development. Interviews and coding exercises will help you shortlist candidates further and assess their prospective fit on your team.
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 quick is the hiring process with Toptal?
Typically, you can hire a Kotlin developer with Toptal in about 48 hours. Our talent matchers are experts 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 expert, 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.
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:
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.
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.
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):
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.
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.
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.