In 2019, Google announced Kotlin as its preferred language for Android development.
Fast to 2024, things changed.
If you're looking to hire Kotlin developers, it's not only for Android anymore - but for everything from cross-platform mobile development (iOS, Android) to server-side applications.
But finding the right Kotlin developer can feel overwhelming. How do you know who's really got the skills you need? What should you look for in a candidate? And most importantly, how much should you budget for your Kotlin team?
In this article, we'll cover them all and walk you through everything you need to know about hiring Kotlin developers.
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Table of contents [ Hide ]
What is a Kotlin developer?
Typical Job Description (JD) of Kotlin developers
Key skills and technologies of Kotlin developers
10 core Kotlin developer interview questions and answers
What types of companies hire Kotlin developers?
How much does it cost to hire Kotlin developers?
Based on developer's location
Based on developer's expertise level
Based on Pricing Model
Based on hiring model
Tips on hiring Kotlin developers
Tip #1: Hire from regions with strong mobile development expertise
Tip #2: Focus on architecture and pattern knowledge
Tip #3: Assess problem-solving through pair programming
At BiPlus, we provide skilled Kotlin developers with competitive rates
What is a Kotlin developer?
A Kotlin developer is a software engineer who specializes in using the Kotlin programming language to develop applications and software solutions. They're particularly crucial in Android app development, where Kotlin is now Google's preferred language, and increasingly important in server-side development.
Typical Job Description (JD) of Kotlin developers
Here are the key responsibilities typically handled by Kotlin developers:
- Design and develop high-performance, scalable applications using Kotlin.
- Create and maintain Android applications using Kotlin and Android SDK.
- Implement clean architecture patterns and SOLID principles.
- Write clean, maintainable, and testable code.
- Integrate RESTful APIs and third-party libraries.
- Optimize application performance and responsiveness.
- Collaborate with cross-functional teams on feature development.
- Conduct code reviews and provide technical mentorship.
- Debug and fix software defects.
- Create technical documentation.
Key skills and technologies of Kotlin developers
To be considered a proficient Kotlin developer, professionals should be skilled in:
Core Technical Skills
- Kotlin fundamentals and language features
- Android SDK and Android Studio
- Kotlin Coroutines for async programming
- Object-Oriented Programming (OOP)
- Functional Programming concepts
- Clean Architecture principles
- REST APIs and web services
- Version control (Git)
Additional Technologies
- Kotlin Multiplatform Mobile (KMM)
- Dependency injection (Koin/Dagger)
- Testing frameworks (JUnit, Mockk)
- Jetpack Compose
- SQL and database management
- CI/CD pipelines
- Agile methodologies
10 core Kotlin developer interview questions and answers
1. What makes Kotlin different from Java?
Kotlin brings several outstanding improvements over Java.
The built-in null safety system helps prevent those dreaded NullPointerExceptions. Extension functions let you add methods to existing classes without inheritance. You'll write less code thanks to features like data classes and smart casts. Plus, Kotlin works seamlessly with existing Java code, making gradual migration possible. It also offers better support for functional programming with features like higher-order functions and lambdas.
Quick answer
Kotlin offers null safety, extension functions, and more concise syntax while maintaining full Java interoperability.
2. Explain Kotlin's coroutines and when you'd use them
Coroutines are super lightweight. Unlike traditional threads that might consume MBs of memory, coroutines only need a few bytes.
They're perfect for handling concurrent operations without blocking your main thread. You'd use them for API calls, database operations, or complex calculations. The 'suspend' keyword marks functions that can be paused and resumed, making async code look almost like synchronous code. They're especially powerful with Kotlin's structured concurrency, which helps manage coroutine lifecycles and prevent memory leaks.
Quick answer
Coroutines are lightweight threads for asynchronous programming, perfect for network calls, database operations, or any long-running tasks.
3. What are sealed classes in Kotlin and when should you use them?
Sealed classes are like enhanced enums that can hold state. When you mark a class as 'sealed', all its subclasses must be defined in the same file.
This restriction makes them perfect for representing a fixed number of possible types. For example, you might use a sealed class to represent your app's states (Loading, Success, Error), or API response types. The Kotlin compiler can verify that you've handled all possible cases in your when expressions, making your code more robust and maintainable.
Quick answer
Sealed classes restrict class hierarchy to a limited set of subclasses, perfect for representing finite sets of options like UI states or API responses.
4. How do you handle null safety in Kotlin?
Kotlin's type system distinguishes between nullable and non-nullable types.
A String can't be null, but a String? can. You handle nullable values using safe call operator (?.) which only executes if the value isn't null. The Elvis operator (?:) provides default values for null cases. While the not-null assertion (!!) exists, it should be used sparingly as it can throw NullPointerException.
Smart casts automatically handle null checks in if statements. This system effectively prevents null-related crashes while making code intentions clear.
Quick answer
Kotlin enforces null safety through its type system using nullable types (?), safe calls (?.), Elvis operator (?:), and not-null assertions (!!).
5. What's the difference between val and var in Kotlin?
With 'val' - once you assign a value, you can't change the reference. However, the object it refers to might still be mutable.
For example, a val list can still have items added to it, but you can't reassign the list variable itself. 'var', on the other hand, lets you reassign values freely. Best practice is to prefer 'val' over 'var' when possible, as immutability helps prevent bugs and makes code easier to reason about, especially in multithreaded environments.
Quick answer
'val' declares immutable references (read-only), while 'var' declares mutable variables that can be reassigned.
6. Explain Kotlin's data classes and their benefits.
When you declare a class with 'data class', Kotlin automatically provides: equals() for structural equality, hashCode() for hash-based collections, toString() for readable string representation, copy() for creating modified copies, and component functions for destructuring.
For example, instead of writing 20+ lines of Java code for a simple POJO, you can write 'data class User(val name: String, val age: Int)'. They're perfect for API responses, database entities, or any class primarily holding data.
Quick answer
Data classes automatically generate equals(), hashCode(), toString(), and copy() methods for classes that mainly hold data.
7. How do you handle object creation in Kotlin? Explain companion objects.
Object declarations create singletons instantly - just use 'object MyObject'. Companion objects serve as containers for factory methods and other members that are tied to a class rather than instances. They're similar to static members in Java but more flexible. You can even have them implement interfaces. The pattern is commonly used for factory methods and constants.
Quick answer
Kotlin uses object declarations for singletons and companion objects for static members, replacing Java's static keyword.
8. What are higher-order functions and when would you use them?
Higher-order functions let you treat functions as first-class citizens. Common use cases include: creating callback mechanisms, implementing the Observer pattern, building decorators, and creating DSLs.
The standard library uses them extensively - map(), filter(), and reduce() are all higher-order functions. This pattern makes code more flexible and reusable.
Quick answer
Higher-order functions take functions as parameters or return functions, enabling powerful functional programming patterns like callbacks and decorators.
9. How does Kotlin support multiplatform development?
Kotlin Multiplatform (KMP) lets you write shared business logic once and use it everywhere. You define 'expect' declarations in common code and 'actual' implementations for each platform. This works great for network calls, data processing, and business rules.
The common code can use coroutines, serialization, and other Kotlin features. Platform-specific code (like UI) stays native to each platform. This approach gives you the best of both worlds: code sharing where it makes sense, and native performance where it matters.
Quick answer
Kotlin Multiplatform allows sharing code between platforms (Android, iOS, Web, Desktop) while keeping platform-specific implementations separate.
10. What's your experience with Kotlin Flow and StateFlow?
Flow is Kotlin's solution for reactive programming, built on coroutines. It's great for handling streams of data, like real-time updates or sensor readings. Flow is cold, meaning it starts producing values only when collected.
StateFlow is hot and stateful - it always holds the latest value and emits to all collectors. It's perfect for UI state management, replacing LiveData in many cases. This pattern is commonly used in ViewModels for modern Android development.
Quick answer
Flow is for reactive programming with cold streams, while StateFlow is for hot streams that maintain state, similar to LiveData but more powerful.
What types of companies hire Kotlin developers?
Mobile-first tech companies
Industry leaders like Uber and Lyft have migrated their Android apps to Kotlin, citing a 50% reduction in crash rates. These companies leverage Kotlin for complex real-time features like location tracking and payment processing. Pinterest also rebuilt their Android app in Kotlin, reporting a 30% decrease in development time for new features.
Financial services companies
Capital One and Revolut use Kotlin not just for their mobile apps but also for backend microservices. The language's null safety and functional programming features make it ideal for handling sensitive financial transactions. JPMorgan Chase employs Kotlin developers for their trading platforms, where type safety and performance are crucial.
E-commerce giants
Amazon and Alibaba utilize Kotlin for both their consumer-facing apps and internal tools. The language's multiplatform capabilities allow them to share business logic between Android, iOS, and web applications. Shopify's point-of-sale Android app is written in Kotlin, showcasing the language's capability in handling complex offline-online synchronization.
Healthcare technology companies
Companies like Epic Systems and Philips Healthcare employ Kotlin developers for building medical record systems and diagnostic tool interfaces. They particularly value Kotlin's strong typing and null safety for building critical healthcare applications where reliability is paramount.
Enterprise software companies
Microsoft and JetBrains (Kotlin's creator) use Kotlin for developing developer tools and enterprise applications. These companies appreciate Kotlin's interoperability with existing Java codebases, allowing gradual migration of legacy systems.
IT services/IT outsourcing sector
Companies like Accenture, Deloitte, and Capgemini, consistently hires Kotlin developers to support their client projects.
How much does it cost to hire Kotlin developers?
Based on developer's location
It's important to understand why location impacts salary ranges so significantly. Market maturity, living costs, and local tech ecosystems all play crucial roles in determining developer compensation.
North America leads with the highest salaries due to its mature tech market and high living costs. Silicon Valley Kotlin developers often command $140,000+ annually, but they bring extensive experience in building scalable applications and deep understanding of mobile architecture.
Western European developers, particularly in tech hubs like London and Berlin, offer a strong combination of technical expertise and product thinking, though at slightly lower rates than their North American counterparts.
Eastern European developers from countries like Poland and Ukraine have gained recognition for their strong computer science fundamentals and Kotlin expertise. They offer an attractive balance of cost and quality.
Southeast Asian developers, especially in Vietnam and Indonesia, provide the most competitive rates while maintaining high technical standards. Many have experience with large-scale Android applications and are increasingly skilled in Kotlin Multiplatform development.
Estimated annual salary for Kotlin developers in different regions
| Region | Median Annual Salary (USD) |
|---|
North America | $80,000 - $100,000 |
Western Europe | $58,000 - $75,000 |
East Asia | $45,000 - $60,000 |
Eastern Europe | $35,000 - $50,000 |
Latin America | $25,000 - $35,000 |
Southeast Asia | $20,000 - $30,000 |
South Asia | $15,000 - $25,000 |
Based on developer's expertise level
Junior Kotlin developers (1-3 years) are typically proficient in basic Android development and Kotlin syntax. They can handle feature implementation but require guidance on architecture and best practices. While they command lower salaries, investment in mentorship and training is essential.
Mid-level Kotlin developers (3-5 years) bring solid understanding of Kotlin's advanced features like coroutines and flow. They can architect smaller applications independently and mentor junior developers. Their salary reflects their ability to deliver complete features with minimal supervision.
Senior Kotlin developers (5+ years) offer expertise in performance optimization, complex architectural decisions, and team leadership. They often have experience migrating large Java codebases to Kotlin and implementing cutting-edge features like Kotlin Multiplatform Mobile.
Estimated annual salary for Kotlin developers based on experience
| Region | Junior (USD) | Middle (USD) | Senior (USD) |
|---|
North America | $60,000 - $80,000 | $80,000 - $100,000 | $100,000 - $130,000 |
Western Europe | $42,000 - $58,000 | $58,000 - $75,000 | $75,000 - $95,000 |
East Asia | $30,000 - $45,000 | $45,000 - $60,000 | $60,000 - $80,000 |
Eastern Europe | $15,000 - $25,000 | $25,000 - $35,000 | $35,000 - $50,000 |
Latin America | $15,000 - $25,000 | $25,000 - $35,000 | $35,000 - $50,000 |
Southeast Asia | $10,000 - $20,000 | $20,000 - $30,000 | $30,000 - $40,000 |
South Asia | $8,000 - $15,000 | $15,000 - $25,000 | $25,000 - $35,000 |
Based on Pricing Model
Fixed-price contracts work best for well-defined projects with clear requirements. They offer budget certainty but require thorough upfront planning.
Hourly rates provide flexibility for evolving projects. Rates vary from $15-120 per hour depending on expertise and location. This model works well for maintenance and ongoing development.
Retainer agreements secure dedicated developer time monthly, usually at a slight discount compared to hourly rates. Monthly retainers range from $4,000-$18,000 depending on time commitment and expertise level.
Comparison of different pricing models for hiring Kotlin developers
Hourly rates | $15-120 per hour |
Fixed price | Project dependent, typically $15,000-90,000+ |
Retainer | $4,000-$18,000 per month |
Based on hiring model
Full-time employment offers the deepest integration with your team and long-term commitment. While requiring higher initial investment in benefits and equipment, it provides stability and cultural alignment.
Freelance developers provide flexibility and specialized expertise for specific projects. They often charge premium hourly rates but don't require long-term commitments or benefits.
Outsourcing companies offer managed teams with established processes. They handle recruitment, management, and quality assurance, making them ideal for companies wanting to scale quickly while minimizing administrative overhead.
Tips on hiring Kotlin developers
Tip #1: Hire from regions with strong mobile development expertise
If you're in APAC region or willing to collaborate with offshore development team, you can consider Southeast Asia as an ideal destination for Kotlin development. With:
- Growing pool of millions of IT professionals
- Strong focus on mobile development
- High English proficiency among tech professionals
- Cost savings of 50-70% compared to Western rates
Tip #2: Focus on architecture and pattern knowledge
Beyond basic Kotlin syntax, you should evaluate candidates' understanding of clean architecture and modern Android development patterns. Create technical assessments that focus on:
- Implementing MVVM architecture with Kotlin Flow
- Handling configuration changes efficiently
- Designing modular applications using Kotlin's powerful feature set
Tip #3: Assess problem-solving through pair programming
Instead of traditional whiteboard interviews, conduct pair programming sessions focused on real-world scenarios. Have candidates:
- Refactor Java code to idiomatic Kotlin
- Implement coroutines for complex async operations
- Debug memory leaks in Android applications
At BiPlus, we provide skilled Kotlin developers with competitive rates
As an IT outsourcing company based in Vietnam, BiPlus offers high-quality Kotlin developers at significant cost savings. Our Kotlin teams have successfully delivered projects across various industries, from mobile apps to enterprise systems.
We offer:
- Experienced Kotlin developers at competitive rates (40-60% cost savings)
- Strong English communication skills.
- Agile development approach.
- Proven track record of mobile app development for multiple sectors.
- Flexible adaptation to work in different timezones.
Ready to build your Kotlin development team?
Book a quick meeting with our Kotlin teams to discuss your needs.
