--- title: "Kotlin Interview Questions (2026): By Level, With Model Answers" url: https://weworkworldwide.com/kotlin-developer-interview-questions/ description: "Kotlin interview questions for junior, mid and senior developers — null safety, coroutines, data classes and extension functions — with model answers and red flags." date: 2026-07-04T15:33:07+00:00 source: https://weworkworldwide.com/llms.txt --- # Kotlin Interview Questions (2026): By Level, With Model Answers How to use this Kotlin fixes a lot of Java’s sharp edges, but only if you use it idiomatically. These questions check whether a candidate writes Kotlin or Java-in-Kotlin. Hiring a Kotlin developer is easy. Telling a real one from a convincing résumé is the hard part — and it’s most of what we do. These are grouped by level, because the same question that stretches a junior is a warm-up for a senior. ## Junior Kotlin interview questions 0–2 years Language fundamentals. ### How does null safety work in Kotlin? What a strong answer covers Types are non-nullable by default; nullable types use `?`, and the compiler forces you to handle nulls with `?.`, `?:` or checks. Red flag Uses `!!` everywhere, defeating null safety. ### What is the difference between `val` and `var`? What a strong answer covers `val` is a read-only reference, `var` is reassignable; prefer `val` for immutability. Red flag Uses `var` by default with no reason. ### What is a data class? What a strong answer covers A class that auto-generates `equals`, `hashCode`, `toString` and `copy` for holding data. Red flag Writes boilerplate equals/hashCode by hand. ### What are the safe-call and Elvis operators? What a strong answer covers `?.` calls only if non-null; `?:` provides a fallback value; together they replace verbose null checks. Red flag Uses nested if-null checks instead. ### What is the difference between `==` and `===`? What a strong answer covers `==` calls `equals` (structural), `===` checks referential identity. Red flag Assumes `==` is reference equality like Java. ### What are `when` expressions? What a strong answer covers A powerful switch that can return a value, match ranges/types, and be exhaustive over sealed types. Red flag Writes long if-else chains instead. ### What is string templating? What a strong answer covers Embedding expressions in strings with `$` and `${}` rather than concatenation. Red flag Concatenates strings with `+` everywhere. ### What are default and named arguments? What a strong answer covers Parameters can have defaults and be passed by name, reducing overloads and improving readability. Red flag Writes many overloads instead of defaults. ## Mid-level Kotlin interview questions 2–5 years Idioms and coroutines. ### What are extension functions? What a strong answer covers Functions that add behaviour to existing types without inheritance, resolved statically. Red flag Wraps classes unnecessarily to add helpers. ### What are coroutines and how do they differ from threads? What a strong answer covers Lightweight, suspendable computations for asynchronous code that read sequentially; many coroutines multiplex onto few threads. Red flag Blocks threads instead of suspending. ### What is `suspend` and structured concurrency? What a strong answer covers `suspend` functions can pause without blocking; structured concurrency scopes coroutines so they’re cancelled together and don’t leak. Red flag Launches coroutines with no scope, leaking them. ### What are sealed classes and why use them? What a strong answer covers Restricted class hierarchies that enable exhaustive `when` handling, great for modelling states/results. Red flag Uses enums or open classes where sealed fits. ### What are higher-order functions and lambdas? What a strong answer covers Functions taking or returning functions; lambdas plus the trailing-lambda syntax make DSL-like APIs readable. Red flag Avoids functional style and writes verbose loops. ### What do `let`, `apply`, `run`, `also`, `with` do? What a strong answer covers Scope functions that differ in receiver (`this` vs `it`) and return value; used for null handling and configuration. Red flag Uses them randomly without knowing the difference. ### What is the difference between a `List` and a `MutableList`? What a strong answer covers The read-only `List` interface vs the mutable one; Kotlin encourages immutable collections by default. Red flag Defaults to mutable collections everywhere. ### How does Kotlin interoperate with Java? What a strong answer covers Seamless two-way interop; platform types from Java can be null, so you add null handling at the boundary. Red flag Trusts Java-returned values as non-null and hits NPEs. ## Senior Kotlin interview questions 5+ years Concurrency, design and Android. ### How do coroutine dispatchers and context work? What a strong answer covers Dispatchers (Main, IO, Default) decide which thread pool runs the work; you switch context with `withContext` for the right workload. Red flag Runs blocking IO on the Main dispatcher. ### What are Flows and how do they differ from suspend functions? What a strong answer covers Cold asynchronous streams of values with operators and backpressure, versus a single suspended result. Red flag Uses one-shot suspend functions where a stream is needed. ### How do you handle coroutine exceptions and cancellation? What a strong answer covers Cancellation is cooperative and propagates through the scope; use `SupervisorJob` and handlers where children should fail independently. Red flag Swallows `CancellationException` and breaks cancellation. ### What are inline functions and reified generics for? What a strong answer covers `inline` avoids lambda allocation overhead and enables `reified` type parameters so generic types are available at runtime. Red flag No idea why some generic functions can inspect types. ### How do you design idiomatic APIs in Kotlin? What a strong answer covers Immutability, null safety at boundaries, sealed results, extension functions and DSLs where they aid readability. Red flag Writes Java-style APIs in Kotlin. ### How do delegated properties work (e.g. `by lazy`)? What a strong answer covers Property access is delegated to an object implementing get/set, enabling lazy initialisation, observables and map-backed properties. Red flag Reimplements lazy initialisation manually and unsafely. ### What is Kotlin Multiplatform and its tradeoffs? What a strong answer covers Sharing business logic across platforms while keeping native UIs; it reduces duplication but adds build and tooling complexity. Red flag Thinks it means one UI for all platforms. ### How do you test coroutine-based code? What a strong answer covers A test dispatcher and runTest to control virtual time, making suspending code deterministic. Red flag Adds real delays and flaky sleeps in tests. **Skip the screening entirely.**We vet Kotlin engineers so you don’t have to — embed one in your team, or have us build it. [Hire Kotlin developers](https://weworkworldwide.com/hire-remote-kotlin-developers/)[Compare us](https://weworkworldwide.com/compare/) Build and score a full interview with our free [interview scorecard tool](https://weworkworldwide.com/developer-interview-scorecard/), browse the [full question hub](https://weworkworldwide.com/interview-questions/), or see [how we interview engineers](https://weworkworldwide.com/how-we-interview-engineers/).