---
title: Kotlin Interview Questions & Answers (2026): Null Safety, Coroutines & More
description: The Kotlin interview questions that get asked in 2026 — null safety, val vs var, data classes, coroutines, extension functions and Kotlin vs Java — with clear answers.
url: https://usegreenroom.app/blog/kotlin-interview-questions
last_updated: 2026-06-19
---

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Technical

# Kotlin interview questions and answers

June 19, 2026 · 9 min read

![Kotlin interview questions and answers — cover from Greenroom, the AI mock interviewer](/assets/blog/kotlin-interview-questions-hero.webp)

Kotlin is the default language for Android and increasingly popular on the backend, and its interviews focus on the features that solve Java's pain points: **null safety**, concise data classes, coroutines for async, and extension functions. Here are the **Kotlin interview questions** that actually get asked, with answers. (See also our Java guide.)

## Null safety & basics

- How does Kotlin's **null safety** work — nullable vs non-null types?
- The safe call `?.`, Elvis `?:`, and `!!` operators.
- `val` vs `var`; type inference.
- What is a data class, and what does it generate?

## Functions & classes

- Extension functions — what and why.
- Higher-order functions and lambdas.
- Companion objects; object declarations (singletons).
- Sealed classes and when to use them.

![Kotlin interview topics — null safety, coroutines, data classes, extensions](/assets/blog/pool-structured-screen.webp)

Kotlin rounds test null safety and coroutines — the features that beat Java's pain points.

## Coroutines & interop

- What are **coroutines** and how do they differ from threads?
- suspend functions; launch vs async.
- Coroutine scopes and dispatchers.
- Kotlin and Java interoperability.

**The core truth:** Kotlin interviews reward knowing what Kotlin fixes about Java — null safety and coroutines especially. The candidate who explains how the type system prevents NullPointerExceptions shows real understanding.

## How to prepare

Kotlin rounds probe its distinctive features. Practise explaining null safety and coroutines out loud. Greenroom runs spoken technical interviews that follow up on your reasoning. Pair it with our Java and Android guides.

## Frequently asked questions

### What are the most common Kotlin interview questions?

Common Kotlin questions cover null safety (nullable vs non-null types, the ?., ?: and !! operators), val vs var and type inference, data classes, extension functions, higher-order functions and lambdas, companion objects and object declarations, sealed classes, coroutines (suspend functions, launch vs async, scopes and dispatchers), and Kotlin-Java interoperability.

### How does null safety work in Kotlin?

Kotlin's type system distinguishes nullable types (String?) from non-null types (String) at compile time, so you can't assign null to a non-null type or call methods on a nullable type without handling null. The safe call operator ?. returns null instead of throwing, the Elvis operator ?: provides a default, and !! asserts non-null (throwing if wrong), largely eliminating NullPointerExceptions.

### What are coroutines in Kotlin?

Coroutines are Kotlin's solution for asynchronous, non-blocking code. A coroutine is a lightweight unit of work that can suspend and resume without blocking a thread, so you can run many concurrently. suspend functions can pause execution, launch starts a coroutine that returns a Job, and async returns a Deferred result. Scopes and dispatchers control lifecycle and which thread pool runs the work.

### How should I prepare for a Kotlin interview?

Focus on the features that distinguish Kotlin from Java — null safety, coroutines, data classes, extension functions and sealed classes — since interviewers want to know what Kotlin improves and how. Practise explaining null safety and coroutines out loud with a voice-based mock interview that follows up, because these rounds probe understanding, not just syntax.

Kotlin rounds reward knowing what it fixes about Java, out loud. Greenroom runs spoken technical interviews that follow up on your reasoning. Free to start.