Kotlin: practice, test and prove it

Kotlin basics: null safety, val and var, data classes, when, extension functions, lambdas and coroutines.

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Kotlin screenings focus on what makes the language different from Java, above all its null safety. Expect snippets that use ?., ?: and !!, questions on val versus var, data class equality and printing, when expressions, and short lambda chains on lists. Android-focused tests usually add a few coroutine basics.

This page helps you practise reading idiomatic Kotlin and predicting its output. The lessons cover null safety and data classes first, then functions, lambdas and coroutines.

Benchmark: The pass mark on this site is 80%, and Kotlin is commonly screened for Android developer roles and for some backend roles on the JVM. Requirements vary by employer.

Lessons

Null safety, val and data classes in KotlinHow nullable types and the ?., ?: and !! operators work, plus val versus var and what data classes give you.Lesson 1

Functions, lambdas and coroutines in Kotlinwhen expressions, extension functions, lambdas on collections and the coroutine basics that Android screenings ask about.Lesson 2

Classes, inheritance, objects and sealed classesPrimary constructors and init, open and override, object and companion object, interfaces with defaults, and sealed classes with when.Lesson 3

Scope functions, read-only collections, ranges and lazyPick between let, apply, also, run and with, know when a list is really mutable, loop over ranges correctly and defer work with lazy.Lesson 4

Interview prep

Kotlin interview questionsKotlin questions that are common in Android and JVM interviews, with answers that show how to reason about the language.6 questions

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Common questions

Do I need to know Java before Kotlin?

No, but it helps to know that Kotlin runs on the JVM and can call Java code directly. Many Android projects mix both languages, so being able to read basic Java is useful.

How much coroutine knowledge do screenings expect?

Usually the basics: what a suspend function is, the difference between launch and async, and why long work should not run on the main thread. Advanced Flow operators are rarely part of entry-level tests.

Is using !! ever acceptable?

It compiles, but it throws a NullPointerException when the value is null, which defeats null safety. Prefer ?., ?:, let or an early return, and treat !! as a sign the code needs another look.

Kotlin test for interviewWhat the screening round looks like, with a 5-question sample.