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A Kotlin course with a compiler, Coroutines lessons, quizzes and interview prep
Learn Kotlin by writing it. This Kotlin tutorial ends every lesson with code you edit and run on a real compiler (kotlinc on the JVM), so the language sticks instead of staying theory. Start from zero or pick up where you are: the course goes from your first val to coroutines, Flow, generics and DSLs. KOTLIN PROGRAMMING, STEP BY STEP • 51 courses and 203 short lessons, from beginner (A1) to advanced (C1) • Basics: variables, string templates, when expressions, ranges, loops and collections • Null safety done right: safe calls, let chains, smart casts, sealed interfaces and Result • Classes and objects: data classes, copy/equals, companion objects, delegation and enum classes • Functional Kotlin: lambdas, map/filter/fold pipelines, sequences, scope functions, inline functions and reified generics • Coroutines: cancellation, timeouts, SupervisorScope, channels, Flow, StateFlow and SharedFlow • Generics and variance, type bounds, custom property delegates, annotations and reflection • Real-world code: Ktor HTTP servers, Exposed SQL, kotlinx serialization, Gradle build scripts, JUnit 5 and MockK testing • Beyond the JVM: Multiplatform (KMP), KSP, Kotlin/Native and Wasm, Arrow and type-safe DSLs CODE EDITOR AND COMPILER • Write code in the built-in editor and run it on a JVM compiler in the cloud • Compiler errors and warnings come back with line numbers, so you learn to read them • Save your code, keep notes and come back to any lesson later THE LAB: DAILY CODING PRACTICE • Kata of the day: a small refactor with hidden tests and an idiom check • NPE Hunter: programs that really crash with a NullPointerException, and you fix them properly • Coroutine Lab: solve launch, async, await and cancel scenarios on a virtual clock • Compose preview: write a Compose-style layout and see it render instantly • Ship It: grow one program into a command-line tool in seven steps • Kotlin Doctor: see what the compiler thinks of your code and get a score • Recall cards with spaced repetition, plus a mistake journal that shows what keeps tripping you up QUIZZES, PROJECTS AND INTERVIEW PREP • Quiz questions with an explanation for every answer • Kotlin interview questions for developer job prep • A cheat sheet to look up syntax fast • 15 guided projects: every step compiles and is checked against the expected output • 60 coding missions to practice what you learned • World Coding Challenge: three global coding rounds every day, with a live leaderboard AI MENTOR • Stuck on an error or a concept? Ask the AI Mentor and get a clear answer in seconds • Free users get one AI Mentor question a day; Pro removes the limit CERTIFICATE • Pro members can take the final exam and earn a certificate of completion HOW IT WORKS 1. Start with the free first course, or jump to the topic you need 2. Read a short lesson, then write and run the Kotlin code yourself 3. Practice every day in the Lab and keep your streak going 4. Test yourself with quizzes, missions and projects WHO IT'S FOR • Beginners learning their first programming language • Java developers moving to Kotlin • Students and developers preparing for job interviews • Anyone who wants hands-on coding practice on the phone The app is available in 22 languages. FREE AND PRO Free to start: the first course, the first guided project, two missions, the daily Lab practice and one AI Mentor question a day are free. The free version shows ads. Pro unlocks every course, project and mission, the certificate exam and unlimited AI Mentor, and removes ads. Kotlin is a trademark of JetBrains s.r.o. This app is an independent learning tool and is not affiliated with or endorsed by JetBrains.