Null-safe greeting
Write a function that returns a user's nickname, or "guest" when nickname is null. Use the Elvis operator.
Languages / Knowledge library
Kotlin
a statically typed language for jvm and multiplatform
Why learn Kotlin?
Kotlin helps teams write safer, shorter code than typical Java while staying fully interoperable with existing JVM ecosystems. It also supports Android, backend, and multiplatform apps from one language, reducing context switching and duplicated business logic.
Where it came from
2010: JetBrains started Kotlin to improve developer productivity on the JVM. 2016: Kotlin 1.0 was officially released. 2017: Google announced Kotlin as a first-class language for Android. 2020s: Kotlin Multiplatform and coroutines adoption grew across mobile and backend teams. 2024: Kotlin 2.0 promoted the new K2 compiler to default status.
- 2010: JetBrains started Kotlin to improve developer productivity on the JVM.
- 2016: Kotlin 1.0 was officially released.
- 2017: Google announced Kotlin as a first-class language for Android.
- 2020s: Kotlin Multiplatform and coroutines adoption grew across mobile and backend teams.
- 2024: Kotlin 2.0 promoted the new K2 compiler to default status.
Build your understanding
Work through these exercises in order. Try the task before opening a reference.
- Write a function that returns a user's nickname, or "guest" when nickname is null. Use the Elvis operator.
- Given a list of Int, return the first 3 squared even numbers using filter + map + take.
- Model API states with a sealed class (Loading, Success, Error) and render a message using when.
- Create a shared validation function in common code and call it from platform-specific entry points.
- Implement a suspend function with timeout and one retry on failure; return a typed Result.
More ways to practice
Collection pipeline
Given a list of Int, return the first 3 squared even numbers using filter + map + take.
Sealed result handler
Model API states with a sealed class (Loading, Success, Error) and render a message using when.
Mini multiplatform domain module
Create a shared validation function in common code and call it from platform-specific entry points.
Coroutine timeout retry
Implement a suspend function with timeout and one retry on failure; return a typed Result.
Documentation and sources
- Kotlin documentation (kotlinlang.org)
- Get started with Kotlin (kotlinlang.org)
- Kotlin Playground (play.kotlinlang.org)
- Coroutines overview (kotlinlang.org)
- Kotlin Multiplatform docs (kotlinlang.org)
Catalog research notes
- Kotlin 2.0 made the K2 compiler the default, improving compile speed and diagnostics.
- Kotlin Multiplatform setup became simpler with clearer source-set defaults in Gradle.
- Kotlin/Wasm progressed further for running Kotlin apps in modern browsers.
Connect the ideas
Explore next
Real World Example
Unified Cross-Platform Inventory Logic
A retail logistics firm managing inventory across mobile Android devices and web-based warehouse management systems.
The business problem
The firm suffers from inconsistent stock calculation logic because separate teams maintain Java-based Android apps and JavaScript-based web services, leading to frequent inventory discrepancies.
How this topic solves it
Kotlin Multiplatform allows the team to consolidate core inventory calculation logic into a single shared module that compiles to both JVM bytecode for the backend and native code for mobile clients, ensuring identical business rules.
- Define the core inventory calculation logic within a shared Kotlin Multiplatform module.
- Configure the build system to target both the JVM for the backend service and native targets for mobile.
- Expose the shared business logic as a library dependency for both the Android application and the web backend.
- Implement platform-specific UI layers that invoke the shared logic for data processing.
This diagram illustrates how shared Kotlin logic flows into different platform targets.
Contains core inventory logic
Consumes shared logic
Consumes shared logic
Data source
- Shared Module → compiles to → JVM Backend
- Shared Module → compiles to → Android App
- JVM Backend → queries → Inventory Database
Implementation example
This function represents the shared inventory logic that is compiled once and used by both the JVM backend and the Android mobile application.
fun calculateStock(items: List<Int>): Int { // Shared logic function
return items.sum() // Consistent calculation across platforms
}Expected business outcome
The firm achieves a single source of truth for inventory math, eliminating logic drift and reducing the time required to update stock calculation rules across all platforms.
Limitations and checks
The primary tradeoff is the increased complexity of the build configuration and the need to manage platform-specific dependencies, which can be verified by inspecting the build output logs for unexpected target-specific compilation errors.