Kotlin — Course (zero → hero)

Every practice exercise from the full Kotlin course ported to the arena — hello-world, lists/sets/maps, control flow, loops, sort, all the way through DSL builders. Each challenge ships an animated in-IDE concept visualizer (memory diagram, active-line highlight, narration) tuned to the concept it teaches. Runs server-side via Piston.

380 challenges191 topics

How it works1Pick a challenge2Read the requirements3Code in a full IDE4Run the tests

380 challenges

easy5 min

Hello, Kotlin!

Total Counts

Set Membership

Number to Word

Dice Game

Button Actions (when)

Words Starting with 'l'

Sort Numbers Ascending

Hello world · Exercise

Hello world · Three-row multiplication snippet

Basic types · Exercise

Basic types · Fuel economy: conversion and division

Collections · Exercise 1

Collections · Exercise 2

Collections · Exercise 3

Collections · Category Revenue Report

Collections · Hashtag Extractor

Collections · Inventory Value Pipeline

Control flow · Conditional: Dice game

Control flow · Conditional: Button actions

Control flow · Loops: Pizza slices (while)

Control flow · Loops: Fizz buzz

Control flow · Loops: Words starting with l

Functions · Exercise 1: circleArea

Functions · Exercise 2: circleArea single-expression

Functions · Exercise 3: intervalInSeconds with defaults

Functions · Lambda: Build URLs

Functions · Lambda: repeatN

Classes · Exercise 1: Employee data class

Classes · Exercise 2: Person, Name, Address, City

Classes · Exercise 3: RandomEmployeeGenerator

Null safety · Exercise: salaryById

Null safety · Exercise: resolveTheme with a nullable chain

Extension functions · Exercise 1

Extension functions · Exercise 2

Extension functions · Null-safe uppercase extension

Extension functions · Generic middle-element extension

Extension functions · Initials extension property

Scope functions · Exercise 1

Scope functions · Exercise 2

Scope functions · Build and configure a server config with apply

Scope functions · Null-safe role lookup with let and elvis

Scope functions · Log a transform pipeline with also, finish with let

Scope functions · Parse a nullable timeout with let and a fallback

Scope functions · Build a Canvas with a receiver lambda + apply

Scope functions · Null-safe transform with let + Elvis

Scope functions · Configure and compute in one block with run

Scope functions · Tap a chain with also to log the prices flowing through

Scope functions · Order summary with with

Lambda expressions with receiver · Exercise 1

Lambda expressions with receiver · Exercise 2

Lambda expressions with receiver · Exercise 3

Lambda expressions with receiver · Exercise 4 — convert normal lambda to receiver

Lambda expressions with receiver · Exercise 5 — html DSL

Lambda expressions with receiver · Exercise 6 — counter DSL

Lambda expressions with receiver · Exercise 7 — apply scope function

Lambda expressions with receiver · Exercise 8 — request builder

Lambda expressions with receiver · Exercise 9 — calculator DSL

Lambda expressions with receiver · Exercise 10 — build your own buildString

Lambda expressions with receiver · Build a Canvas DSL with a lambda receiver

Lambda expressions with receiver · Build the StringBuilder DSL with a receiver lambda

Lambda expressions with receiver · Button.onEvent: write a lambda with receiver

Lambda expressions with receiver · Write a receiver lambda: List<Int>.incremented() with buildList

Lambda expressions with receiver · Build the Menu DSL with a lambda with receiver

Lambda expressions with receiver · Build a type-safe HTML DSL with receiver lambdas

Lambda expressions with receiver · Join numbers with buildString { }

Lambda expressions with receiver · Build a Config DSL with a Receiver Lambda

Classes and interfaces · Exercise 1

Classes and interfaces · Exercise 2

Classes and interfaces · Exercise 3

Classes and interfaces · Exercise 4

Objects · Exercise 1

Objects · Exercise 2

Objects · Exercise 3

Objects · Singleton seat-booking registry

Objects · Companion factory with a private constructor

Objects · Anonymous Comparator object expression

Open and special classes · Exercise 1

Open and special classes · Exercise 2

Properties · Exercise 1