Setting Up Kotlin: REPL, IntelliJ & Online Playground
Explore the Kotlin playground, REPL, and IntelliJ IDEA to run your first snippets.
Setting Up Kotlin: REPL, IntelliJ & Online Playground is a free Kotlin Academy lesson on CoddyKit — lesson 1 of 4. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the Kotlin Academy learning path, one of 4 lessons in the course, and your progress syncs across the web and the CoddyKit app.
Why Set Up an Environment?
Before writing real Kotlin code you need a way to edit, compile, and run it. Kotlin supports three common entry points: the online Playground, the command-line REPL, and IntelliJ IDEA.
Kotlin Playground
The Kotlin Playground at play.kotlinlang.org runs Kotlin in your browser — no install required. Perfect for tiny snippets and learning.
// Try this in the Playground:
fun main() {
println("Hello from Playground!")
}Your First Snippet
The simplest Kotlin program is a main function that prints to the console. Run this snippet and watch the output appear.
fun main() {
println("Hello, Kotlin!")
}Installing IntelliJ IDEA
IntelliJ IDEA Community Edition is the recommended local IDE: free, Kotlin-native, and bundled with the Kotlin compiler. Download from jetbrains.com/idea.
Creating a Kotlin Project
In IntelliJ: File → New → Project, pick Kotlin, JVM, and a JDK. IntelliJ scaffolds a src/main/kotlin folder where your .kt files live.
Running Code in IntelliJ
Add a file Main.kt, paste a main function, and click the green ▷ in the gutter. IntelliJ compiles and runs in one step.
fun main() {
println("Running in IntelliJ")
println("Kotlin version active")
}The Kotlin REPL
The REPL (Read-Eval-Print Loop) lets you type Kotlin expressions and see results immediately. Launch it from IntelliJ (Tools → Kotlin → REPL) or via kotlinc on the command line.
// Sample REPL session:
// >>> val x = 10
// >>> x * 5
// res1: kotlin.Int = 50REPL-Style Expressions
Anything you can type in the REPL also works in a main function. Try arithmetic, string operations, and variable declarations.
fun main() {
val x = 10
val y = 5
println(x + y) // 15
println("Hello".length) // 5
}Command-Line kotlinc
Install the Kotlin compiler via Homebrew (brew install kotlin) or SDKMAN. Then kotlinc Hello.kt -include-runtime -d Hello.jar builds a runnable JAR.
Multiple println Calls
Each println prints one line. Use it freely to trace what your program is doing.
fun main() {
println("Line 1")
println("Line 2")
println("Line 3")
}Picking Your Tool
For tiny experiments: Playground. For learning and projects: IntelliJ. For quick one-liners: REPL. Most production work happens in IntelliJ.
Quick Check
Which tool requires no installation and runs Kotlin entirely in the browser?
Recap
Three ways to run Kotlin: Playground (zero install), IntelliJ (full IDE), and the REPL (quick experiments). Pick the lightest tool that fits the task.
Frequently asked questions
Is the “Setting Up Kotlin: REPL, IntelliJ & Online Playground” lesson free?
Yes — the full text of “Setting Up Kotlin: REPL, IntelliJ & Online Playground” is free to read here on the web, and the Kotlin Academy course includes 4 lessons in total. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the Kotlin Academy course, upgrade to CoddyKit PRO.
What will I learn in “Setting Up Kotlin: REPL, IntelliJ & Online Playground”?
Explore the Kotlin playground, REPL, and IntelliJ IDEA to run your first snippets. You practise Kotlin Academy with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.
Do I need any experience to start Kotlin Academy?
No prior experience is required. Kotlin Academy on CoddyKit is structured for beginners through advanced learners; this is — lesson 1 of 4, so you can start here or from the beginning and move at your own pace.
How long does the “Setting Up Kotlin: REPL, IntelliJ & Online Playground” lesson take?
Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.
Can I write and run code in this Kotlin Academy lesson?
Yes. Every Kotlin Academy lesson includes a built-in code editor, so you write and run real code right in your browser and get instant AI feedback — no local setup required.
All lessons in this course
- Setting Up Kotlin: REPL, IntelliJ & Online Playground
- val vs var: Immutability From Day One
- Kotlin Basic Types: Int, Double, Boolean, Char
- Your First Kotlin Program: main, println & Input