Build Tools & the Compiler

Real projects are built with Gradle, but it helps to understand the compiler underneath. This chapter covers kotlinc, the REPL, and your first Gradle project.

The command-line compiler (kotlinc)

If you installed kotlinc in Setup, you can compile and run a file directly.

Create Hello.kt:

fun main() {
    println("Compiled with kotlinc")
}

Compile to a JAR and run it:

kotlinc Hello.kt -include-runtime -d hello.jar
java -jar hello.jar

Or run a .kt file as a script without producing a JAR:

kotlinc -script script.kts

The REPL

The REPL (Read-Eval-Print Loop) lets you evaluate Kotlin interactively — great for experimenting:

$ kotlinc
Welcome to Kotlin version 2.0 (...)
>>> val x = 21
>>> x * 2
res1: kotlin.Int = 42
>>> :quit

Your first Gradle project

Gradle automates compiling, testing, packaging, and dependency management. Most Kotlin and all Android projects use it.

Generate a project skeleton:

mkdir my-app && cd my-app
gradle init --type kotlin-application --dsl kotlin

A typical Gradle Kotlin project looks like this:

my-app/
├── settings.gradle.kts        # project name, module list
├── build.gradle.kts           # plugins & dependencies (root)
├── gradle/
│   └── libs.versions.toml      # version catalog
├── gradlew, gradlew.bat        # the Gradle wrapper (run without installing Gradle)
└── app/
    ├── build.gradle.kts
    └── src/
        ├── main/kotlin/...     # production code
        └── test/kotlin/...     # tests

Build and run with the wrapper (./gradlew), which downloads the exact Gradle version the project expects:

./gradlew build      # compile + test
./gradlew run        # run the application

Always commit the Gradle wrapper (gradlew, gradlew.bat, gradle/wrapper/). It lets anyone build the project with the right Gradle version, without installing Gradle themselves.

The guide’s examples

This guide’s runnable code lives in the examples/ directory as two Gradle builds: examples/core/ (plain Kotlin/JVM, JDK 17 only) and examples/android/ (the Android apps). Each defines its dependency versions in a gradle/libs.versions.toml version catalog. See the Examples & Exercises page for how to build them.

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