Kotlin Functions

How it works

A function declares parameters and a return type; expression-bodied functions can infer the result type. Kotlin also has top-level functions, so not every helper needs a class. Write a narrow contract and validate invalid input where the function owns that responsibility.

Run this Kotlin example

These are local Kotlin/JVM programs. Install a JDK and the compiler using the official Kotlin compiler guide, or use an IDE with Kotlin support. Check java -version and kotlinc -version in a new terminal. The reviewed compiler is Kotlin 2.4.21 with a JVM 21 target, executed on Temurin JDK 25.0.5. Use JDK 21 or newer for this target. Android, Kotlin/JS and Kotlin/Native require separate platform projects and are not the tested environment here.

Save the complete source as functions.kt in its own learning folder. The package line gives this example a namespace, not a network address. Bhylu has no Kotlin browser runner; do not use the JavaScript or Python editor. Apache, PHP and MySQL are not required.

Compile with kotlinc functions.kt -jvm-target 21 -include-runtime -d lesson.jar, then run java -jar lesson.jar. Use a learning folder you own and keep generated JARs outside the website. These sources require no third-party application libraries.

Download the complete functions.kt · Download all source files with hash checks

For the optional collection, save download-examples.mjs in a new learning folder and run node download-examples.mjs. A local preview uses node download-examples.mjs http://127.0.0.1:8000/examples/kotlin/. It refuses an existing bhylu-kotlin-examples folder and installs or executes no Kotlin code. Compile one ordinary lesson at a time by filename.

Example

package bhylu.lessons.k_functions

fun freeBikes(total: Int, hired: Int): Int {
    require(total >= 0 && hired in 0..total) { "Invalid bicycle counts" }
    return total - hired
}

fun main() {
    println(freeBikes(7, 2))
}

Expected result

5

freeBikes checks the capacity relationship before subtraction. Its caller controls output, making the helper reusable in tests and other applications. The result remains an Int because its checked domain fits.