Kotlin Online Compiler – Run Kotlin Code Online Free

Code

fun main(args: Array<String>) {
    println("Hello, Kotlin!")
    if (args.isNotEmpty()) {
        println("First arg: ${args[0]}")
    }
}

STDIN (optional)

Args (space separated)

Output

About this runner: Kotlin

Kotlin is easiest to reason about when the experiment is small. Put the code in the code panel, add input only if the snippet reads input, and run it once before you start editing around the problem.

Runs Kotlin through Piston from Main.kt, with the current runner deciding the Kotlin/JVM version. We chose that setup so the page can answer quick syntax and behavior questions without pretending to be your whole development machine.

Kotlin's null-safety helps, but platform types from Java interop can still surprise you when you move code into a mixed project. Tiny repros are honest. They either show the issue, or they tell you the issue lives somewhere else.

Good uses

The page is most useful before a change becomes expensive. Try the syntax here, learn the error message, then carry the cleaned-up idea back to your project.

This is a nice place to practice data classes and collections because the feedback loop stays short. That opinion is not subtle, but it saves time: online runners are for confidence, not final authority.

If a result surprises you, write down the exact input and the exact output before editing again. Tiny records like that make bug reports better, and they also keep your own memory from smoothing over the inconvenient detail that caused the failure.

The run path

The runner does only a few things. The work area holds the source, the input controls hold the data, and the output panel shows what the runner captured.

Syntax errors usually come back before your program starts.
Runtime errors are more interesting: they mean the code got far enough to execute, so the next question is about data, environment, or a Kotlin rule rather than typing.
Timeouts are a signal too. Reduce the loop or the input and run again.

The best follow-up is usually boring: rename a variable, change one input value, or remove one branch. If the behavior changes, you found the sensitive part of the Kotlin example; if it does not, you can cross that idea off the list.

A worked check

The saved example below is intentionally left unchanged. Run it once as written, then make a small edit and run it again; that gives you a known-good baseline before you test your own idea.

data class Person(val name: String, val age: Int)

fun String.toPerson(): Person? {
    val parts = split(",").map { it.trim() }
    if (parts.size != 2) return null
    val age = parts[1].toIntOrNull() ?: return null
    return Person(parts[0], age)
}

fun main() {
    val input = readLine() ?: ""
    val people = input.split(";")
        .mapNotNull { it.toPerson() }
        .sortedBy { it.age }

    people.forEach { println("${it.name} is ${it.age} years old") }
    println("Average age: ${people.map { it.age }.average()}")
}

The next preserved block belongs to the same example. Keep it nearby when you are comparing input, output, or the shape of the result. Small examples expose mistakes quickly.

Bob is 25 years old
Alice is 30 years old
Carol is 35 years old
Average age: 30.0

After the sample works, try one edge case that exercises the page's limits. Kotlin's null-safety helps, but platform types from Java interop can still surprise you when you move code into a mixed project. That single change often teaches more than pasting a large program and trying to guess which part failed.

Known gaps

The runner is not a production environment. It will not carry durable state between serious sessions, and it should not be asked to handle Gradle plugins, Android APIs, coroutines with external libraries, and multi-module builds. Good. Boundaries make bugs easier to see.

The sweet spot is data classes, null checks, collection transforms, lambdas, and console input. If your example needs a package manager, a database full of private rows, a web server listening on a port, or special build flags, move it to a local workspace before you draw conclusions.

A small runner should feel disposable. Try the idea, keep the useful lesson, and leave the accidental environment behind when the next step needs real project context.

FAQ

This FAQ is intentionally mixed: short answers for quick checks, longer notes where Kotlin has environment traps. Scan it once before assuming the runner and your local setup behave the same.

Does Kotlin compile to Java?

This page runs Kotlin through Piston using Main.kt; the wildcard version means the runner supplies its available kotlin runtime. For this question, keep the test small and concrete: one input, one expected result, and one change at a time. If the snippet depends on packages, server settings, files, or exact versions, use this page for the idea and confirm the final behavior locally.

Is Kotlin a compiled language?

Is Kotlin interpreted or compiled?

What does Kotlin compile to?

Does Kotlin compile faster than Java?

How to use the Kotlin compiler?

Version?

This page runs Kotlin through Piston using Main.kt; the wildcard version means the runner supplies its available kotlin runtime. I would treat that as good enough for syntax checks, but not as a promise about your production version. If a feature depends on a release, run the same snippet locally and check the official release notes. A quick local check is still worth doing when you use newer Kotlin syntax or APIs.

Can I use packages with Kotlin?

Assume package installs are not available here unless the page already loads a library for the demo. That keeps the run predictable and quick. If your example needs a package manager, create a local project and add the dependency there, then use this page only for the small language part. That keeps the example focused on the bug instead of package installation or module resolution.

Show me stdin.

Use the STDIN box for the text your program would normally read from the terminal. Put one input value per line. For example, enter Alice on the first line and 42 on the second, then read two values in your code. If parsing fails, test one line first before adding more cases. If the first read works, add the second line; if it fails, the parsing code is the part to fix.

Why did my run stop?

The runner may stop code that loops too long, waits for input you did not provide, or tries work outside the sandbox. Start by shrinking the input and adding one print statement before the slow section. If the smaller case finishes, the original program probably has a loop, scale, or environment problem. Kotlin null-safety helps, but Java interop and platform types can still need a local project check.

How to run Kotlin code online?

Paste your Kotlin program into the editor. Make sure there's a fun main() somewhere - top-level or inside a class. If you read from readLine(), supply each input line in the STDIN box. Click Run to compile and execute. Compile-time and runtime errors both print to the Output panel.

Docs worth opening

Reference pages are better than folklore when an error message gets specific. Start with official docs, then use tutorials for context once the rule is clear.

Free Online Kotlin Compiler