Online Kotlin Compiler – Write and Run Kotlin Code in Your Browser

Test and run Kotlin code directly in your browser using our free online Kotlin compiler. Great for Android developers and Kotlin learners — no installation required.

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📚 Everyone's learning Kotlin – what about you?

💡 Kotlin Basics Guide for Beginners

1. Declaring Variables and Constants

Kotlin uses val for immutable variables and var for mutable ones.

val name: String = "Alice"
var age: Int = 30
val pi = 3.14  // type inferred

// Constants
const val MAX_USERS = 100

2. Conditionals (if / when)

Use if expressions or when for multiple branches.

val x = 2
if (x == 1) {
    println("One")
} else if (x == 2) {
    println("Two")
} else {
    println("Other")
}

when (x) {
    1 -> println("One")
    2 -> println("Two")
    else -> println("Other")
}

3. Loops

Use for, while, and do-while for iteration.

for (i in 0..2) {
    println(i)
}

var n = 3
while (n > 0) {
    println(n)
    n--
}

4. Arrays

Kotlin supports arrays using the arrayOf() function.

val numbers = arrayOf(10, 20, 30)
println(numbers[1])

5. List Manipulation

Use mutableListOf for dynamic lists.

val nums = mutableListOf(1, 2, 3)
nums.add(4)
nums.removeAt(0)

for (n in nums) {
    print("$n ")
}

6. Console Input/Output

Use readLine() for input and println() for output.

print("Enter your name: ")
val name = readLine()
println("Hello, $name")

7. Functions

Functions use fun keyword with optional return type.

fun add(a: Int, b: Int): Int {
    return a + b
}

println(add(3, 4))

8. Maps

mutableMapOf stores key-value pairs.

val ages = mutableMapOf("Alice" to 30)
println(ages["Alice"])

9. Exception Handling

Use try, catch, and finally to handle errors.

try {
    val result = 10 / 0
} catch (e: ArithmeticException) {
    println("Error: ${e.message}")
}

10. File I/O

Use File from java.io for file operations.

import java.io.File

File("file.txt").writeText("Hello File")
val text = File("file.txt").readText()
println(text)

11. String Manipulation

Kotlin strings support many methods and interpolation.

val text = "Hello World"
println(text.length)
println(text.substring(0, 5))
println(text.contains("World"))

12. Classes & Objects

Kotlin classes are concise and support default constructors.

class Person(val name: String) {
    fun greet() = println("Hi, I'm $name")
}

val p = Person("Alice")
p.greet()

13. Null Safety

Kotlin distinguishes nullable and non-nullable types. Use ? and ?: for safety.

val name: String? = null
println(name?.length ?: 0)