Your First Kotlin Program

Every Kotlin program starts in a function called main:

fun main() {
    println("Hello, world!")
}

Run it with the ▶ button in your IDE, or from a terminal:

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

Expected output

Hello, world!

Line by line:

• fun main() declares the main function — the program's starting point. fun is the keyword for a function.
• The curly braces { } hold the function's code.
• println prints a value followed by a new line. (print prints without the new line.)
• "Hello, world!" is a string — text in double quotes.
• No semicolons are needed at the end of lines.

A slightly longer program runs line by line, top to bottom:

fun main() {
    println("Hello, world!")
    println("I am learning Kotlin.")
    println(2 + 3)
}

Hello, world!
I am learning Kotlin.
5

Common mistakes

• Writing Println or PrintLn. Kotlin is case-sensitive: Unresolved reference 'Println'.
• Using single quotes for text. 'Hello' is an error — single quotes are only for a single character, such as 'A'.

Practice

Write a program that prints your name on one line and your favorite food on the next.

fun main() {
    println("Ada")
    println("Pizza")
}