Your first program

Learn to write your first Kotlin program with fun main and println.

Your first program: fun main and println

The heart of a Kotlin program is the fun main() function. The fun keyword declares a function; the name main is the entry point. To write text to the screen you use println(...).

You put a string between the parentheses, written between double quotes: "Hello". Kotlin does not require semicolons at the end of statements.

fun main() {
    println("Hello Kotlin")
}

Exercise Write your first program

Write a fun main(). Print Hello Kotlin with println.

Show example solution

fun main() {
    println("Hello Kotlin")
}

Compile and run

Kotlin source code (a .kt file) is first compiled into JVM bytecode, then run on the JVM. From the command line you compile with kotlinc Hello.kt -include-runtime -d Hello.jar and run with java -jar Hello.jar.

IntelliJ IDEA and Android Studio do these steps automatically; you just write the code.

Test yourself

Which is a program's entry point?
- fun main
- println
- String
- JVM
Show answer

fun main — fun main is the entry point of a Kotlin program.

Your first program: fill in the missing part

fun ____() { println("Hello") }

Show answer

fun main() { println("Hello") }

What does println do?
- Declares a variable
- Writes a line to the screen
- Declares a class
- Declares a function
Show answer

Writes a line to the screen — println writes a line to the screen and moves to a new line.

Next lesson: The main function

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