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Your First Kotlin Program

Your first Kotlin program is a tiny set of instructions that tells the computer to display a message on the screen.

In this page:

  1. The main() Function
  2. Printing Hello, World
  3. Understanding the Syntax
  4. Compiling and Running
Syntax
kotlin 
fun main() {
    println("text")
}

The main() Function

Every runnable Kotlin program needs an entry point function called main, marked with the fun keyword. When you run the compiled program, execution starts at the first line inside main's curly braces.

Example: The main() Function

markup 
// Entry point: execution of the program starts here
fun main() {
    // Print "Hello from main()" to the console, with a trailing newline
    println("Hello from main()")
}

Printing Hello, World

The classic first program in any language prints a greeting to the screen. In Kotlin, println("Hello, World!") does exactly that, sending the text to standard output followed by a newline.

Note: println adds a newline after the text; print does not.

Example: Printing Hello, World

markup 
// Entry point: execution of the program starts here
fun main() {
    // Print "Hello, World!" to the console, with a trailing newline
    println("Hello, World!")
}

Understanding the Syntax

Curly braces { } mark the start and end of the function body, and each statement inside typically sits on its own line. Unlike some languages, Kotlin does not require a semicolon at the end of each line.

Example: Understanding the Syntax

markup 
// Entry point: execution of the program starts here
fun main() {
    // Print "Curly braces group statements together" to the console, with a trailing newline
    println("Curly braces group statements together")
    // Print "No semicolon needed at the end of a line" to the console, with a trailing newline
    println("No semicolon needed at the end of a line")
}

Compiling and Running

Save the code in a file such as Hello.kt, then compile it with kotlinc Hello.kt -include-runtime -d hello.jar and run the result with java -jar hello.jar to see the output printed in your terminal.

Example: Compiling and Running

markup 
// Entry point: execution of the program starts here
fun main() {
    // Print "Saved as Hello.kt, compiled, then run" to the console, with a trailing newline
    println("Saved as Hello.kt, compiled, then run")
}
Related Topics

Setting Up IntelliJ IDEA · The main() Function · Installing Kotlin · print() vs println()

{# common_mistakes/chapter_summary/browser_support: on Hindi pages the view already swaps in the hi_ translation fields (or blanks these out if untranslated), so this renders correctly for both languages without a lang_code check here. #}
Common Mistakes
  1. Forgetting the fun keyword before main, which is required to define any function in Kotlin.
  2. Misspelling println as print_ln or Println, causing a compiler error since Kotlin is case-sensitive.
  3. Leaving off the closing } for the main function body and getting a confusing 'expecting a top level declaration' error.
Chapter Summary
  • Every standalone Kotlin program needs a fun main() function as its entry point.
  • println() prints text followed by a newline; text must be wrapped in double quotes as a string.
  • Kotlin is case-sensitive, so Main, main, and MAIN are three different identifiers.
  • Kotlin source files typically end in .kt and are compiled with kotlinc.
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