Kotlin is a modern programming language created by JetBrains, the company behind the IntelliJ IDEA editor. In 2019 Google made it the preferred language for Android apps, and today most new Android apps are written in it. Kotlin is also used to build web servers, desktop tools and apps that share code between Android and iOS.
Kotlin runs on the Java Virtual Machine (JVM), the same engine that runs Java, so it can use every Java library ever written. But it was designed to fix many of Java's frustrations: code is shorter, null errors are caught before the program runs, and common tasks need far less typing.
In this lesson you'll learn where to run Kotlin, write your first program, print text to the screen and add comments. You don't need to know any other language first, though if you've used Java, Kotlin will feel familiar.
Where to write and run Kotlin
You have three easy options:
- Kotlin Playground (play.kotlinlang.org): runs Kotlin in your browser with nothing to install. Perfect for this course's examples.
- IntelliJ IDEA Community Edition: JetBrains' editor, with Kotlin built in. Choose New Project, pick Kotlin, and click the green run arrow next to
main. - Android Studio: based on IntelliJ, used for Android apps.
Kotlin source files end in .kt. Behind the scenes the Kotlin compiler turns your code into JVM bytecode, which the Java runtime then runs. You don't need to manage those steps yourself; the run button does it for you.
The main function
Every Kotlin program starts in a function called main:
fun main() {
println("Hello, Kotlin!")
}fundeclares a function, andmainis its name.- The empty brackets
()mean it takes no input. - The curly braces
{ }hold the code that runs, one statement per line. - Unlike Java, there's no class around it and no semicolons at the end of lines. Kotlin lets you leave them out.
println, print and comments
println prints its value and then moves to a new line. print prints without the new line, so the next output continues on the same line:
print("Ready, ")
print("steady, ")
println("go!")Comments are notes for people that the compiler ignores. // starts a comment that runs to the end of the line, and /* ... */ can span several lines. Use them to explain why code does something, not to repeat what it obviously does.
How Kotlin runs: the JVM, Android and beyond
Kotlin code is compiled before it runs. The most common target is the Java Virtual Machine (JVM): the Kotlin compiler turns your .kt files into the same bytecode that Java produces, so Kotlin programs run anywhere Java does, on servers, desktops and Android phones.
Because both languages share the JVM, Kotlin and Java work together in the same project. A Kotlin file can call any Java library, and Java code can call Kotlin. That's why companies could adopt Kotlin gradually, one file at a time, instead of rewriting everything.
The same language also compiles to other targets:
| Target | Used for |
|---|---|
| Kotlin/JVM | Android apps, backend services with Spring or Ktor, desktop tools |
| Kotlin/JS and Kotlin/Wasm | Code that runs in web browsers |
| Kotlin/Native | Programs that run without a JVM, such as iOS code |
| Kotlin Multiplatform | Sharing business logic between Android, iOS, web and desktop |
This course focuses on the language itself, which is the same on every target, and runs its examples on the JVM.
Kotlin Basics Syntax
A Kotlin program
fun main() {
// a one-line comment
println("Text on its own line")
print("No new line after this")
}Kotlin Basics Summary
- Kotlin is a modern JVM language used for Android, servers and multiplatform apps.
- Programs start in
fun main(); files end in.kt. printlnprints and adds a new line;printdoesn't.- Semicolons are optional, and
maindoesn't need a class around it. //and/* */write comments the compiler ignores.
Kotlin Basics Examples
Each example shows the code, the output it produces and what each part does.
Your first program
fun main() {
println("Hello, Kotlin!")
println("I'm learning to code.")
}Hello, Kotlin!
I'm learning to code.Line-by-line explanation
- Line 1Every program starts in main.
- Lines 2 to 3Each println prints its text on a new line.
print versus println
fun main() {
print("Loading")
print(".")
print(".")
println(".")
println("Done!")
}Loading...
Done!Line-by-line explanation
- Lines 2 to 4print keeps the output on the same line.
- Line 5println finishes the line, so Done! starts on the next one.
Numbers and comments
fun main() {
// Kotlin can calculate as well as print text
println(7 + 5)
println("7 + 5")
/* The first line prints the result of the sum.
The second prints the characters exactly as written. */
println(10 * 3)
}12
7 + 5
30Line-by-line explanation
- Line 3Without quotes, Kotlin calculates the sum and prints 12.
- Line 4Inside quotes it's just text, printed exactly as written.
- Lines 5 to 6A multi-line comment is ignored when the program runs.
Common Kotlin Basics Mistakes
Capital letters in println or main
fun Main() {
Println("Hi")
}fun main() {
println("Hi")
}Kotlin is case-sensitive: main and println must be lowercase.
Forgetting the quotes around text
println(Hello) // error: Unresolved reference: Helloprintln("Hello")Code outside main
println("Hi")
fun main() { }fun main() {
println("Hi")
}Statements like println must be inside a function.
Kotlin Basics in Real Projects
Kotlin in practice
Apps such as Google Maps, Pinterest, Trello and Duolingo use Kotlin on Android. Companies including Netflix, Atlassian and Amazon use it on their servers, often with frameworks like Spring Boot and Ktor. Kotlin Multiplatform lets teams write business logic once and share it between Android, iOS and desktop apps.
Kotlin Basics FAQ
Is Kotlin better than Java?
Kotlin was designed to fix common pain points in Java: it needs less boilerplate code, builds null safety into the type system and adds features such as data classes and extension functions. Java remains excellent and widely used, and the two work together in one project. For new Android apps, Google recommends Kotlin.
Do I need to learn Java before Kotlin?
No. This course assumes no programming experience. Knowing Java helps when you read older Android code or use Java libraries, but you can learn Kotlin first and pick up the Java you need later.
Is Kotlin only for Android?
No. Android is the best-known use, but Kotlin is also used for backend services with Spring Boot and Ktor, command-line tools, data processing and, through Kotlin Multiplatform, sharing code between Android, iOS, web and desktop apps.
Which Kotlin version should I learn?
Learn a current Kotlin 2 release. Kotlin 2.0 introduced the faster K2 compiler, and the language you write is almost the same as in late Kotlin 1.x versions, so tutorials from the last few years still apply.
Is Kotlin free to use?
Yes. Kotlin is open source under the Apache 2.0 licence and free for any use, including commercial apps. IntelliJ IDEA Community Edition and Android Studio, the main editors for Kotlin, are free too.
Kotlin Basics Exercises
Answer each one and check it instantly. If you get stuck, open the hint before the answer.
Complete the program so it prints Hi.
main() {
("Hi")
}What does this print?
fun main() {
print("A")
print("B")
println("C")
}What does this print?
fun main() {
println("2 + 2")
println(2 + 2)
}Which file extension do Kotlin source files use?
Why can Kotlin programs use Java libraries?
Kotlin Basics Quiz
5 questions to check what you learned.
Where does every Kotlin program start?
- In a class called Program
- On the first line of the file
- In a function called start
- In the main function
Which company created Kotlin?
- JetBrains
- Oracle
- Microsoft
Are semicolons required at the end of Kotlin lines?
- No, they're optional
- Yes, always
- Only inside main
- Only after println
What does
//start?- A division
- A string
- A one-line comment
- A new function
What's the difference between print and println?
- println is faster
- print only works with numbers
- println moves to a new line after printing; print doesn't
- There's none