Kotlin Introduction

Kotlin is a modern, safe and easy-to-read programming language, used most for Android apps.

What is Kotlin?

Kotlin was created by JetBrains and released in 2016
In 2019 Google announced that Kotlin is the preferred language for Android
Kotlin runs on the JVM (Java Virtual Machine), so it works with all Java libraries
Kotlin code is shorter than Java code and protects you from many errors

Where is Kotlin Used?

Area	Example
Android apps	Food delivery, banking and shopping apps
Server side	Web APIs with Ktor or Spring Boot
Desktop	Programs built with Compose for Desktop
Multiplatform	Sharing code between Android and iPhone apps

How to Run Kotlin

You have two easy choices:

Kotlin Playground: an online editor at play.kotlinlang.org. Nothing to install.
IntelliJ IDEA or Android Studio: free editors from JetBrains and Google. Create a new Kotlin project and write code in a file that ends with .kt.

For this tutorial the online Playground is enough. You will need Android Studio later when you start building apps.

Your First Program

Example

kotlin

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

Output

Hello, Workownix!

The example explained

Code	Meaning
`fun`	Starts a function
`main()`	The function where every Kotlin program starts
`{ }`	Curly braces hold the code of the function
`println()`	Prints text and then moves to a new line
`"Hello, Workownix!"`	A string: text inside double quotes

Unlike Java, you do not need a semicolon at the end of each line.

print and println

println() adds a new line after the text. print() does not, so the next output continues on the same line.

Example

kotlin

fun main() {
    print("Karachi ")
    print("is a big city. ")
    println("It is by the sea.")
    println(5 + 3)
    println("5 + 3")
}

Output

Karachi is a big city. It is by the sea.
8
5 + 3

Notice the difference: 5 + 3 without quotes is a calculation, so Kotlin prints 8. With quotes it is just text.

Comments

Comments are notes for people who read your code. Kotlin ignores them.

Example

kotlin

fun main() {
    // This is a single-line comment
    println("Comments are not printed") // a comment at the end of a line

    /* This comment
       can span many lines */
}

Output

Comments are not printed

Exercise

Which function is the starting point of every Kotlin program?

start()main()run()println()