Kotlin Introduction

What is Kotlin?

Kotlin logo

Kotlin is a modern, statically typed programming language that runs on the Java Virtual Machine (JVM). Developed by JetBrains (the company behind popular IDEs like IntelliJ IDEA), Kotlin was officially announced in 2011 and reached version 1.0 in 2016. In 2017, Google announced first-class support for Kotlin on Android, making it an official language for Android development.

Kotlin combines object-oriented and functional programming features, offering a more concise, safer, and more expressive alternative to Java while maintaining full interoperability with Java code.

Why Learn Kotlin?

There are several compelling reasons to learn Kotlin:

Android Development: Kotlin is now the preferred language for Android app development.
Concise Syntax: Kotlin reduces boilerplate code significantly compared to Java.
Safety Features: Null safety is built into the type system, helping eliminate NullPointerExceptions.
Interoperability: Kotlin works seamlessly with existing Java code and libraries.
Modern Features: Kotlin includes modern programming features like lambda expressions, extension functions, and coroutines.
Versatility: You can use Kotlin for Android, server-side, web development, and even native applications.

Setting Up Kotlin

Before we dive into coding, let's set up a Kotlin development environment.

Online Playground

The quickest way to start experimenting with Kotlin is through the Kotlin Playground, an online editor that allows you to write and run Kotlin code directly in your browser.

Local Setup

For a local development environment, you have several options:

IntelliJ IDEA: Download the Community Edition (free) or Ultimate Edition from JetBrains website.
Android Studio: If you're interested in Android development, Android Studio comes with Kotlin support.
Command Line: Install the Kotlin compiler following the official documentation.

Your First Kotlin Program

Let's start with the traditional "Hello, World!" program:

kotlin
fun main() {
    println("Hello, World!")
}

When you run this code, you'll see:

Hello, World!

Let's break down what's happening:

fun is the keyword used to declare a function in Kotlin.
main() is the entry point of a Kotlin program.
println() is a built-in function that prints text to the console followed by a line break.

Basic Syntax

Variables

Kotlin has two main types of variables:

Read-only variables (declared with val)
Mutable variables (declared with var)

kotlin
fun main() {
    val name = "John"  // Read-only (cannot be reassigned)
    var age = 25       // Mutable (can be changed)
    
    // age = 26        // This is valid
    // name = "Jane"   // This would cause an error
    
    println("Name: $name, Age: $age")
}

Output:

Name: John, Age: 25

Type Inference and Explicit Types

Kotlin can infer types automatically, but you can also specify them explicitly:

kotlin
fun main() {
    val implicitInt = 42            // Type is inferred as Int
    val explicitInt: Int = 42       // Type is explicitly declared
    
    val implicitDouble = 3.14       // Type is inferred as Double
    val explicitDouble: Double = 3.14
    
    println("Implicit Int: $implicitInt, Explicit Int: $explicitInt")
    println("Implicit Double: $implicitDouble, Explicit Double: $explicitDouble")
}

Output:

Implicit Int: 42, Explicit Int: 42
Implicit Double: 3.14, Explicit Double: 3.14

Basic Data Types

Kotlin provides several basic data types:

kotlin
fun main() {
    val myInt: Int = 100
    val myLong: Long = 1234567890L
    val myDouble: Double = 3.14
    val myFloat: Float = 2.71f
    val myBoolean: Boolean = true
    val myChar: Char = 'A'
    val myString: String = "Hello Kotlin"
    
    println("Integer: $myInt")
    println("Long: $myLong")
    println("Double: $myDouble")
    println("Float: $myFloat")
    println("Boolean: $myBoolean")
    println("Character: $myChar")
    println("String: $myString")
}

String Templates

As you've seen in the previous examples, Kotlin allows for string interpolation using the $ symbol:

kotlin
fun main() {
    val name = "Alice"
    val age = 30
    
    // Simple variable reference
    println("Hello, $name!")
    
    // Expression in string template
    println("Next year, you will be ${age + 1} years old.")
    
    // Accessing properties
    println("Your name has ${name.length} characters.")
}

Output:

Hello, Alice!
Next year, you will be 31 years old.
Your name has 5 characters.

Control Flow

Conditional Expressions

Kotlin uses if, when, for, and while for control flow.

If Expression

In Kotlin, if is an expression (returns a value) rather than just a statement:

kotlin
fun main() {
    val number = 42
    
    // Traditional usage
    if (number > 0) {
        println("Positive number")
    } else if (number < 0) {
        println("Negative number")
    } else {
        println("Zero")
    }
    
    // Using if as an expression
    val result = if (number % 2 == 0) "Even" else "Odd"
    println("The number is $result")
}

Output:

Positive number
The number is Even

When Expression

The when expression is Kotlin's replacement for the switch statement in other languages, but more powerful:

kotlin
fun main() {
    val day = 3
    
    val dayName = when (day) {
        1 -> "Monday"
        2 -> "Tuesday"
        3 -> "Wednesday"
        4 -> "Thursday"
        5 -> "Friday"
        6 -> "Saturday"
        7 -> "Sunday"
        else -> "Invalid day"
    }
    
    println("Day $day is $dayName")
}

Output:

Day 3 is Wednesday

Loops

For Loop

kotlin
fun main() {
    // Loop through a range
    println("Counting from 1 to 5:")
    for (i in 1..5) {
        println(i)
    }
    
    // Loop through an array
    println("\nFruits:")
    val fruits = arrayOf("Apple", "Banana", "Cherry")
    for (fruit in fruits) {
        println(fruit)
    }
    
    // Loop with index
    println("\nIndexed fruits:")
    for ((index, fruit) in fruits.withIndex()) {
        println("$index: $fruit")
    }
}

Output:

Counting from 1 to 5:
1
2
3
4
5

Fruits:
Apple
Banana
Cherry

Indexed fruits:
0: Apple
1: Banana
2: Cherry

While and Do-While Loops

kotlin
fun main() {
    // While loop
    var counter = 0
    println("While loop:")
    while (counter < 3) {
        println("Counter: $counter")
        counter++
    }
    
    // Do-while loop
    counter = 0
    println("\nDo-while loop:")
    do {
        println("Counter: $counter")
        counter++
    } while (counter < 3)
}

Output:

While loop:
Counter: 0
Counter: 1
Counter: 2

Do-while loop:
Counter: 0
Counter: 1
Counter: 2

Functions

Functions in Kotlin are declared using the fun keyword:

kotlin
fun main() {
    // Call function without parameters
    greet()
    
    // Call function with parameters
    greetPerson("Alice")
    
    // Function with return value
    val sum = add(5, 7)
    println("5 + 7 = $sum")
    
    // Single-expression function
    val product = multiply(4, 3)
    println("4 * 3 = $product")
}

// Function with no parameters and no return value
fun greet() {
    println("Hello, world!")
}

// Function with parameter
fun greetPerson(name: String) {
    println("Hello, $name!")
}

// Function with parameters and return value
fun add(a: Int, b: Int): Int {
    return a + b
}

// Single-expression function
fun multiply(a: Int, b: Int): Int = a * b

Output:

Hello, world!
Hello, Alice!
5 + 7 = 12
4 * 3 = 12

Real-World Example: Simple Temperature Converter

Let's create a practical example that converts temperatures between Celsius and Fahrenheit:

kotlin
fun main() {
    println("Temperature Converter")
    println("=====================")
    
    val celsiusTemperatures = listOf(0, 25, 100)
    
    println("Celsius to Fahrenheit:")
    for (celsius in celsiusTemperatures) {
        val fahrenheit = celsiusToFahrenheit(celsius)
        println("$celsius°C = $fahrenheit°F")
    }
    
    println("\nFahrenheit to Celsius:")
    val fahrenheitTemperatures = listOf(32, 77, 212)
    for (fahrenheit in fahrenheitTemperatures) {
        val celsius = fahrenheitToCelsius(fahrenheit)
        println("$fahrenheit°F = $celsius°C")
    }
}

// Convert Celsius to Fahrenheit
fun celsiusToFahrenheit(celsius: Int): Double {
    return celsius * 9.0 / 5.0 + 32.0
}

// Convert Fahrenheit to Celsius
fun fahrenheitToCelsius(fahrenheit: Int): Double {
    return (fahrenheit - 32.0) * 5.0 / 9.0
}

Output:

Temperature Converter
=====================
Celsius to Fahrenheit:
0°C = 32.0°F
25°C = 77.0°F
100°C = 212.0°F

Fahrenheit to Celsius:
32°F = 0.0°C
77°F = 25.0°C
212°F = 100.0°C

This example demonstrates:

Function definitions with parameters and return values
List creation and iteration
String templates for formatted output
Mathematical operations

Summary

In this introduction to Kotlin, we've covered:

What Kotlin is and why it's worth learning
How to set up a Kotlin development environment
Basic syntax including variables, data types, and string templates
Control flow statements like if, when, and loops
Function declarations and usage
A practical example with temperature conversion

Kotlin's modern features, safety guarantees, and concise syntax make it an excellent choice for both beginners and experienced programmers.

Additional Resources

If you want to learn more about Kotlin, check out these resources:

Official Kotlin Documentation
Kotlin Koans - Interactive coding exercises
Google's Kotlin Codelab
Kotlin Playground - Write and run Kotlin code online

Exercises

To solidify your understanding, try these exercises:

Write a program that checks if a number is prime.
Create a function that reverses a string.
Implement a simple calculator that can add, subtract, multiply, and divide.
Write a program that converts a decimal number to binary.
Create a function that calculates the factorial of a number.

If you spot any mistakes on this website, please let me know at [email protected]. I’d greatly appreciate your feedback! :)

What is Kotlin?​

Why Learn Kotlin?​

Setting Up Kotlin​

Online Playground​

Local Setup​

Your First Kotlin Program​

Basic Syntax​

Variables​

Type Inference and Explicit Types​

Basic Data Types​

String Templates​

Control Flow​

Conditional Expressions​

If Expression​

When Expression​

Loops​

For Loop​

While and Do-While Loops​

Functions​

Real-World Example: Simple Temperature Converter​

Summary​

Additional Resources​

Exercises​

What is Kotlin?

Why Learn Kotlin?

Setting Up Kotlin

Online Playground

Local Setup

Your First Kotlin Program

Basic Syntax

Variables

Type Inference and Explicit Types

Basic Data Types

String Templates

Control Flow

Conditional Expressions

If Expression

When Expression

Loops

For Loop

While and Do-While Loops

Functions

Real-World Example: Simple Temperature Converter

Summary

Additional Resources

Exercises