Learn Kotlin: The Complete Beginner’s Guide

Kotlin is a modern, concise, and safe programming language developed by JetBrains. It’s officially supported by Google for Android development and can also be used for web, desktop, and backend apps.

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Why Learn Kotlin?

• Concise → Less boilerplate than Java.
• Safe → Built-in null safety prevents many errors.
• Interoperable → Works with existing Java code.
• Versatile → Use it for Android, web, and server apps.

Hello World

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

Variables

• val → Immutable (like constants).
• var → Mutable (can change).

val name = "Alice"   // Cannot be changed
var age = 25         // Can be changed

Data Types

Kotlin is strongly typed but can infer types automatically.

• String → "Hello"
• Int → 42
• Double → 3.14
• Boolean → true
• List → listOf("a", "b")
• MutableList → mutableListOf(1, 2, 3)

Functions

fun greet(name: String): String {
    return "Hello, $name!"
}

fun main() {
    println(greet("Alice"))
}

Conditionals

val age = 18

if (age >= 18) {
    println("Adult")
} else {
    println("Minor")
}

When Expression

val day = 3
when (day) {
    1 -> println("Monday")
    2 -> println("Tuesday")
    3 -> println("Wednesday")
    else -> println("Another day")
}

Loops

for (i in 1..5) {
    println(i)
}

var x = 0
while (x < 3) {
    println(x)
    x++
}

Classes and Objects

class Person(val name: String, var age: Int) {
    fun greet() {
        println("Hi, my name is $name and I'm $age years old.")
    }
}

fun main() {
    val p = Person("Alice", 25)
    p.greet()
}

Null Safety

Kotlin prevents null pointer errors by default.

var name: String? = null  // Nullable type
println(name?.length)     // Safe call (prints null)

Collections

val numbers = listOf(1, 2, 3)
for (n in numbers) {
    println(n)
}

val mutable = mutableListOf(10, 20)
mutable.add(30)

Mini Project: Simple Calculator

fun add(a: Int, b: Int) = a + b
fun subtract(a: Int, b: Int) = a - b

fun main() {
    println("Sum: ${add(5, 3)}")
    println("Difference: ${subtract(10, 4)}")
}

Best Practices

• Prefer val unless a variable must change.
• Use string templates ("Hello $name") instead of concatenation.
• Take advantage of Kotlin’s null safety.
• Write small, reusable functions.

Next Steps

1. Android Development → Use Kotlin in Android Studio.
2. Kotlin Coroutines → Learn async programming.
3. Backend → Explore Ktor or Spring Boot with Kotlin.

Conclusion

Kotlin is a modern, safe, and fun programming language that simplifies development. It’s especially powerful for Android apps, but you can use it almost anywhere.

Start small, practice daily, and build amazing apps with Kotlin 🚀🔥