CodeHub

🐰 Kotlin — Learn From Zero

Playground: Kotlin Playground — click, type, run.

What is Kotlin?

Kotlin is a modern, concise language on the JVM, fully interoperable with Java. Official language for Android.

  • Android apps
  • Backend (Ktor, Spring)
  • Multiplatform (KMM)
  • Scripting

Run Online — No Install

Use the playground links above.

🟢 Beginner — Core Basics

Your First Program

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

Top-level fun main().

Variables

kotlin
val name = "Ada"
var age = 36
val height = 1.78

Prefer val over var.

If / Else

kotlin
val age = 18
val msg = if (age >= 18) "Vote" else "Too young"
println(msg)

If is an expression.

Loops

kotlin
for (i in 0..5) println(i)
for (n in list) println(n)

Ranges with 0..5 (exclusive end).

Functions

kotlin
fun greet(name: String): String = "Hello, $name!"

Expression-bodied functions.

Null Safety

kotlin
var s: String? = null
s?.length
val len = s?.length ?: 0

? marks nullable types.

🟡 Intermediate — Building Real Programs

Classes & Data

kotlin
data class Person(val name: String, val age: Int)

Data classes auto-generate methods.

OOP

kotlin
interface Animal { fun speak() }
class Dog : Animal { override fun speak() {} }

Inheritance with :.

Collections

kotlin
val nums = listOf(1, 2, 3)
val even = nums.filter { it % 2 == 0 }

Functional collection ops.

Lambdas

kotlin
val add: (Int, Int) -> Int = { a, b -> a + b }

First-class functions.

Scope Functions

kotlin
val p = Person("Ada", 36).apply { println(name) }

let, run, apply.

🔴 Advanced — Pro Concepts

Coroutines

kotlin
suspend fun load(): String { delay(100); return "x" }
launch { val d = load() }

Lightweight concurrency.

Generics

kotlin
class Box(val value: T)

Type parameters.

Sealed Classes

kotlin
sealed class Result { data class Ok(val v: Int): Result()
  object Err: Result() }

Restricted hierarchies for when.

Delegation

kotlin
interface Base { fun f() }
class Impl : Base { override fun f() {} }
class Wrapper(b: Base) : Base by b

Class delegation via by.

What's Next?

  • Build a tiny project: Calculator, To-Do App, Weather Widget
  • Read the official docs and a good book
  • Solve Exercism exercises
  • Join a community & share your code

Practice daily for 20 minutes. Consistency > intensity.