# Kotlin Quick Reference

*Null safety, coroutines, data classes, functional programming essentials*

> Source: Kotlin Documentation (kotlinlang.org) · MIT

## Basics

### Hello World

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

### Variables

```
val name = "Kotlin"        // immutable (prefer)
var count = 0               // mutable
val pi: Double = 3.14159   // explicit type
const val MAX = 100         // compile-time constant
```

### Basic Types

| Command | Description |
|---------|-------------|
| `Int, Long` | 32-bit / 64-bit signed integers |
| `Double, Float` | 64-bit / 32-bit floating point |
| `Boolean` | `true` / `false` |
| `Char` | Single Unicode character |
| `String` | Immutable text, supports templates |
| `Unit` | Equivalent to `void` (single value) |
| `Nothing` | Function never returns (e.g., throws) |

### String Templates

```
val name = "World"
println("Hello, $name!")
println("Length: ${name.length}")
val raw = """line 1
    |line 2""".trimMargin()
```

## Functions

### Function Declaration

```
fun add(a: Int, b: Int): Int {
    return a + b
}
fun add(a: Int, b: Int) = a + b  // single expression
```

### Default & Named Arguments

```
fun greet(name: String, greeting: String = "Hello") {
    println("$greeting, $name!")
}
greet("Alice")                    // Hello, Alice!
greet("Bob", greeting = "Hi")     // Hi, Bob!
```

### Higher-Order Functions

```
fun operate(a: Int, b: Int, op: (Int, Int) -> Int): Int {
    return op(a, b)
}
val sum = operate(3, 4) { a, b -> a + b }
```

### Varargs

```
fun sum(vararg nums: Int): Int = nums.sum()
sum(1, 2, 3)
val arr = intArrayOf(1, 2, 3)
sum(*arr)                         // spread operator
```

## Classes

### Class Definition

```
class Person(val name: String, var age: Int) {
    fun greet() = "Hi, I'm $name"
}
val p = Person("Alice", 30)
println(p.name)
```

### Inheritance

```
open class Shape(val sides: Int) { open fun area(): Double = 0.0 }
class Circle(val r: Double) : Shape(0) {
    override fun area() = Math.PI * r * r
}
```

### Visibility Modifiers

| Command | Description |
|---------|-------------|
| `public` | Visible everywhere (default) |
| `private` | Visible within the class / file |
| `protected` | Class and subclasses |
| `internal` | Same module only |

### Abstract & Interfaces

```
interface Drawable { fun draw() }
abstract class Widget : Drawable { abstract val label: String }
class Button(override val label: String) : Widget() {
    override fun draw() = println("Drawing $label")
}
```

## Null Safety

### Nullable Types

```
var name: String? = null     // nullable
val len = name?.length       // safe call: null
val len2 = name?.length ?: 0 // Elvis operator: 0
val len3 = name!!.length     // assert non-null (throws)
```

### Safe Operations

| Command | Description |
|---------|-------------|
| `?.` | Safe call — returns null if receiver is null |
| `?:` | Elvis — default value when null |
| `!!` | Non-null assertion (throws if null) |
| `?.let { }` | Execute block only if non-null |
| `as?` | Safe cast — returns null on failure |

### Smart Casts

```
if (obj is String) println(obj.length)  // auto-cast
when (obj) {
    is Int    -> println(obj + 1)
    is String -> println(obj.uppercase())
}
```

## Collections

### Creating Collections

```
val list = listOf(1, 2, 3)         // immutable
val mList = mutableListOf(1, 2, 3) // mutable
val map = mapOf("a" to 1, "b" to 2)
val set = setOf("x", "y", "z")
```

### Collection Operations

```
val nums = listOf(1, 2, 3, 4, 5)
nums.filter { it > 2 }             // [3, 4, 5]
nums.map { it * 2 }                // [2, 4, 6, 8, 10]
nums.firstOrNull { it > 3 }        // 4
nums.sumOf { it }                  // 15
```

### Common Operations

| Command | Description |
|---------|-------------|
| `.filter { }` | Keep elements matching predicate |
| `.map { }` | Transform each element |
| `.flatMap { }` | Map and flatten |
| `.groupBy { }` | Group by key into Map |
| `.sortedBy { }` | Sort by selector |
| `.associate { }` | Transform to Map (key-value pairs) |
| `.any { } / .all { }` | Check if any/all match predicate |
| `.fold(init) { }` | Reduce with initial accumulator |

## Coroutines

### Basic Coroutine

```
import kotlinx.coroutines.*
fun main() = runBlocking {
    launch { delay(1000); println("World") }
    println("Hello")
}
```

### Async / Await

```
val deferred = async { fetchData() }
val result = deferred.await()
// parallel: launch multiple async, await all
val (a, b) = awaitAll(async { fetchA() }, async { fetchB() })
```

### Coroutine Builders

| Command | Description |
|---------|-------------|
| `launch { }` | Fire-and-forget coroutine (returns Job) |
| `async { }` | Returns Deferred<T> with result |
| `runBlocking { }` | Bridges blocking and suspending code |
| `withContext(dispatcher)` | Switch coroutine context |
| `coroutineScope { }` | Structured concurrency scope |

### Dispatchers

| Command | Description |
|---------|-------------|
| `Dispatchers.Default` | CPU-intensive work (thread pool) |
| `Dispatchers.IO` | Blocking I/O operations |
| `Dispatchers.Main` | Main/UI thread (Android, Swing) |
| `Dispatchers.Unconfined` | Starts in caller thread, resumes in any |

## Extensions

### Extension Functions

```
fun String.isPalindrome(): Boolean {
    return this == this.reversed()
}
println("racecar".isPalindrome())  // true
```

### Extension Properties

```
val String.wordCount: Int
    get() = this.split("\\s+".toRegex()).size
println("hello world".wordCount)   // 2
```

### Operator Overloading

```
data class Vec(val x: Double, val y: Double) {
    operator fun plus(other: Vec) = Vec(x + other.x, y + other.y)
}
val v = Vec(1.0, 2.0) + Vec(3.0, 4.0)  // Vec(4.0, 6.0)
```

## Data Classes

### Data Class

```
data class User(val name: String, val age: Int)
val u1 = User("Alice", 30)
val u2 = u1.copy(age = 31)        // non-destructive copy
val (name, age) = u1               // destructuring
```

### Auto-Generated Members

| Command | Description |
|---------|-------------|
| `equals()` | Structural equality based on properties |
| `hashCode()` | Consistent with `equals()` |
| `toString()` | `User(name=Alice, age=30)` |
| `copy()` | Create modified copy |
| `componentN()` | Destructuring support |

### Enum Classes

```
enum class Direction { NORTH, SOUTH, EAST, WEST }
val dir = Direction.NORTH
when (dir) { Direction.NORTH -> "up"; else -> "other" }
```

## Sealed Classes

### Sealed Class Hierarchy

```
sealed class Result<out T> {
    data class Success<T>(val data: T) : Result<T>()
    data class Error(val message: String) : Result<Nothing>()
    data object Loading : Result<Nothing>()
}
```

### Exhaustive When

```
fun handle(result: Result<String>): String = when (result) {
    is Result.Success -> result.data
    is Result.Error   -> "Error: ${result.message}"
    is Result.Loading -> "Loading..."
}   // no else needed — compiler checks exhaustiveness
```

### Sealed vs Enum

| Command | Description |
|---------|-------------|
| `Sealed class` | Subclasses can hold different state |
| `Sealed interface` | Allows multiple inheritance |
| `Enum class` | Fixed set of singleton instances |
| `data object` | Singleton with `toString()` override |

## Scope Functions

### Scope Function Comparison

| Command | Description |
|---------|-------------|
| `let` | Context as `it`, returns lambda result |
| `run` | Context as `this`, returns lambda result |
| `with(obj)` | Context as `this`, returns lambda result |
| `apply` | Context as `this`, returns context object |
| `also` | Context as `it`, returns context object |

### let & apply

```
val name: String? = "Alice"
name?.let { println("Name is $it") }
val person = Person("Bob", 25).apply {
    age = 26                     // configure object
}
```

### run & with

```
val result = "Hello".run { uppercase() + " WORLD" }
val info = with(person) { "$name is $age years old" }
```

### also

```
val numbers = mutableListOf(1, 2, 3)
    .also { println("Original: $it") }
    .also { it.add(4) }
// also is useful for side effects (logging, validation)
```