Kotlin Cheat Sheet (For My Own Reference)

val str: String? = "ABC"
str.length // ERROR
str?.length // OK

val str: String? = "ABC"
str?.length ?: 0 // either str.length or 0 
str?.length ?: throw IllegalArgumentException("null이 들어왔습니다.") // or throw exception
str?.length ?: return 0 // can also be used for early return

// If @Nullable, @NonNull are present they get converted, otherwise platform type
// In Java 
private final String name; << When no annotation

val name : String? = javaPerson.name // OK
val name : String = javaPerson.name  // OK

val number1 : Int = 4
val number2 : Long = number1.toLong()

val number1 : Int? = 3 
val number2 : Long = number1?.toLong() ?: 0L // for nullable variables, handle as needed

fun printAgeIfPerson(obj: Any){
	// is = Java's instanceof
	if(obj is Person){
			// Smart cast makes obj's type Person
			println(obj.age)
	}
}

// OR
// Same as Java's (Person) obj
val person = obj as Person

// !is, as? are also usable
// as? : the whole expression becomes null

fun fail(message : String): Nothing{
	throw IllegalArgumentException(message)
}

val person = Person(name = "lemon", age = 20)
val log = "이름 : ${person.name}, 나이 : ${person.age}"

val str = "ABC"
// Instead of Java's str.get(0)
str[1] //ok

val numbers = 1..100 // creates an IntRange object from 1 to 100
println(1 in numbers) // true
println(0 in numbers) // false

// These two are equivalent
if(0 <= score && score <= 100) {..}
if(score in 0..100)

data class Money(val amount : Long){
	
	operator fun plus(other : Money): Money{
		return Money(this.amount + other.amount)
	}
}

// Can be used as below
val sumMoney = money1 + money2

// Not possible in Java, but works in Kotlin since if is an expression
fun getPassOrFail(score : Int): String{
	return if (score >= 50){ "P" } else { "F" }
}

// Basic usage, in place of Switch
fun getGradeWithSwitch(score: Int): String{
	return when(score / 10){
		9 -> "A"
		8 -> "B"
		7 -> "C"
		else -> "D"
	}
}

// Expressions on the left side are also fine, including is Type, etc.
when(score){
in 90..99 -> "A"
in 80..89 -> "B"
...

// Multiple conditions at once
when (number){
1,0,-1 -> println("1,0,-1입니다")
else -> println("아닙니다")
}

// Can be used like an if-chain
fun printOdd(score: Int): Int{
	when{
		score % 2 == 1 -> println("홀수")
		score % 2 == 0 -> println("짝수")
		else -> println("넌머임")
	}
	return 1
}

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

val intRange = 1..100

// Java's try-with-resources
// Auto-closes when try block ends. Must implement AutoCloseable. 
public void readFile(String path) throws IOException{
	try(BufferedReader reader = new BufferedReader(new FileReader(path))){
		System.out.println(reader.readLine());
	}
}

// In Kotlin, use behaves similarly to try-with-resources
fun readFile(path: String){
	BufferedReader(FileReader(path)).use { reader ->
		println(reader.readLine())
	}
}

// If a function body is just a single expression, you can use =
fun max(a: Int, b: Int) : Int = if(a > b) {a} else {b}

fun repeat(
	str : String,
	num : Int = 3,
	useNewLine : Boolean = true
){
	for(i in 1..num){
	if(useNewLine) { println(str)} else{ print(str) }
	}
}

repeat(str = "Hello, World", useNewLine = true)
// Here, num isn't specified so its default (3) is used
// Acts like a builder without writing one

// Java
public void printAll(String... strings){
	for(String str : strings){
		System.out.println(str);	
	}
}
String[] array = new String[]{"Hello", "World", "!!"};
printAll(array)
printAll("Hello", "World", "!!")

// Kotlin
printAll(vararg strings : String){
	for (str in strings){
		println(str)
	}
}

val array = arrayOf("Hello", "World", "!!")
printAll(*array) // spread operator
printAll("Hello", "World", "!!")

class Person(
	val name:String,
	var age:Int
)

// Access getter/setter as if accessing a property
person.age = 10
println(person.age)

class Person(
	val name:String = "Patrick",
	var age:Int = 1
){
	init{
		if(age <= 0){
			throw IllegalArgumentException("나이는 ${age}일 수 없습니다.")
		}
	}
}

class Person(
	val name:String,
	var age:Int
){
	
	// Used like a property; single expression form uses =
	val isAdult: Boolean
		get() = this.age >= 20
	
	// Same expression as above
	fun isAdult(): Boolean{
		return this.age >= 20
	}
}

// public class by default
abstract class Animal(
	protected val species:String,
	protected val legCount:Int
){
	abstract fun move()
}

//Java
public class JavaPenguin extends JavaAnimal{
	
	// Field added in subclass
	private final int wingCount;
	
	public JavaPenguin(String species){
		super(species, 2)
		this.wingCount = 2;
	}

	@Override
	public void move(){
		System.out.println("penguin is moving");
	}

	// Override the parent's getter
	@Override
	public int getLegCount(){
		return super.legCount + this.wingCount;
	}
}

//Kotlin
class penguin(
	species : String // primary constructor
) : Animal(species, 2) // Inherit Animal and call its constructor directly {

	private val wingCount: Int = 2
	
	// Use a keyword instead of an annotation
	override fun move(){
		println("cat is moving")
	}

	override val legCount: Int
		get() = super.legCount + this.wingCount
}

// Note: Kotlin classes are final by default, so you need open to override
abstract class Animal(
	protected val species:String,
	protected open val legCount: Int
){
	abstract fun move()
}

// Java
public interface JavaSwimable{
	default void act(){System.out.println("swim!"); }
}
public interface JavaFlyable{
	default void act(){System.out.println("swim!"); }
}

// Kotlin
interface Swimable{
	// No default keyword needed
	fun act(){ println("swim!") } 
}
interface Flyable{
	fun act(){ println("fly!") }
}

// Java
public final class JavaPenguin extends JavaAnimal implements JavaFlyable, JavaSwimable{
	
	// Constructor implementation.. (omitted)
	
	@Override
	public void act(){
		JavaSwimable.super.act();
		JavaFlyable.super.act();
	}
}

// Kotlin
class Penguin(): Animal(species, 2), Swimable, Flyable{
	override fun act(){
		super<Swimable>.act()
		super<Flyable>.act()
	}
}

// Kotlin
interface Swimable{
	// Property expected to be implemented by subclasses
	val swimAbility: Int 
}

class Penguin(): Swimable{
	// Implement like this
	override val swimAbility: Int
		get() = 3
}

open class Base(
	open val number : Int = 100
){
	init{
		println("Base Class")
		println(number)
	}
}

class Derived(
	override val number: Int
): Base(number){
	init{
		println("Derived class")
	}
}

// Output when instantiating Derived
/*
	Output : 
	Base Class
	0
	Derived Class 
*/

// Reason: when the superclass (Base) accesses number, it pulls the subclass's number.
// But the superclass (Base) constructor is still running, so the subclass (Derived) hasn't been initialized yet.
// So 0 is printed.

class Car(
	// internal getter for name
	internal val name : String,
	var owner : String,
	_price : Int
){
	// Only the setter is private
	var price = _price
		private set
}

//Java
public class JavaPerson{
    private static final int MIN_AGE = 1;

    // Static factory method
    public static JavaPerson newBaby(String name){
        return new JavaPerson(name, MIN_AGE);
    }

    private String name;
    private int age;

    private JavaPerson(String name, int age){ this.name = name; this.age = age}
}

//Kotlin
class Person private constructor(
    var name : String,
    var age : Int,
){
		// companion object instead of static
    companion object{
				// const = assigned at compile time, otherwise at runtime
				// Only allowed for primitive types + String
        const val MIN_AGE = 1
        fun newBaby(name: String): Person{
            return Person(name, MIN_AGE)
        }
    }
}

// Callable as Person.Companion.newBaby("ABC"),
// or via @JvmStatic annotation to access like a Java static directly

/*
	@JvmStatic
	fun newBaby(name: String) : Person{...}

	After this, you can call:
	Person.newBaby("ABC")
*/

// Kotlin
interface Log{ fun log() }

class Person private constructor(
    var name : String,
    var age : Int,
){
		// You can name it
    companion object Factory{
        const val MIN_AGE = 1
        fun newBaby(name: String): Person{
            return Person(name, MIN_AGE)
        }
				override fun log(){ println("I am companion object!") }
    }
}

// If named, you can access via the name
// Person.Factory.newBaby("ABC");

//Java
public class JavaSingleton{
	private static final JavaSingleton INSTANCE = new JavaSingleton();
	private JavaSingleton(){}

	public static Javasingleton getInstance(){ return INSTANCE; }
}

//Kotlin
object Singleton{
	var a: Int = 0
}

// Use as Singleton.a

// Java
private static void moveSomething(Movable movable){ movable.move(); movable.fly(); }

// Anonymous class
moveSomething(new Movable(){
	@Override
	public void move(){System.out.println("move~");}
	@Override
	public void fly(){System.out.println("fly~");}
}

// Kotlin
private fun moveSomething(movable:Movable){ movable.move(); movable.fly();}

// Use the object keyword for an anonymous class
moveSomething(object : Movable{
	override fun move(){ println("move~") }
	override fun fly(){println("fly~")}
})

//Kotlin
data class PersonDto(val name: String, val age: Int)

// With named arguments, it's like using a builder
PersonDto(name="Patrick", age=20)

//Java
public enum JavaCountry{
	KOREA("KO"),
	AMERICA("US");

	private final String code;
	JavaCountry(String code) { this.code=code; }
	public String getCode() { return this.code; }
}

// Kotlin (equivalent)
enum class Country(
	private val code: String
){
	KOREA("KO"),
	AMERICA("US");
}

// Handle easily with when
// Bonus: when adding a new value, the compiler will tell you about non-exhaustive when
fun handleCountry(country: Country){
	when(country){
		Country.KOREA -> LOGIC()
		Country.AMERICA -> LOGIC()
		// works without an else!
	}
}

//Kotlin
sealed class Car(
	val name:String,
	val price:Long
)

class Avante: Car("Avante",1_000L)
class Sonata: Car("Sonata",2_000L)
class Grandeur: Car("Grandeur",3_000L)
// Like enum class above, you can use when

/* Default implementation: ArrayList */
// Immutable list
val numbers = listOf(100,200)
// Mutable list
val mutableNumbers = mutableListOf(100,200)
// Empty list
val emptyList = emptyList<Int>()

// Same as numbers.get(0)
numbers[0]

for(number in numbers){println(number)}
// When you also need the index
for((idx, number) in numbers.withIndex()){println("${idx}, ${number}")}

/* Default implementation: LinkedHashSet */
val numbers = setOf(100,200)

val weekMap= mutableMapOf<Int, String>()
// Access like an array
weekMap[1] = "MONDAY"
weekMap[2] = "TUESDAY"

// Initialize with `to`
mapOf(1 to "MONDAY", 2 to "TUESDAY")

for(key in weekMap.keys){println(key) println(weekMap[key]) }
for((key, value) in weekMap.entries){println(key) println(value)} 

// Kotlin

// Extending the String class
fun String.lastChar(){ return this[this.length - 1] }

// Use it in Kotlin like this
val str = "ABC"
println(str.lastChar());

infix fun Int.add(other: Int): Int{
	return this + other
}

// Callable like this
3 add 4

inline fun Int.add(other: Int): Int{

// Kotlin
fun createPerson(firstName: String, lastName:String): Person{
	if(firstName.isEmpty()){ throw IllegalArgumentException("에러!!")}
	if(lastName:String.isEmpty()){ throw IllegalArgumentException("에러!!")}
	return Person(firstName, lastName)
}

// With local functions
fun createPerson(firstName: String, lastName:String): Person{
	// Function inside function
	fun validateName(name: String){if(name.isEmpty()){throw IllegalArgumentException("에러!!")}
	
	validateName(firstName)
	validateName(lastName)
	return Person(firstName, lastName)
}

// Kotlin

// You can specify the function type
val isApple : (Fruit) -> Boolean 
= fun(fruit: Fruit): Boolean {
	return fruit.name == "사과"
}

// Same as above
val isApple2 = {fruit: Fruit -> fruit.name == "사과" }

val isApple3 = { fruit ->
println("사과만 받는 함수입니다.")
fruit.name == "사과" // In multi-line lambdas, omitting return makes the last line the return value
}

// Calling the lambda, equivalent
isApple(Fruit("사과", 1000))
// Explicit invoke
isApple.invoke(Fruit("사과", 1000))

// You can write the function type to accept functions as parameters
fun filterFruits(fruits: List<Fruit>, filterFunc : (Fruit) -> Boolean) : List<Fruit>{
	val results = mutableListOf<Fruit>()
	for(fruit in fruits){
		if(filterFunc(fruit)){ results.add(fruit)}
	}
	return results
}

// Call by passing a lambda
filterFruits(fruit, {fruit -> fruit.name =="사과"} )

// If the lambda is the last parameter, you can write it like this (same behavior as above)
filterFruits(fruit) {fruit -> fruit.name =="사과"}

// With a single parameter, you can replace the part before the arrow with `it`
filterFruits(fruit) {it.name =="사과"}

//Java
String targetFruitName = "바나나"
targetFruitName = "수박"

// In Java, variables used in a lambda must be final, 
// or effectively final (not changed after assignment, even without the final keyword)
filterFruits(fruits, (fruit) -> targetFruitName.equals(fruit.getName())); // ERROR!

//Kotlin
var targetFruitName = "바나나"
targetFruitName = "수박"
filterFruits(fruits) { it.name == targetFruitName } // It Works!

// Take only apples
val apples = fruits.filter { fruit -> fruit.name == "사과" }

val apples = fruits.filterIndexed { idx, fruit -> 
println(idx)
fruit.name == "사과"
} 

val values = fruits.filter { it.name == "사과" }
	.mapNotNull { it.nullOrValue() } // only non-null values

// Check whether all are apples
val isAllApple = fruits.all { it.name == "사과" }

// Check whether there are no apples
val isNoApple = fruits.none { it.name =="사과" }

// Check whether there's any fruit over 10,000 won
val hasExpensiveFruit = fruits.any { it.price > 10_000 }

// Same as Java's list.size()
val fruitCount = fruits.count()

val sortedFruit = fruit.sortedBy{it.price} // ascending
val desendSortedFruit = fruit.sortedByDescending{it.price} // descending

// See what kinds of fruits exist
val distinctFruitNames = fruits.distinctBy{ it.name } // dedupe by name
	.map{it.name}

fruits.first()
fruits.lastOrNull()

// Group items by fruit name
// key: 사과, value: {사과1, 사과2, 사과3...}
val map: Map<String, List<Fruit>> = fruits.groupBy{fruit -> fruit.name } 

// You can also process key and value at once
// Example: name as key, list of prices as value
val map2 : Map<String, List<Long>> = fruits
	.groupBy({it.name}, {it.price})

val map: Map<Long, Fruit> = fruits.associateBy { fruit -> fruit.id }

val fruitList = listOf<Fruit>(
			// id is the same
      Fruit(1,"1"),
      Fruit(1,"2"),
      Fruit(1,"3"),
    )

  val associateBy = fruitList.associateBy { it.id }

  println(associateBy.count()) // 1
  for (entry in associateBy) {
      println(entry) // 1=Fruit(id=1, name=3)
  }

// Likewise can process key and value together
val map: Map<Long, Long> = fruits
	.associateBy({it.id}, {it.price})

val map: Map<String, List<Fruit>> = fruits.groupBy{fruit -> fruit.name } 
	.filter { (key, value) -> key == "사과" } 

var fruitsInList: List<List<Fruit>> = listOf(
	listOf(Fruit(1,"사과"), Fruit(2,"수박"), Fruit(3,"사과")),
	listOf(Fruit(4,"바나나"), Fruit(5,"사과"), Fruit(6,"바나나")),
)

// Pull all apples regardless of the list structure
// output: listOf(Fruit(1,"사과"), Fruit(3,"사과"), Fruit(5,"사과"))
val appleList : List<Fruit>
	= fruitsInList.flatMap { list -> list.filter{it.name == "사과"}

// Flatten 2D list to 1D
val flattenList : List<Fruit>
	= fruitsInList.flatten();

// This function
fun getFruitOrNull(fruit: Fruit): Fruit?{
	return if(fruit.name == "사과"){fruit} else {null}
}
// Can be written as
val getfruit = fruit.takeIf { it.name == "사과" }
Fruit(1L, "사과") // returns the fruit in this case
Fruit(1L, "바나나") // returns null in this case

// These two are equivalent
fun printPerson(person: Person?){
	if(person != null){
		println(person.name)
		println(person.age)
	}
}

fun printPerson(person: Person?){
	person?.let{
		println(person.name)
		println(person.age)
	}
}

val value1 = person.let{ it.age }  // value1: person.age
// You can rename it: person.let{ person -> person.age }
val value2 = person.run{ this.age } // value2: person.age
// You can omit this: person.run{ age }

val value3 = person.also{ it.age } // value3: person
val value4 = person.apply{ this.age } // value4: person 

with(person){
	println(name)
	println(age)
}

/* 
Using `it`: takes a regular function as a parameter, block: (T) -> R : R
Using `this`: takes an extension function as a parameter, block: T.() -> R : R
If you're curious, check the actual implementation....
*/