some quick notes that might comes handy for beginners is a new language which at least most of Android developers heard about it or are using it instead of . Kotlin Java Out of my curiosity start learning it to know what’s attractive inside that makes some developer crazy about it! so during my learning process, I have made some notes which might look too minified but contains keywords for beginners and your later searches. This is the notes which I prefer to have at the beginning of my learning process: Variables: val immutableName: String = "Mohsen" var mutableName: String = "Mohsen"var mutableName = "Mohsen" var nullableName: String? = null val numbers = arrayListOf(1, 2, 3, 4, 5)print("Even Numbers are ${numbers.filter { i -> i % 2 == 0 }}") val countries = listOf(Pair("Iran", "Tehran"), Pair("France", "Paris"), Pair("Germany", "Berlin")) for ((country, city) in countries) {print("Country: $country - City: $city")} Functions: // fun stands for function! fun addNumbers(first: Int, second: Int): Int {return first + second}addNumbers(10, 20)addNumbers(first = 10, second = 20)addNumbers(second = 1, first = 2) // !!! // you can have multiple paramsfun addNumbers(vararg numbers: Int): Int {return numbers.sum()}addNumbers(1, 2, 3, 4, 5, 6) // default valuefun addTwoNumbers(first: Int, secound: Int = 0): Int {return first + secound}addTwoNumbers(2) Classes: // no fields in classes! only properties and functionsclass Customer(val id: Int = 0, var name: String) { // secondary constructor  
constructor(id: Int) : this(id, "")  
  
// initialize method  
init() {  
}  
  
// public by default property  
val test = 0  
  
// visible inside Customer.kt  
private fun foo() {}  
  
// property is visible everywhere  
// setter is visible only in Customer.kt  
public var bar: Int = 5  
	private set  
  
// visible inside the same module  
internal val baz = 6 } Data Classes: // mutable data classes similar to AutoValue library which provide equals, toString, copy, etc.data class Customer(val id: Int = 0, var name: String) Extension Function: fun String.convertSpacesToUnderscores(): String {return this.replace(" ", "_")} print("This is my sample String.".convertSpacesToUnderscores()) Null: val x: String = null //error!val x: String? = null // if x is not null range to 10x?.rangeTo(10) Let: var customer: Customer? = nullcustomer?.let {// it = user and not null and only read once!println(it.name)} someMethod().let { result ->//method only run once!} Any: fun getStringLen(obj: Any): Int? = if (obj is String) obj.length else null Companion: // the same as Static methods and you have to access the// method like this: Customer.Companion.what() class Customer {companion object {fun what(customer: Customer): String {return customer.id.toString() + customer.name}}} // If you want the function looks like java static methods// like this: Customer.what() have to add JvmStatic// It is useful for calling static methods from NDK librariesclass Customer {companion object {@JvmStaticfun what(customer: Customer): String {return customer.id.toString() + customer.name}}} Package function: // functions with no classes, just the same as java import staticpackage com.google.test fun packageFunction() {println("I am a package function!!")} List, ArrayList, Map: val list = listOf("a", "b")val mutableList = mutableListOf("a", "b") for (s in list) {// ...} val map = mapOf("key" to "value")val mutableMap = mutableMapOf("key" to "value") for ((key, value) in map) {// ...} Interfaces: interface TestInterface {fun run(t:Int): String fun test(){  
	// default body!  
} } Generic: class Box<T>(t: T) {// ...}Val box: Box(1) // box: Box<Int> Enums: enum class Direction(val degree: Int) {NORTH(0), EAST(90), SOUTH(180), WEST(270)} Type Casting: val y = x as Intval y = x as? Int // nullable // smart castIf (x is String)x.toUpperCase() Operator Overloading: operator fun plus(other: Point): Point = Point(x + other.x, y + other.y) Inheritance: // all classes are final by default and for inheritance should be openopen class Shape {} class Circle: Shape() {} infix: infix fun String.hello(name:String): String{return "Hello, $name. $this"} "Test" hello "test" // == "Test".hello("test") Delegates: interface Repository<T : Record> {fun getById(id: Int): Tfun getAll(): List<T>} class Controller(repository: Repository<Customer>) : Repository<Customer> by repository {fun customerList(): List<Customer> {return getAll()} fun customerById(id: Int): Customer {  
	return getById(id)  
} } var test: String by Delegates.observable("") {prop, old, new -> ("$old -> $new")} println Sealed: // sealed means there is no PageResult child more than ones that are here in this filesealed class PageResult class Success(val url: String) : PageResult()class Error(val code: Int, val message: String) : PageResult() fun getPage(url: String): PageResult {val someOperationIsSuccessfull = false if (someOperationIsSuccessfull)  
	return Success(url)  
else  
	return Error(404, "Not found") } fun callingPage() {val result = getPage("http://google.com")when (result) {is Success -> println(result.url)is Error -> println(result.message)}} Lazy: class User(firstName: String, lastName: String) {val fullName: String by lazy { "$firstName $lastName" } fun printFullName() {  
	println("User full name is: $fullName")  
} } Local return: myFunction() { numbers = 1..100numbers. ( %5==0) @forEach ( ) //Hello will be printed!} fun val forEach {if it return } println "Hello!!" Tailrec: //tailrec convert a recursive function call to a goto loop factorial(number: Int, accumulator: Int = 1): Int { (number) {0 -> accumulator -> (number - 1, accumulator * number)}} tailrec fun when return else return factorial Some unusual syntaxes for Java developers!!: // multiline string literalsprint("""1. test2. unit test3. what a test""") print("""1. test2. unit test3. what a test""".trimIndent()) print("""|1. test|2. unit test|3. what a test""".trimMargin()) fun max(a: Int, b: Int) = if (a > b) a else b // no for (int i=0;i<10;I++) loop!!!val myArray = ("A","B","C","D")for (index in 0 myArray.size){ (myArray.get(index))} arrayOf until println for ( i in 1..10 ) (myArray.get(index))} println for ( i in 100 downTo 1 step 5) (myArray.get(index))} println var i = 0while (i < 10)println (i++) // no switch case defaultwhen (i) {0 -> println("zero")1..5 -> println("number")is String -> println("$i is string")!is String -> println("is not string")else -> println("unknown!!!")} val isString = when ("Hello") {is String -> trueelse -> false} val isInRange = i in 5..9val isMember = "hello" in array // set alias while importing!Import com.google.Test as MyTest // loop operatorswhile (true){if(false) breakif(false) continueif(false) return} // labelsouter@ for (l in list) {for (m in map) {if (l == m.key) break@outer}} // Pair and Triple data typesval myPair = Pair(1, "Mohse")val myTriple = Triple(1, "A", "Mohsen")val (id, name) = myPair // measure runtime in ms!!val took = measureTimeMillis {// ...} // deprecate a method@Deprecated ("Use another method!", level = WARNING/ERROR/HIDDEN, replaceWith = ReplaceWith("newPrintFullName()", imports = "com.google.test"))fun printFullName() {// …} // rename method for Java@JvmName("methodNameInJava")fun printFullName() {// …} // inject using qualifier annotation@field:[Inject ApplicationContext]lateinit var context: Context // exclude field from serialization and gson@Transientvar id: String? = null Kotlin Android Extensions: // add plugin to build.gradleapply plugin: 'kotlin-android-extensions' import kotlinx.android.synthetic.main.<layout>.* // access views without findViewById!!myText.text = "Hello world!" I look forward for your comments, experiences with , and any unusual syntaxes which I might miss as a beginner. Share this article if you think it is useful by tapping 💚 button, and follow me for more articles . Kotlin Mohsen Mirhoseini _Senior Android Developer_mohsenoid.com Mohsen Mirhoseini

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