Kotlin 1.2.71 or earlier

  
  Awesome Kotlin 1.3

  
  fun foo(s: String?) {
    s?.let {
      s.length // smart cast is here
    }
    
    if (s.isNotNull()) {
      s?.length // no smart cast here :(
    }
  }

  fun String?.isNotNull() = this != null

  
  fun foo(s: String?) {
    s?.let {
      s.length // smart cast here
    }

    if (isNotNull(s)) {
      s.length // also smart cast here
    }
  }

  fun isNotNull(value: Any?): Boolean {
    contract { returns(true) implies (value != null) }
    return value != null
  }

  
  
  val myValue: String
  
  myRun {
    myValue = "Hello, World!"   // Error here ... :(
  }
  
  fun <R> myRun(block: () -> R): R {
    return block()
  }

  
  
  val myValue: String
  
  myRun {
    myValue = "Hello, World!"   // Woohoo it works!
  }
  
  fun <R> myRun(block: () -> R): R {
    contract {
      callsInPlace(block, InvocationKind.EXACTLY_ONCE)
    }
    return block()
  }

  
  val answer = calculate()
  
  return when(answer) {
    42 -> "Right answer!"
    123 -> "ARGHHHHH!"
    else -> "Sorry, $answer is not correct"
  }

  answer // I don't want to see you here

  

  

  return when(val answer = calculate()) {
    42 -> "Right answer!"
    123 -> "ARGHHHHH!"
    else -> "Sorry, $answer is not correct"
  }


  //answer you are not being seen here

  
interface Foo {
    
  companion object {
        
    @JvmField
    val bar: Int = 42
        
    @JvmStatic
    fun sayHello(name: String = "") {
      println("Hello, $name")
    }
  }
}
  

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


  // compare to the full-blown version

  fun main(args : Array<String>) { 
    println("Hello, world") 
  }

  

  
  
  fun onlyRealManCanUnderstandThis(
     block: (Any, Any, Any, /* 250 more */, Any, Any
  ) {
    block(Any(), Any(), /* 250 more */, Any(), Any())
  }

  
  
  @SinceKotlin("1.3")
  interface FunctionN<out R> : Function<R> {
    operator fun invoke(vararg args: Any?): R
    
    override val arity: Int
  }

  
  
  class FirstNameInKana(n: String) 
  class LastNameInKana(n: String)

  fun formattedName(f: FirstName, l: LastName) {
    println("$l $f")
  }

  formattedName(
    FirstNameInKana("やまだ"), 
    LastNameInKana("たろう")
  )

  typealias FirstName = String // OK, but meh :|
  typealias LastName = String // OK, but meh :|

  

  inline class FirstNameInKana(n: String) 
  inline class LastNameInKana(n: String)

  fun formattedName(f: FirstName, l: LastName) {
    println("$l $f")
  }

  formattedName(
    FirstNameInKana("やまだ"), 
    LastNameInKana("たろう")
  )
  
  

  
  

  
  
  // Literal shortcode for UInt
  val aUInt: UInt = 3u
  val aULong: ULong = 3uL

  // And from Hex...
  val hexUByte: UByte = 0xFFu  // 255
  val hexUShort: UShort = 0xFFFFu  // 65535
  val hexUInt: UInt = 0xFFFF_FFFFu  // 4294967295
  val hexULong: ULong = 0xFFFF_FFFF_FFFF_FFFFu  // xD

  
  
  fun calculateArray(): Array<Int>? {
    // some calculation

    
  }

  val array = calculateArray()

  array.isNullOrEmpty() // ERRRAARGGHHH, there is none

  
  
  fun calculateArray(): Array<Int>? {
    // heavy calculation

    
  }

  val array = calculateArray()

  if (array.isNullOrEmpty()) {
    // :) Joy
  }

  
  
  val maybeArray: Array<T>
  maybeArray.isNullOrEmpty()


  val maybeCollection: Collection<T>
  maybeCollection.isNullOrEmpty()


  val maybeMap: Map<T, U>
  maybeMap.isNullOrEmpty()
  
  
  

  
  
  val listOfFeatures: List<String> = ...
  println(listOfFeatures.ifEmpty { 
     listOf("Default1", "Default2") 
  })

  
  val townProvidedByUser: String = ...
  println(townProvidedByUser.ifBlank { "東京" })

  
  @Serializable
  data class Destination(
        val name: String? = "",
        val country : String? = "",
        val code : Int = 0,
        @Optional
        val isMetro : Boolean = false,
        @Transient
        val favorite : Boolean = false
  )
  
  val tokyoAsString = JSON.stringify(
    Destination(name = "東京", country = "Japan") 
  ) // { "name":"東京","country":"Japan","code":0 }
  

  val tokyo = JSON.unquoted.parse<Destination>(
    "{ name:東京,country:Japan, code:12 }"
  ) // Destination(name = "東京", country = "Japan", code = 12)