Kotlin: First Impressions

I played around with Kotlin recently and was pretty impressed. It seems like they took the best parts of C#, Scala, and Go. Here's a quick rundown of some features.

• Expression bodied functions

  
  

  fun sum(a: Int, b: Int) = a + b

  
  

• Immutable variables with type inference

  
  

  val b = 2

  
  

• String templates

  
  

  val s2 = "${s1.replace("is", "was")}, but now is $a"

  
  

• Type checks and automatic casts

  
  

  if (obj is String && obj.length > 0) // obj casted to a string

  
  

• Pattern matching,

  
  

  when (obj) {

  1 -> "One"

  "Hello" -> "Greeting"

  is Long -> "Long"

  !is String -> "Not a string"

  else -> "Unknown"

  }

  
  

• Ranges

  
  

  for (x in 1..5) ...

  
  

• Immutable collections with lambdas

  
  

  val fruits = listOf("banana", "avocado", "apple", "kiwifruit")

  fruits

  .filter { it.startsWith("a") }

  .sortedBy { it }

  .map { it.toUpperCase() }

  .forEach { println(it) }

  
  

• DTOs with equals, copy, toString; can be created without new

  
  

  data class Customer(val name: String, val email: String)

  
  

• Default parameter values

  
  

  fun foo(a: Int = 0, b: String = "")

  
  

• Extension functions

  
  

  fun String.spaceToCamelCase() { ... }

  "Convert this to camelCase".spaceToCamelCase()

  Singletons,

  object Resource {

  val name = "Name"

  }

  
  

• Elvis operator aka "if not null" shorthand

  
  

  println(files?.size)

  
  

• "If not null and else" shorthand

  
  

  println(files?.size ?: "empty")

  
  

• Get item of possibly empty list

  
  

  emails.firstOrNull() ?: ""

  
  

• if expressions

  
  

  val result = if (param == 1) "one" else "other"

  
  

• Nullables without the .Get and .HasValue in C#

  
  

  val b: Boolean? = ...

  if (b == true) { ... } else { /* b is false or null */ }

  
  

• The main Go influence seems to be coroutines (lightweight threads) for async code. They also reminded me of C#'s async/await, here's a good explanation.