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.