Kotlin: First Impressions

March 03, 2020

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.