Kotlin idioms

/* bad */
var counter = 0
for(i in 0..20){
  counter += i
}

/* preferred */
val sum = (0..20).sum()

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

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

if ("john@example.com" in emailsList) { ... }

if ("jane@example.com" !in emailsList) { ... }

println("Name $name")

when (x) {
    is Foo -> ...
    is Bar -> ...
    else   -> ...
}

for ((k, v) in map) {
    println("$k -> $v")
}

val (a,b) = Pair(1, 2)

for (i in 1..100) { ... }  // closed range: includes 100
for (i in 1 until 100) { ... } // half-open range: does not include 100
for (x in 2..10 step 2) { ... }
for (x in 10 downTo 1) { ... }
if (x in 1..10) { ... }

val list = listOf("a", "b", "c")

println(map["key"])
map["key"] = value

val p: String by lazy {
    // compute the string
}

fun String.spaceToCamelCase() { ... }

"Convert this to camelcase".spaceToCamelCase()

object Resource {
    val name = "Name"
}

val files = File("Test").listFiles()

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

val value = ...

value?.let {
    ... // execute this block if not null
}

fun transform(color: String): Int {
    return when (color) {
        "Red" -> 0
        "Green" -> 1
        "Blue" -> 2
        else -> throw IllegalArgumentException("Invalid color param value")
    }
}

fun test() {
    val result = try {
        count()
    } catch (e: ArithmeticException) {
        throw IllegalStateException(e)
    }

    // Working with result
}

fun arrayOfMinusOnes(size: Int): IntArray {
    return IntArray(size).apply { fill(-1) }
}

fun theAnswer() = 42

val myRectangle = Rectangle().apply {
    length = 4
    breadth = 5
    color = 0xFAFAFA
}

val stream = Files.newInputStream(Paths.get("/some/file.txt"))
stream.buffered().reader().use { reader ->
    println(reader.readText())
}

//  public final class Gson {
//     ...
//     public <T> T fromJson(JsonElement json, Class<T> classOfT) throws JsonSyntaxException {
//     ...

inline fun <reified T: Any> Gson.fromJson(json: JsonElement): T = this.fromJson(json, T::class.java)

fun foo() {
    listOf(1, 2, 3, 4, 5).forEach {
        if (it == 3) return // non-local return directly to the caller of foo()
        print(it)
    }
    println("this point is unreachable")
}

val list: List<Int> = listOf(1,2,3,4,5,6)

val result = list
	.filter{it%2 == 0} //select even numbers
    .map{it*it} // get square of each element
    .sumByDouble{it.toDouble()} //use one of reduce operations

class Doubles(val list: DoubleArray)

@Suppress("FunctionName")
fun Doubles(vararg numbers: Number) = Doubles(numbers.map{it.toDouble()}.toDoubleArray())

Doubles(1,2,3)

var x: String? = null
x = x ?: "Hello"

var a: Int = 2
	protected set // accessible only in descendents

ptivate val _list = ArrayList<Int>()
val list: List<Int> get() = _list

fun integrate(from: Double = 0.0, to: Double = 1.0, function: (Double)→Double): Double {...}
integrate{x→x*x + 2*x + 1}
integrate(0.0, Pi){ sin(it) }

val x = foo() ?: return notFound()

val y = if(conditionSatisfied()){
  1.0
} else {
  error("Condition is not satisfied"}//throws inside, the type is Nothing
}

a?.b == true 
//instead of 
a?.b ?: false

val a = if(b>0) 0 else 1

val ioResult: IOResult? = try{
  readFromFile() //assign result if it read is successful
} catch(t: FileNotFoundException){
  null // return null if file not found
}

val map: MutableMap<String, Int>
val (a, b) = readlLine()?.split(" ") ?: error("No line to read") // read line and split it by given delimeter and use destructuring declaration
map[a] = b.toInt() // Convert the value on-flight and assign it

fun doSomething(additionalArguments: List<String> = emptyList())

import java.util.Calendar

var Calendar.year
    get() = get(Calendar.YEAR)
    set(value) = set(Calendar.YEAR, value)

val calendar = Calendar.getInstance()
println(calendar.year) // 2020
calendar.year = 2021
println(calendar.year) // 2021