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  1. https://kotlinlang.org/docs/reference/idioms.html#string-interpolation

    Code Block
    println("Name $name")


  2. Prefer read-only `val` over mutable `var`

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

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


  3. Unit as a return type

    Code Block
    fun doSomething(): Unit{
  println("do something")
}


  4. https://kotlinlang.org/docs/reference/idioms.html#single-expression-functions

    Code Block
    fun theAnswer() = 42


  5. https://kotlinlang.org/docs/reference/idioms.html#default-values-for-function-parameters

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


  6. https://kotlinlang.org/docs/reference/idioms.html#creating-a-singleton

    Code Block
    object Resource {
    val name = "Name"
}


  7. https://kotlinlang.org/docs/reference/idioms.html#instance-checks

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


  8. https://kotlinlang.org/docs/reference/idioms.html#creating-dtos-pojospocos

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


  9. Use the fact that `return` and `throw` have type `Nothing`, https://kotlinlang.org/docs/reference/idioms.html#todo-marking-code-as-incomplete

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

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


  10. Nullable truth.

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


  11. Safe chain and elvis (https://kotlinlang.org/docs/reference/idioms.html#if-not-null-and-else-shorthand)

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

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


  12. https://kotlinlang.org/docs/reference/idioms.html#execute-if-not-null

    Code Block
    val value = ...

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


  13. https://kotlinlang.org/docs/reference/idioms.html#extension-functions

    Code Block
    fun String.spaceToCamelCase() { ... }

"Convert this to camelcase".spaceToCamelCase()


  14. Extension properties

    Code Block
    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

    https://kotlinlang.org/docs/reference/idioms.html#creating-dtos-pojospocos

    Code Blockdata class Customer(val name: String, val email: String)


  15.  With block

https://kotlinlang.org/docs/reference/idioms.html#checking-element-presence-in-a-collection

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

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

...

https://kotlinlang.org/docs/reference/idioms.html#instance-checks

...

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

Destructuring declaration (https://kotlinlang.org/docs/reference/idioms.html#traversing-a-maplist-of-pairs)
 Code Block
for ((k, v) in map) {
    println("$k -> $v")
}

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

ranges instead of for loops (https://kotlinlang.org/docs/reference/idioms.html#using-ranges):
 Code Block
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) { ... }

https://kotlinlang.org/docs/reference/idioms.html#read-only-list
 Code Block
val list = listOf("a", "b", "c")

https://kotlinlang.org/docs/reference/idioms.html#accessing-a-map
 Code Block
println(map["key"])
map["key"] = value

https://kotlinlang.org/docs/reference/idioms.html#lazy-property
 Code Block
val p: String by lazy {
    // compute the string
...
https://kotlinlang.org/docs/reference/idioms.html#creating-a-singleton
...

}
...
https://kotlinlang.org/docs/reference/idioms.
...
 Code Block
val files = File("Test").listFiles()

println(files?.size ?: "empty")
https://kotlinlang.org/docs/reference/idioms.html#execute-if-not-null
 Code Block
val value = ...

value?.let {
    ... // execute this block if not null
}
...
html#return-on-when-statement
 Code Block
fun transform(color: String): Int {
    return when (color) {
        "Red" -> 0
        "Green" -> 1
        "Blue" -> 2
        else -> throw IllegalArgumentException("Invalid color param value")
    }
}

https://kotlinlang.org/docs/reference/idioms.html#trycatch-expression
 Code Block
fun test() {
    val result = try {
        count()
    } catch (e: ArithmeticException) {
        throw IllegalStateException(e)
    }

    // Working with result
}

https://kotlinlang.org/docs/reference/idioms.html#builder-style-usage-of-methods-that-return-unit
 Code Block
fun arrayOfMinusOnes(size: Int): IntArray {
    return IntArray(size).apply { fill(-1) }
}

https://kotlinlang.org/docs/reference/idioms.html#configuring-properties-of-an-object-apply
 Code Block
val myRectangle = Rectangle().apply {
    length = 4
    breadth = 5
    color = 0xFAFAFA
}

Scoped use of objects (https://kotlinlang.org/docs/reference/idioms.html#java-7s-try-with-resources)
 Code Block
val stream = Files.newInputStream(Paths.get("/some/file.txt"))
stream.buffered().reader().use { reader ->
    println(reader.readText())
}

https://kotlinlang.org/docs/reference/idioms.html#convenient-form-for-a-generic-function-that-requires-the-generic-type-information

 Code Block
//  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)

Non-local return
 Code Block
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")
}

.filter.map.reduce (collection processing, higher order functions) (https://kotlinlang.org/docs/reference/idioms.html#filtering-a-list)
 Code Block
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

Factory method instead of constructors
 Code Block
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?; x = x ?: "Hello"; (analog for dart ?=)
 Code Block
var x: String? = null
x = x ?: "Hello"

Public var with a private/protected setter
 Code Block
var a: Int = 2
	protected set // accessible only in descendents

Mutable private - read-only public collections.
 Code Block
ptivate val _list = ArrayList<Int>()
val list: List<Int> get() = _list

Trailing lambdas (place lambda the last in the function parameters list, try to provide default values for parameters). Try to make such functions inline (to remove lambda creating overhead).
 Code Block
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
...
Use the fact that `return` and `throw` have type `Nothing`, https://kotlinlang.org/docs/reference/idioms.html#todo-marking-code-as-incomplete
...
)
...
 
...
}
...


...
Nullable truth. 
...

Assignment to `if`/`when`/`try`
 Code Block
val a = if(b>0) 0 else 1

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

Read line
 Code Block
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

Simple empty collection
 Code Block
fun doSomething(additionalArguments: List<String> = emptyList())