https://kotlinlang.org/docs/reference/idioms.html#string-interpolation
println("Name $name")Prefer read-only `val` over mutable `var`
/* bad */ var counter = 0 for(i in 0..20){ counter += i } /* preferred */ val sum = (0..20).sum()Unit as a return type
fun doSomething(): Unit{ println("do something") }https://kotlinlang.org/docs/reference/idioms.html#single-expression-functions
fun theAnswer() = 42
https://kotlinlang.org/docs/reference/idioms.html#default-values-for-function-parameters
fun foo(a: Int = 0, b: String = "") { ... }https://kotlinlang.org/docs/reference/idioms.html#creating-a-singleton
object Resource { val name = "Name" }https://kotlinlang.org/docs/reference/idioms.html#instance-checks
when (x) { is Foo -> ... is Bar -> ... else -> ... }https://kotlinlang.org/docs/reference/idioms.html#creating-dtos-pojospocos
data class Customer(val name: String, val email: String)
Use the fact that `return` and `throw` have type `Nothing`, https://kotlinlang.org/docs/reference/idioms.html#todo-marking-code-as-incomplete
val x = foo() ?: return notFound() val y = if(conditionSatisfied()){ 1.0 } else { error("Condition is not satisfied"}//throws inside, the type is Nothing }Nullable truth.
a?.b == true //instead of a?.b ?: false
Safe chain and elvis (https://kotlinlang.org/docs/reference/idioms.html#if-not-null-and-else-shorthand)
val files = File("Test").listFiles() println(files?.size ?: "empty")https://kotlinlang.org/docs/reference/idioms.html#execute-if-not-null
val value = ... value?.let { ... // execute this block if not null }https://kotlinlang.org/docs/reference/idioms.html#extension-functions
fun String.spaceToCamelCase() { ... } "Convert this to camelcase".spaceToCamelCase()Extension properties
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- With block
https://kotlinlang.org/docs/reference/idioms.html#checking-element-presence-in-a-collection
if ("john@example.com" in emailsList) { ... }
if ("jane@example.com" !in emailsList) { ... }
Destructuring declaration (https://kotlinlang.org/docs/reference/idioms.html#traversing-a-maplist-of-pairs)
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):
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
val list = listOf("a", "b", "c")
https://kotlinlang.org/docs/reference/idioms.html#accessing-a-map
println(map["key"]) map["key"] = value
https://kotlinlang.org/docs/reference/idioms.html#lazy-property
val p: String by lazy {
// compute the string
}
https://kotlinlang.org/docs/reference/idioms.html#return-on-when-statement
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
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
fun arrayOfMinusOnes(size: Int): IntArray {
return IntArray(size).apply { fill(-1) }
}
https://kotlinlang.org/docs/reference/idioms.html#configuring-properties-of-an-object-apply
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)
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)
Non-local return
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)
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
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 ?=)
var x: String? = null x = x ?: "Hello"
Public var with a private/protected setter
var a: Int = 2 protected set // accessible only in descendents
Mutable private - read-only public collections.
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).
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) }
Assignment to `if`/`when`/`try`
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
}
Read line
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
fun doSomething(additionalArguments: List<String> = emptyList())