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()
https://kotlinlang.org/docs/reference/idioms.html#creating-dtos-pojospocos
Code Block data class Customer(val name: String, val email: String)
https://kotlinlang.org/docs/reference/idioms.html#default-values-for-function-parameters
Code Block fun foo(a: Int = 0, b: String = "") { ... }
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#string-interpolation
Code Block println("Name $name")
https://kotlinlang.org/docs/reference/idioms.html#instance-checks
Code Block when (x) { is Foo -> ... is Bar -> ... else -> ... }
Destructuring declaration (https://kotlinlang.org/docs/reference/idioms.html#traversing-a-maplist-of-pairs)
Code Block for ((k, v) in map) { println("$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#extension-functions
Code Block fun String.spaceToCamelCase() { ... } "Convert this to camelcase".spaceToCamelCase()
https://kotlinlang.org/docs/reference/idioms.html#creating-a-singleton
Code Block object Resource { val name = "Name" }
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")
https://kotlinlang.org/docs/reference/idioms.html#execute-if-not-null
Code Block val value = ... value?.let { ... // execute this block if not null }
https://kotlinlang.org/docs/reference/idioms.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#single-expression-functions
Code Block fun theAnswer() = 42
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()) }
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
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 }
Nullable truth.
Code Block a?.b == true //instead of a?.b ?: false
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())
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