type: doclayout: referencecategory: "Basics"title: "Idioms"

Idioms

Creating DTOs (POJOs/POCOs)

data classCustomer(valname:String,valemail:String)

Default values for function parameters

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

Filtering a list

val positives= list.filter { x-> x>0 }

val positives= list.filter { it>0 }

String Interpolation

println("Name$name")

Instance Checks

when (x) {isFoo->...isBar->...else->...}

Traversing a map/list of pairs

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

Using ranges

for (iin1..100) {... }// closed range: includes 100for (iin1 until100) {... }// half-open range: does not include 100for (xin2..10 step2) {... }for (xin10 downTo1) {... }if (xin1..10) {... }

Read-only list

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

Read-only map

val map=mapOf("a" to1,"b" to2,"c" to3)

Accessing a map

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

Lazy property

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

Extension Functions

fun String.spaceToCamelCase() {... }"Convert this to camelcase".spaceToCamelCase()

Creating a singleton

object Resource {val name="Name"}

If not null shorthand

val files=File("Test").listFiles()println(files?.size)

If not null and else shorthand

val files=File("Test").listFiles()println(files?.size?:"empty")

Executing a statement if null

val data=...val email= data["email"]?:throwIllegalStateException("Email is missing!")

Execute if not null

val data=...data?.let {...// execute this block if not null}

Map nullable value if not null

val data=...val mapped= data?.let { transformData(it) }?: defaultValueIfDataIsNull

Return on when statement

funtransform(color:String):Int {returnwhen (color) {"Red"->0"Green"->1"Blue"->2else->throwIllegalArgumentException("Invalid color param value")    }}

'try/catch' expression

funtest() {val result=try {        count()    }catch (e:ArithmeticException) {throwIllegalStateException(e)    }// Working with result}

'if' expression

funfoo(param:Int) {val result=if (param==1) {"one"    }elseif (param==2) {"two"    }else {"three"    }}

Builder-style usage of methods that returnUnit

funarrayOfMinusOnes(size:Int):IntArray {returnIntArray(size).apply { fill(-1) }}

Single-expression functions

funtheAnswer()=42

funtheAnswer():Int {return42}

funtransform(color:String):Int=when (color) {"Red"->0"Green"->1"Blue"->2else->throwIllegalArgumentException("Invalid color param value")}

Calling multiple methods on an object instance ('with')

classTurtle {funpenDown()funpenUp()funturn(degrees:Double)funforward(pixels:Double)}val myTurtle=Turtle()with(myTurtle) {//draw a 100 pix square    penDown()for(iin1..4) {        forward(100.0)        turn(90.0)    }    penUp()}

Java 7's try with resources

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

Convenient form for a generic function that requires the generic type information

//  public final class Gson {//     ...//     public <T> T fromJson(JsonElement json, Class<T> classOfT) throws JsonSyntaxException {//     ...inlinefun <reifiedT:Any> Gson.fromJson(json:JsonElement):T=this.fromJson(json,T::class.java)

Consuming a nullable Boolean

val b:Boolean?=...if (b==true) {...}else {// `b` is false or null}