import kotlin.math.PI
import kotlin.system.measureTimeMillis

fun test1(a : FloatArray) : Float {
    val angles = a.map { it.toDegrees() }
    val yaw = angles[0]
    val pitch = angles[1]
    return yaw + pitch
}

fun test2(angles : FloatArray) : Float {
    val yaw = angles[0].toDegrees()
    val pitch = angles[1].toDegrees()
    return yaw + pitch
}

fun Float.toDegrees() : Float {
    return (this * 180 / PI).toFloat()
}


fun main(args: Array<String>) {
    val a = floatArrayOf(0.5f, 0.7f)

    val test2Time = measureTimeMillis {
        var out = 0L
        for (i in 0..1000000) {
            out += test2(a).toLong()
        }
    }
    val test1Time = measureTimeMillis {
        var out = 0L
        for (i in 0..1000000) {
            out += test1(a).toLong()
        }
    }
    println("Test1: $test1Time ms")
    println("Test2: $test2Time ms")
}

Test1: 42 ms Test2: 7 ms Using map is pretty costy. Am i doing something wrong?

What is the difference between an interface and a data class?

What’s the idiomatic way in Kotlin to handle events in non-UI code? In JS you declare an EventTarget, in Swift you use the delegate pattern. What could I use in Kotlin to be notified when a specific event occurs in a class instance? In Jetpack Compose, they have a simple

onClick

parameter for the

Button

class. My feeling here is there is no idiomatic way to handle events, am I wrong?

Hello! I've (wrongly) asked this on #eap already, and moving here as requested. Here's the previous thread: https://kotlinlang.slack.com/archives/C0KLZSCHF/p1673469970819299 I'm playing around with the new context receivers prototype (really exciting!) and I've hit a little snag with function references. I'm trying to run a function of the following type:

suspend context(Foo, Bar) () -> Unit

, however its

.invoke

method is requiring two arguments, one for each receiver. The thing is: I explicitly have the receiver for one of them, but the other one is also a context receiver of the callee, so it isn't available as

this

. Is there any way I can call this function?

this@Foo

and

this@Bar

are not options, according to IntelliJ. Thank you!

In rust iterators cost nothing. How kotlin compares to this? Let's say i have an array of floats and i want to do something on them, like multiply each item by 2. And sum the results. Using simple for loop in kotlin would be faster then say: array.map(it -> it*2).sum() ?

I've come across an issue where some Java code is using org.apache.commons.beanutils.PropertyUtils.setProperty to try and set a property on a Kotlin class. I've annotated the field with @JvmField to get past the first issue I had, and now I'm getting

Unknown property 'someField' on class 'class rubbish.SomeKotlinClass'

Is there a way of making Kotlin POJO type classes 100% compatible with code like PropertyUtils that relies on Java reflection ?

// <https://mvnrepository.com/artifact/commons-beanutils/commons-beanutils>
implementation("commons-beanutils:commons-beanutils:1.9.3")

package rubbish

import io.kotest.matchers.nulls.shouldNotBeNull
import io.kotest.matchers.shouldBe
import org.apache.commons.beanutils.PropertyUtils
import org.junit.jupiter.api.Test

data class SomeKotlinClass(
    @JvmField
    var someField: String? = null,
)

class SetTest {

    @Test
    fun kotlinTestData() {

        val x = SomeKotlinClass::class.java.getDeclaredConstructor().newInstance()

        x.shouldNotBeNull()

        val fields = SomeKotlinClass::class.java.declaredFields
        fields.forEach {
            println("Field is $it")
            println("trying to get field")
            SomeKotlinClass::class.java.getField(it.name)
            println("got field")
        }

        PropertyUtils.setProperty(x,"someField","hello")

        x.someField shouldBe "hello"
    }
}

Hi, small question. Is there a way to pattern match a lambda that receives a Map<K,V>. For example:

onSomething { (key,value) : Map<K,V> -> ... }

Hello, Anyone knows about how can I open streetview from app google maps through URI, like: google.streetview:cbll=$lat,$long When I open with this, just be loadding a black screen, but when tap in map, then load the view through the lat/long

I have a small program, kind of like a collection of scripts, and I need to have a different config for when I run it normally vs when I run unit tests. Is there a way create a global config so that when I run the script it loads and uses the real config file and when I run the test it loads and uses a test config file? I know every script and test could start by creating a new config, but that seems a bit much 🤔

Is there any channel to share projects written in Kotlin? I would be curious to see what the community creates.

Hi everyone, any recommendations for tools that can be used either as pre-commit hooks or (preferably) with Github Actions for opinionated code formatting? In the Python world I'm used to Black (https://black.readthedocs.io/en/stable/) which can be used in so many ways to make sure everyone is on the same page w.r.t. code formatting. Is there something like this in the Kotlin world other than making everyone on a project use IntelliJ?

Kotlin/JVM: after adding a package directive to my code, I can no longer access any resource files. No matter what path I try giving them, it will just not find them. Folders look like this:

src

main

kotlin

*.kt

resources

file.txt

prior to this, just "file.txt" worked.

When I got an interface say like this

interface Binding<T> {
    var value: T
}

And later I have a function which takes a

Binding<String?>

like this

fun foo(asd: Binding<String?>) {}

If on a function, I got an instance of

Binding<String>

, I am unable to pass it since it sees

Binding<String>

and

Binding<String?>

as different types. Changing my interface

Binding

to take in

out T

instead fixes this problem on the call site, but gives me an error on

val value: T

as it tells me that it is delcared as

out

but it occurs in

invariant

position in type T. If it were a

val value: T

it’d work, but it’s a

var

now and I can’t change that. Right now what I am doing, is that in call sites, if I got a

Binding<T>

I convert it into a

Binding<T?>

manually to pass it in. Is this the best I can do in this case?

Hello everyone, I have just started using Kotlin at work. Can someone explain what this line of code mean (declaring a method argument): interface Contract fun someMethod(someLambda: Contract.() -> Unit) { } Here we are declaring a method that takes a lambda

someLambda

, where the lambda must take in a

Contract.()

and return nothing (

Unit

). But what does

Contract.()

mean? Are we dot-accessing something on an interface?

Hello; currently in our application, at some locations, we use a custom 'range' class with bounds like:

date1..null // Range.BoundedLeft<LocalDateTime>
null..date2 // Range.BoundedRight<LocalDateTime>
date1..date2 // Range.Closed<LocalDateTime>

For this we made an operator fun on the rangeTo operator for comparable classes. This works fine for things like dates and custom classes. But if I want to use the rangeTo on an Int, there is no way for me to set the precedence of my own utility function over that of the construction of an IntRange, is this fixable? I know I can import my custom

rangeTo

as

somethingElse

, but I'd like to be able to do the following:

val range: Range.Closed = 1..3

Hello recently I come across a strange thing to me. I declare a variable name with val and try to print it to the console the code look pretty much like this:

fun main() {
	val cup = 25
	println(cup + " cup of water")
}

However this snippet of code face a compile error and the way to work around this is as follow:

fun main() {
	val cup = 25
	println("" + cup + " cup of water")
}

I dont't know that in Java/Kotlin world this is normal or not but I think it's a little weird and can be improved to make a developer experience better (I come from JS field)

What's a good way to configure logging in a kotlin project by code?

is there a nice way to have an overloaded function that accepts all (nullable) unsigned types, and to call that function with

0u

? At the moment Kotlin doesn’t know which unsigned type to use, and errors with

Overload resolution ambiguity

.

Is it possible to delegate an interface to a

lateinit

var? My attempts at doing so result in an Initialization exception, and I can't find anything else about it on formus/SO. Is there some way to foce the compiler to allow this?

Is there a way for a super function to perform a check and return if a condition is met or continue otherwise? The block to continue will be in the overriding function

class VeganFood: Food()

interface Seller<out T>

class FoodSeller: Seller<Food>
class VeganFoodSeller: Seller<VeganFood>

interface Consumer<in T>

class Person: Consumer<Food>
class Vegan: Consumer<VeganFood>

fun main() {
    var foodSeller: Seller<Food>
    foodSeller = FoodSeller()
    foodSeller = VeganFoodSeller()
    var veganFoodConsumer: Consumer<VeganFood>
    veganFoodConsumer = Vegan()
    veganFoodConsumer = Person()
}

Does anyone explain covariant and contravariant in Kotlin, please recommend any discussion on this topic

when defining extension functions for a class (outside of the class definition), is it idiomatic to use

this

? or should I omit it?

In subclasses, one often needs to explicitly cast a supertype to the type required by the current context, e.g.

class Cat : Animal {
  override var owner: CatOwner

  override fun switchOwner(owner: AnimalOwner) {
    owner = owner as CatOwner
  }
}

The cast to

CatOwner

is semantically redundant because it's quite clear what the resulting type should be, only the compiler can't/shouldn't guess it. Still, these casts soon seem like a boilerplate (especially with more complex generic types) so I devised the following helper function:

inline fun <reified TOut : T, T : Any> T.asTOut() = this as TOut

The usage would then simply be

override fun switchOwner(owner: AnimalOwner) {
    owner = owner.asTOut()
  }

This basically emulates what the compiler does when inferring generic parameter types. Of course, the difference in my example is that it's generally unsafe (that's why the compiler doesn't do it automatically). What do you think about this? Neat or horrible?

Kotlin documentation makes no mention on whether parameter default values have to be constant or not and there's no equivalent in java to compare to. This appears to work:

var defaultValue = 4
fun foo(number: Int = defaultValue) { /*something*/ }

even when I change the var, it appears to call the function with the new value. Is there any pitfalls of doing this that I'm unaware of or is it perfectly fine?

I have a class

Processor

that processes some data and I have a function

process()

that instantiates the class and returns the result. I don’t want to have intermediate state, so I want to disallow creation of this class outside of the file it is declared in. however, I do want to call the function as

Processor.process()

. currently I have

class Processor private constructor(...) {
  //...
  private fun finalResults(): Int

  companion object {
    fun process(...): Int {
      return Processor(...).finalResults()
    }
  }
}

is there another way to do this? EDIT: basically, I'm asking for a way to emulate namespaces here.

Hi quick question re write this codes without if statement

val x: int? = null
val y: string? = if (x != null) x.toString() else null

easiest way to cast a mutable list into a list?

Hi everyone, I am newbie to Kotlin. The front part is not rendering on the web page. What might be the issue? Atatched my code. I appreciate for any help!

Newbie (ish) question .... How do I do what the comment in that asks (or something similar) ? Updated link: https://pl.kotl.in/QerMFeLbh

Hi everyone, is there a way to simplify this bit of code ?

sealed interface Namespaced : Argument {
    val name: String
    val namespace: String

    override fun asString() = "$namespace:$name"
}


interface Biome : Namespaced {
    companion object {
        operator fun invoke(biome: String, namespace: String = "minecraft") = object : Biome {
            override val name = biome
            override val namespace = namespace
        }
    }
}

interface MobEffect : Namespaced {
    companion object {
        operator fun invoke(name: String, namespace: String = "minecraft") = object : MobEffect {
            override val name = name
            override val namespace = namespace
        }
    }
}
// etc

So I can avoid having the same companion object again and again, some of the interfaces might change so I would prefer to have an interface the

companion object

implements, but I can't figure out how to do it I have to stick with interfaces because every one of them are implemented by an enum listing the native data of each in the game (for example there is a

Biome

enum implementing the

Biome

interface so you can use the raw values of the enum as an

Argument

object). It helps me for serialization also with the

asString()

method which is custom in enums to include the name of the enum entry as lowercase.