When returning a new instance of an anonymous class, in Java it's obvious that it's a new instance:

return new InterfaceXXX() {
    @Override public void foo() {}
    @Override public void bar() {}
}

But Kotlin uses the

object

keyword:

return object : InterfaceXXX {
    override fun foo() {}
    override fun bar() {}
}

And because it uses the

object

keyword, it looks as if you are returning the same singleton instance every time. I have to do a double-take to notice it's not actually a singleton. Does this confuse others too?

any elegant way to get

z = null

here instead of an exception? (try)

fun main() {
    val (x, y, z) = "A/B".split("/")
}

Hi, trying to work out some generic code to add listeners to all buttons using view Bindings 'pattern'. I will put full class below but the bit that does not work is

for (i in binding) {
    if (i is Button) i.btnPlayVideo.setOnClickListener(this)
}

I get why as you cant do a for iteration on bindings but what would be alternative code for this?

package com.funkytwig.youtubeplayer

import android.os.Bundle
import android.os.PersistableBundle
import android.util.Log
import android.view.View
import android.widget.Button
import androidx.appcompat.app.AppCompatActivity
import com.funkytwig.youtubeplayer.databinding.ActivityStandaloneBinding
import com.google.android.youtube.player.YouTubeStandalonePlayer

class StandaloneActivity : AppCompatActivity(), View.OnClickListener { // android.view.View
    private val TAG = "StandaloneActivity"
    private lateinit var binding: ActivityStandaloneBinding

    override fun onCreate(savedInstanceState: Bundle?) {
        Log.d(TAG, "onCreate")
        super.onCreate(savedInstanceState)
        binding = ActivityStandaloneBinding.inflate(layoutInflater)
        setContentView(binding.root)

        for (i in binding) {
            if (i is Button) i.btnPlayVideo.setOnClickListener(this)
        }

        // Long form version of code
        // binding.btnPlayVideo.setOnClickListener(this)
        // binding.btnPlayPlaylist.setOnClickListener(this)
    }

    override fun onClick(view: View) { // Ctrl+I to implement interface
        TODO("Needs implementing")    
    }
}

Ben

Hello here, I was reading about the mixed kotlin/java support and was wondering. In the docs it advises to run Kotlin compiler first (i use maven) and later java compiler. I tried to create a kotlin class that depends on java class and it worked. My question is why? Java compilation happens after. If it would be the other way around (java class depending on kotlin class and running java compiler without compiling kotlin first) it wouldnt work. So why does it work when you compile kotlin classes that depend on java classes that are not compiled (yet)? I also tried running the kotlin compilation alone and in target folder I saw only compiled kotlin class but not the java class it depends upon. A quick look over compiler documentation didnt provide me with an answer. So maybe someone here knows or could point me in the right direction. Thanks in advance.

I'm writing a database in Kotlin, and don't have a solid understanding of concurrency fundamentals Traditionally, you'd use Read-Write Locks for I/O operations like reading/writing to disk If I'm using Kotlin Coroutines, what's the best practice for ensuring that multiple concurrent reads can happen but only a single write?

class DiskManagerImpl(file: File) : DiskManager {
    private val channel = AsynchronousFileChannel.open(file.toPath(), StandardOpenOption.READ, StandardOpenOption.WRITE)

    override suspend fun readPage(pageId: PageId, buffer: MemorySegment) {
        val offset = pageId * PAGE_SIZE
        val read = channel.readAsync(buffer.asByteBuffer(), offset.toLong())
        require(read == PAGE_SIZE) { "Failed to read page $pageId" }
    }

    override suspend fun writePage(pageId: PageId, buffer: MemorySegment) {
        val offset = pageId * PAGE_SIZE
        val written = channel.writeAsync(buffer.asByteBuffer(), offset.toLong())
        require(written == PAGE_SIZE) { "Failed to write page $pageId" }
    }
}

https://pl.kotl.in/urxAUO4rz 🙂

val a: Double = Double.NaN
println(a == a) // false
val b: Number = a
println(a == b) // true
val c: Double = 0.0
println(c == -c) // true
val d: Number = c
println(d == -c) // false

(yes, I know why this has to happen for IEEE 754 and boxed equals() reasons, it's a fun puzzler to figure out if you don't know why though)

Ime having as problem with AsyncTask (I know it is depreciated, but I am doing an exercise). Basically, doInBackground is running fine but onPostExecute never runs.

@file:Suppress("DEPRECATION")

package com.funkytwig.flickerbrowser

import android.os.AsyncTask
import android.util.Log
import java.io.IOException
import java.net.MalformedURLException
import java.net.URL

enum class DowsloadStatus {
    OK, IDLE, NOT_INSTALLED, FAILED_OR_EMPTY, PERMISSIONS_ERROR, ERROR
}
private const val TAG = "GetRawData"

@Suppress("DEPRECATION")
class GetRawData : AsyncTask<String, Void, String>() { // class
    private var downloadStatus = DowsloadStatus.IDLE

    @Deprecated("Deprecated in Java")
    override fun doInBackground(vararg params: String?): String {
        // params[0] = JSON URL
        val funct = "doInBackground"
        Log.d(TAG, funct)
        Log.d(TAG, "$funct ${params[0]}")
        if (params[0] == null) {
            Log.d(TAG, "$funct No URL")
            downloadStatus = DowsloadStatus.NOT_INSTALLED
            return "No URL Specified"
        }

        try {
            downloadStatus = DowsloadStatus.OK
            Log.d(TAG, "$funct: DowsloadStatus.OK")
            return URL(params[0]).readText()
        } catch (e: Exception) {
            val errorMessage = when (e) {
                is MalformedURLException -> {
                    downloadStatus = DowsloadStatus.NOT_INSTALLED
                    "Invalid URL ${e.message}"
                }
                is IOException -> {
                    downloadStatus = DowsloadStatus.FAILED_OR_EMPTY
                    "IO Exception ${e.message}"
                }
                is SecurityException -> {
                    downloadStatus = DowsloadStatus.PERMISSIONS_ERROR
                    "Needs Permission ${e.message}"
                }
                else -> {
                    downloadStatus = DowsloadStatus.ERROR
                    "Unknown Error ${e.message}"
                }

            }
            Log.d(TAG, "$funct dome errorMessage")

            return errorMessage
        } // try/catch
    } // doInBackground

    @Deprecated("Deprecated in Java")
    override fun onPostExecute(result: String?) {
        val funct = "onPostExecute"
        Log.d(TAG, funct)
        super.onPostExecute(result)
    }

    @Deprecated("Deprecated in Java")
    override fun onCancelled() {
        val funct = "onCancelled"
        Log.d(TAG, funct)
        super.onCancelled()
    }

    @Deprecated("Deprecated in Java")
    override fun onPreExecute() {
        val funct = "onPreExecute"
        Log.d(TAG, funct)
        super.onPreExecute()
    }

    @Deprecated("Deprecated in Java")
    override fun onProgressUpdate(vararg values: Void?) {
        val funct = "onProgressUpdate"
        Log.d(TAG, funct)
        super.onProgressUpdate(*values)
    }
}

Any ideas?

Hi all, is there a way to write a data class that compiles to the same as the following Java class?

public class Pet {

    private List<String> names;

    public @NonNull List<@NonNull String> getNames() {
        return names;
    }

    // constructor, setter, etc. omitted
}

I tried to write the following data class:

data class Pet(val names: @get:NonNull List<@get:NonNull String>)

For each of the annotations I get two compile errors: - This annotation is not applicable to target 'undefined target' and use site target '@get' - This annotation is not applicable to target 'type usage' and use site target '@get' The NonNull annotation is from the Java library microprofile-graphql-api

@Retention(RetentionPolicy.RUNTIME)
@Target({ElementType.TYPE_USE, ElementType.METHOD, ElementType.FIELD, ElementType.PARAMETER})
@Documented
public @interface NonNull {
}

I run into the same problem when I try to set the annotations on the field:

data class Pet(val names: @field:NonNull List<@field:NonNull String>)

Is there a way to get annotations on the type and generic type of a getter method or field?

I am using kotlin in Intellij with a maven build. I have JUnit 5 Version 1.7.10 and the bundled Kotlin compiler. My simple Kotlin code is working fine but I cannot get the JUnit Tests to work in the Test runner. In the Maven build the tests work (goal install). When starting test in the runner I get this error message: Sep 12, 2022 112058 AM org.junit.platform.launcher.core.DefaultLauncher handleThrowable WARNING: TestEngine with ID 'junit-jupiter' failed to discover tests java.lang.NoClassDefFoundError: org/junit/jupiter/api/ClassOrderer at org.junit.jupiter.engine.config.DefaultJupiterConfiguration.<clinit>(DefaultJupiterConfiguration.java:50) at org.junit.jupiter.engine.JupiterTestEngine.discover(JupiterTestEngine.java:66) at org.junit.platform.launcher.core.DefaultLauncher.discoverEngineRoot(DefaultLauncher.java:168) at org.junit.platform.launcher.core.DefaultLauncher.discoverRoot(DefaultLauncher.java:155) at org.junit.platform.launcher.core.DefaultLauncher.discover(DefaultLauncher.java:120) at org.apache.maven.surefire.junitplatform.TestPlanScannerFilter.accept(TestPlanScannerFilter.java:56) has anyone seen this problem before??

interface Criteria<T: Criteria<T>> { fun test(): T }

open class Filter1(val data: String = ""): Criteria<Filter1> {
    override fun test() = Filter1("Basic")
}

object Standard: Filter1() {}

// How can I provide some correct type here?
fun whatever(): Criteria<*> {
    // Some custom code...
    return Standard
}

fun main() {
    println(whatever().data) 
}

Could someone help me with my

whatever

function here, so it returns the correct type? (playground: https://pl.kotl.in/z4lPq_kWE)

Each month sees new server side developers beginning their journeys with Kotlin. Some just want to give the language a try, while others have specific goals in mind. 🧑‍💻 Let us know how your Kotlin journey began so we can make it easier. 👉 👉 👉 https://surveys.jetbrains.com/s3/kotlin-usage-survey

Is it possible to use builder inference with variables? (See thread)

Hi here, how do I reference a constant declared above a kotlin class from the java code?

Is there an easy way to make chunked split on spaces and not just size?

When using the use function on autoClosable classes that implement close(). Is there a way I can get the suppressed exceptions to log them?

I would like to express this

enum class TextRowState {
    Enabled,
    Disabled,
    ActiveAlpha{
        fun get(){
            if (Enabled){
                0.5f
            }else{
                1f
            }
        }
    }
}

What would you suggest for massively parallel programming in Kotlin ? (library, framework) single computer (no need for MPI & co) purely asynchronous (no need to synchronize a known state at any moment) context : a bunch of asynchronous “artificial creature” living in a “shared world”. There will be race condition and its an integral part of the simulation. (whatever happens in case of race condition, happens). a thread pool would be good as i want to create 1 thread per creature.

Is there any library for image/gif manipulation? such as resizing, blurring, color filters, etc

How do I convert a float to a Duration? there's extension values for other number types but not one for floats

I'm getting

Type argument is not within its bounds: should be subtype of 'PostgreSQLContainer<PostgreSQLContainer<*>!>!'

when instantiating it via

PostgreSQLContainer<PostgreSQLContainer<*>>

this used to work in Kotlin 1.6. Java signature is

PostgreSQLContainer<SELF extends PostgreSQLContainer<SELF>> extends JdbcDatabaseContainer<SELF>

removed the type parameters and it works; still probably something wrong with type checking here

hey, coming from other languages, I’m struggling to see use cases for

return@

. why is it needed? does it allow you to do something you previously couldn’t, or is it just syntax sugar?

what would be the equivalent of lua for a kotlin/jvm application please ? (to provide modding/extensibility)

How can i print word like this -> Hello World in terminal? using kotlin

Trying to get my head around what internal means. Been doing a bit of reading (https://kotlinlang.org/docs/visibility-modifiers.html, https://kotlinlang.org/spec/packages-and-imports.html#modules, https://www.digitalocean.com/community/tutorials/kotlin-visibility-modifiers-public-protected-internal-private). The digitalocean.com seems to sum it all up and is consistent with the kotlinlang.org docs. Come with me down the rabbit hole :). I understand everything here apart from internal and exactly what a module is. It says "Setting a declaration as internal means that it’ll be available in the same module only. By module in Kotlin, we mean a group of files that are compiled together.". I think the sticking point for me is "Compiled together". What exactly does that mean? Is this something to do with packages? Or is it simply referring to the current .kt file? The other thing that got me wondering is the use of

private const val TAG = "BaseActivity"

Which can put just after the imports. Its not defined within a class which seems inconsistent with private being '`private` means that the member is visible inside this class only (including all its members).'. Does it effectively become private to all classes in the file? To go further down the rabbit hole what does 'including all its members' mean? I guess it means function/methods but if so why not just say this?

listOf(1,2,3).sumOf { it.toDouble() } //works fine
listOf(1,2,3).sumOf { it.toFloat() } //error: no applicable overload

can someone explain why?

fun <T> List<Int>.transform(
    block: (Int) -> T = { it }
): List<T> {
    return map { block(it) }
}

fun main() {
    val items = listOf(1, 2, 3)
    
    println(items.transform { "$it, " })
    println(items.transform())
}

Does anyone know if there is a issue on youtrack somewhere discussing making something like this possible?

Is it possible to use JvmMultifileClass to separate api (as class/interface) and implementation (class/interface/extensionfuncs/packagefuncs)?

is it OK to have a constructor throw an exception or is it bad practice ?

err, i just realized, if my kotlin code isn’t meant to be called from java, i don’t need to use the @throws annotation in my class declaration, right ?