Hi, is there any way I can optimize this java code in Kotlin to have a something in one line ? I can't find anything right now, and I almost sure I have to do something with

.let

or similar functions but I can't come up with something

Hi, i'm having this kind of data class structure:

abstract class AuthorizeSalesAdditionalSecurity {
    @SerializedName("lang")
    // Using like below is fine too
    // abstract val lang: String
    open val lang: String? = null
    @SerializedName("POSTransactionID")
    // abstract val POSTransactionID: String
    open val POSTransactionID: String? = null
}
data class CreditAuthorizeSalesAdditionalSecurity(
        @SerializedName("lang") override val lang: String,
        @SerializedName("POSTransactionID") override val POSTransactionID: String
) : AuthorizeSalesAdditionalSecurity() {}
data class UnionPayAuthorizeSalesAdditionalSecurity(
        @SerializedName("lang") override val lang: String? = null,
        @SerializedName("POSTransactionID") override val POSTransactionID: String? = null
) : AuthorizeSalesAdditionalSecurity() {}

I realize that I can use keyword "open" or "abstract" in AuthorizeSalesAdditionalSecurity and Kotlin accept both. Can someone explain for me the difference between 2 use case?

Followup to that "NO" reply for inline classes. Is there java interop?

Hi everyone, been playing with the language some time ago and loved it, now i have an opportunity to write a small microservice in tech stack of my choice, so i planned using kotlin with ktor. Any pitfalls to be specially aware of? Also, what are other libraries/tools or compiler plugins that are must have with kotlin projects (things like linting, any special formatting libraries or something lke that)? Thanks for the help 🙂

I am trying to serialize (and eventually deserialize) a

koltin.time.Duration

property of a data class in Kotlin 1.5.30 using Jackson 2.12.5. One immediate problem is that - because Duration is a value class - Jackson sees just a long value and thus serializes a

Duration.seconds(10)

as

20000000000

. This is obviously not what I want because it exposes the internal "use high bit to differentiate nano from milli". I tried to register a custom serializer which extends

JsonSerializer<Duration>()

using

@JsonSerialize

but that is ignored (presumably because Jackson just sees a long value). The only way I see right now is to have an explicit

durationAsString

property and keep that in sync with the

duration

property. Is there a better way? Note: I think I cannot use Kotlin serialization because this is part of a mixed Java/Kotlin project. But if I could: would that solve the issue?

I have a mutableMap and two functions the queue function is getting called from different origins. But somehow the map is always empty if I modify it in these functions? Code:

val buckets = mutableMapOf<String, RestClient.Bucket>()

suspend fun queue(endpoint: String, task: suspend () -> HttpResponse): HttpResponse {
     println(buckets.size) //always 0
     //....
     send(endpoint, task)
}

private suspend fun send(endpoint: String, task: suspend () -> HttpResponse): HttpResponse {
     buckets[endpoint] = bucket
     println(buckets.size) //always one
}

As long as we pass the variable as a parameter, it will declare as

val

?

Best way to filter a list using other list? I want to filter a list of objects with list of ids. As of now, I am just doing this

ids.forEach { id ->
   val person = persons.find { it.id == id}
   filteredList.add(person)
}

Hey guys! I was quite surprised with this code:

repeat(3) {
   if (it == 0) return@repeat
   println("inside repeat")
}

it prints “inside repeat” twice instead of 0 times Why is that?

Hey guys! I am at the very beginning of using Kotlin and understanding certain mechanics, if I have the following:

data class Foo(
   var id: Int?, 
   var name: String = "bar", 
   var anotherVar
)

I thought I could have calls like

Foo()

or

Foo(id=1)

without needing to pass all the parameters to the constructor, am I wrong? Where can I read more about constructors in Kotlin?

When creating a file with only top-level declarations, I know that there is an annotation so the automatically generated class isn't named “<File name>Kt”, but I can't remember which annotation it is, nor find the good search terms...

is there a way to name case in

when

, so that i could write something like this:

when (expression generating nullable){
  null -> handle null
  x    -> handle x for which compiler knows its not null anymore
}

What the idiomatic way of doing this in Kotlin?

if (mNotificationManager == null) {
                mNotificationManager = this.getSystemService(Context.NOTIFICATION_SERVICE) as NotificationManager
 }

Hey peepz, how are you? I have a question regarding Tests,

@Nested

,

inner

classes,

ParametrizedTests + MethodSource

I have the following

class MyServiceTest {
   data class TestData(...)

   @Nested
   inner class MyServiceSpecific{
      private fun prepareTestData() = Stream.of(...)

      @ParameterizedTest
      @MethodSource("prepareTestData")
      fun `GIVEN this example WHEN executing test THEN should run`(data: Testdata) {
         // EVERYTHING I WANT TO TEST
      }
   }
}

With this structure I get the following error

Cannot invoke non-static method [private final java.util.stream.Stream... on a null target.

How can I gave all this working, I am missing a glue probably

How to write an internal extension function for an existing class? See thread.

Hello Community, At the moment, I have two functions, which are able to add and evaluate a lambda from a MutableList. Now, I want to save this List to a file in a human-readable format and later load them again and evaluate the lambdas. What is the best approach? Can I use JSON to save them? And edit them externaly. I want to avoid the ScriptEngine because I'm using linux aarch as host so kotlin/native did not work nor is the jna library compatible. Here is my code so far:

typealias func = ()-> Unit

@JvmField val command = mutableListOf<func>()

fun main() {
	
	add {
		val i =4
		println("${i}")
	}
	
	eval(command[0])
}

fun add(action: () -> Unit): Unit {
	command.add(action)
}

fun eval(action: () -> Unit) {
	action()
}

Thank you

Not sure if this is the right channel, but: is there something new on that topic https://discuss.kotlinlang.org/t/what-is-an-idiomatic-way-to-create-a-read-only-wrapper-for-a-class/10929 ? I.e., I'd like to have a class

data class MutableClass(var my1, var my2, var my3...)

and a fitting

data class ImmutableClass(val my1, val my2, val my3...)

. So the only difference is, that all members of one class are

var

and the members of the other one are

val

and of course I don't want to write the list of members twice 🙂

Given a class

Base

, Is there a way to run some code before and after every method call on Base's subclasses?

This message was deleted.

With all the new advances in Kotlin, Flow, Coroutine etc. what is a good book/online course to get up to speed on latest and advanced Kotlin topics?

why am I getting this?

is this behavior expected of reified parameters?  Given (this is as simple as I could drive down my example):

inline fun <reified R, reified S> reifiedGenericIsSame(r: R, s: S): Boolean = (r is S) && (s is R)
interface FooBar<out A: Any, out B: Any>
typealias FBOI<A> = FooBar<Int,A>
typealias FBOS<A> = FooBar<String,A>

and the following:

open class Base<out A: Any, out B: Any>(val a: A, val b: B): FooBar<A, B> 

class SpecialOfInt<out B: Any>(b: B) : Base<Int, B>(1, b)
class SpecialOfStr<out B: Any>(b: B) : Base<String, B>("1", b)

// val soitd: FBOI<Int> = SpecialOfInt(2)
// val sostd: FBOS<Int> = SpecialOfStr(2)
val soi = SpecialOfInt(2)
val sos = SpecialOfStr(2)
//
// EDIT
//
val soitd: FBOI<Int> = soi
val sostd: FBOS<Int> = sos

then this holds:

reifiedGenericIsSame(soi, sos) shouldBe false     // passes
reifiedGenericIsSame(soi, sos) shouldBe true      // FAILS
reifiedGenericIsSame(soitd, sostd) shouldBe false // FAILS
reifiedGenericIsSame(soitd, sostd) shouldBe true  // passes

which 🤞is VERY counterintuitive, as it appears a typealias (allegedly syntax sugar) is changing a result semantically. I suppose type erasure side effects may be invoked as the mechanics, but this question is about the intrinsic behavior. Could please anyone provide me with great kotlin wisdom?

I have a polymorphic class tree. I want to enumerate a list of leaf classes, and evaluate each one with an argument and let each one tell me how suited it is for that context. The sorting and selection is straightforward to me. It's how to get abstract class side methods and a list of class instances that is confounding me.

listOf<DragGesture>(DragCyclePositionGesture.class, DragMoveGesture.class, DragBeginGesture.class, DragEndGesture.class)

is not legal. Number one, there needs to be way a way to make the template not be instances of DragGesture, but rather subtypes of DragGesture? Is that even specifiable? And the compiler isn't happy after the first .class either, i start getting all kinds of red after that. Am I supposed to use companions in this case? I'm not even sure how companions fit in the scheme of inheritance. 😕

How can I solve this? Gradle: 7.2, Intellij: 2021.2.2, Kotlin: 1.5.30, Kotlin IDE plugin: 212-1.5.30-release-409-IJ4638.7 I tried,

rm -rf ~/.gradle/caches

and Invalidate Caches and restart on Intellij

What would be the equivalent in Java of

Array<IntArray>

? I'm trying to store a 2D matrix of ints/floats in a compact way, would this Array of IntArrays be the way to go? Would that be equivalent to

int[][]

?

What does this mean, ViewBinding has type safety and null safety?

May I ask why this smart cast does not work? Thanks.

fun <T : Any> isNotNull(value: T?): Boolean {
    contract {
        returns(true) implies (value != null)
        returns(false) implies (value == null)
    }
    return value != null
}


class ContractTest {
    @Test
    fun test1() {
        val a: String? = null
        if (isNotNull(a)) {
            println(a.uppercase()) // Works as expected: "Smart cast to kotlin.String"
        }

        val notNull = isNotNull(a)
        if (notNull) {
            println(a.uppercase()) // Compilation error: "Only safe (?.) or non-null asserted (!!.) calls are allowed on a nullable receiver of type String?"
        }
    }
}

how am I supposed to create a

kotlin.time.Duration

from something like

d-hh:mm:ss

? Combining

d.toDuration(DAYS) + hh.toDuration(HOURS)

and so on? If yes, I find java

Duration

better in this regards, because you can parse it from a `String`:

Duration.parse("..")

what is the most efficient way to round

Double

up to certain decimal points, there is

round()

function in Kotlin but it always completely rounds up to 0 decimals? I want something like

round(1.4999,decimalPrecision = 2) // 1.50