In the following data, [MyData(name=mike, country=mm), MyData(name=tike, country=bkk)] how to retrieves all member of MyData and transform as one list? what i want to do is, [“mike”, “mm”, “tike”, “bkk”]

What is the best way to deepcopy a list, I am using the following way

studentList.forEach {

if(it.id ==student.id){

list.add(it.copy(isFailed = false))

}else{

list.add(it)

}

}

Why would this code not work?

val f: (String) -> Unit = { it -> println(it.length) }
val x = "hello".also(::f)

This prints out these error messages:

error: type inference failed: inline fun <T> T.also(block: (T) -> Unit): T
cannot be applied to
receiver: String  arguments: (KProperty0<(String) -> Unit>)

val x = "hello".also(::f)
                ^
error: type mismatch: inferred type is KProperty0<(String) -> Unit> but (String) -> Unit was expected
val x = "hello".also(::f)
                     ^

While

val x = "hello".also(::println)

would work, why would above not? What is

KProperty0<>

?

Another question: Can one override a function with lambda expression? I was following the koans, and below code

import java.util.*

fun getList(): List<Int> {
    val arrayList = arrayListOf(1, 5, 2)
    Collections.sort(arrayList, object : Comparator<Int> {
        override fun compare = {x: Int, y: Int -> y - x}
    })
    return arrayList
}

gave me error messages:

Object is not abstract and does not implement abstract member public abstract fun compare(p0: Int!, p1: Int!): Int defined in java.util.Comparator
'compare' overrides nothing
Expecting '('

Can someone elaborate these messages? edit: additionally when I tried different codes in the line starting with

override fun compare

, 1.

override fun compare(x: Int, y: Int) {
            return y - x
        }

this did not work as well, producing error messages

Return type of 'compare' is not a subtype of the return type of the overridden member 'public abstract fun compare(p0: Int!, p1: Int!): Int defined in java.util.Comparator'
Type mismatch: inferred type is Int but Unit was expected

I don’t understand the error message. Why was the expected type

Unit

, not

Int

? Shouldn’t the compiler expect the return type as

Int

? BUT 2. with a slight modification,

override fun compare(x: Int, y: Int): Int {
            return y - x
        }

this worked. 3.

override fun compare(x: Int, y: Int) = y - x

This works also. But in this case, I also didn’t explicitly write the return type, same as approach 1. What are the differences between them? Why did 1. produce error while the other two didn’t? What’s happening under the hood?

Hiya, I'm new to kotlin (coming from Typescript) and was curious what best practices people are using to managing app state? I've got a several projects I'm considering kotlin for (server, mobile, and web) and I'd love to have a shared core across all -- so this query isn't strictly about one setup. I am leaning towards the Flux/Redux design pattern as I am familiar with it and I think it makes sense once you've wrapped your head around unidirectional data flow. But I can't find much written about Kotlin + Flux. Is there a preferred approach someone here might be able to direct me to?

I couldn’t understand inline function explanation. Can anyone explain ?

Using higher-order functions imposes certain runtime penalties: each function is an object, and it captures a closure, i.e. those variables that are accessed in the body of the function. Memory allocations (both for function objects and classes) and virtual calls introduce runtime overhead.
But it appears that in many cases this kind of overhead can be eliminated by inlining the lambda expressions.

I might understand. Thank you! 🙂 Is such overhead so large that I should often care about ?

What is the underlying List type for Kotlins read only list?

AbstractList ?

I’d like to read in a file and produce a

Set<String>

containing all of the words in the file. Is there a way to

map

or

flatMap

over the lines of a file? I’ve gotten as far as

bufferedReader().useLines()

.

if I have a function at the top level of a file, not inside a class or an object, is there a way to get the name of the file , or the name of the function or if that doesn't work maybe the package ? (would like to log it)

Also asking here hoping to understand that part

Hi to all, I started using Kotlin little over two month to write a library, since then I started listening to Talking Kotlin and I started to hear the concept of DSL. The more I hear DSL in a conversation, seems to me that what it means is a specific library that is going to be used within a specific language. I would like for someone to explain what are the cases where to can call some code base a DSL.

with Kotlin IDEA seems to prefer this myMap[mykey] over this myMap.get(mykey) , is that the more idiomatic way to do things?

Hi all, I was wondering if there's something like

myMap.mapWithIndex { (index, key, value ) -> .... }

available? I know it's possible with arrays, but I "need" to have an index counter (my current approach is just to have a counter var, but I was hoping there would be something more elegant)

like a must read

Is there a way to get the keypaths to a class’s companion object such as this:

class Test {    
    companion object {
        const val A: Int = 0

        const val B: Int = 1

        const val C: Int = 2

        const val DInt = 3

        const val E: Int = 4
    }
}

This companion object will only have constant vals

What is the Java equivalent of this code:

interface MyInterface<out T> {
    fun process(): T
}

I need to convert this interface to Java because of SAM conversion, but I'm failing a bit here.

What is the java equivalent of this code:

Why is Java code almost always notably harder to understand that Kotlin code? Whenever i look up code online (be it stackoverflow or a github repo) i am happy with the kotlin examples most of the time but always find myself heavily refactoring converted java samples. I know Java is very verbose by design, but even when using IntelliJ to convert the Java code to Kotlin I often find myself doing heavy refactoring. Everything is needlessly wrapped twice or thrice and the exception handling is completely overkill. is it convention in java to always write code that is very verbose not only syntax wise (which luckily the converter can get rid of) but also structure wise? example: I am currently looking into software testing and came accross assertions. Let's compare the implementations of

assert()

from the kotlin stdlib and

assertEquals()

from junit. the former is basically implemented as

if (!condition)
    throw Exception(message)

its as simple and intuitive as can be. meanwhile the implementation of the latter goes so far down the rabbit hole (I have to navigate through 4 different files for what is actually just 5 lines of code) that I'd have trouble understanding what the function even does if it wasn't for the descriptive name. And this isn't just some random snippet from a stranger on the internet, its one of the most popular testing frameworks out there. just why?? Edit: I don't just mean to vent, I mean this as an actual question. Why is this? There's gotta be a reason for it

Hi! I’m exploring EduTools plugin trying to create a new course, but whenever I click on “Check” button to run a task the IDE goes like “No tests were found” error but the plugin reports a “✅ Correct”

If I have a function that requires not null variables, what is the best way to null check them before calling the function?

message has been deleted

is there a way to compare elements to each other in a list? For example if I have:

listOf(A, B, C)

I want a way to compare A to B, B to C and A to C so they are all evaluated against each other

anyone can help me with my project? I’m trying to target ios9, but I cant seem to get it right using the build.gradle

any thoughts?

we decide to not use any of theses, we just name an interface as MyStuffInterface, then used it in the implementation. Tks for inputs ;D

Hi, can anyone explain why in this code:

fun List<User>.aggregate(): List<User> {
    val result = this
        .groupingBy { it.login } // Grouping<User, String>
        .aggregate { key: String, accumulator: User?, element: User, first: Boolean
            ->
            if (first) element
            else User(
                accumulator!!.login,
                accumulator!!.contributions + element.contributions
            )
        }
    return result.values.toList()
}

result

’s type becomes

Map<String, User?>

, while I was expecting it to be

Map<String, User>

?

I’ve got what may be an interop nightmare. I’m converting a Java library to Kotlin, and the library has a dependency on a different library implemented in Clojure. The Clojure library throws an IllegalStateException but the Kotlin re-write seems to completely discard that exception when thrown. Is this expected behavior in Kotlin? Is there a way to get the previously expected behavior back?