kotlinlang #getting-started

Just published an article on Jetpack compose. About how it works, why it was introduced, what problems it solves, etc. Please go through it whenever you get time and provide your feedback. Thanks https://medium.com/@ajinkyasaswade09/jetpack-compose-what-problem-does-it-solves-110ed9314dbe

Steve

03/18/2020, 7:51 PM

How do I OR together constants in Kotlin?

Czar

03/19/2020, 6:11 PM

Hi, does this make sense? I know it's generalizable with comparables, I'm just interested if this simple version can be written better.

fun Sequence<Int>.isOrderedAscending(): Boolean =
	zipWithNext { prev, next -> prev <= next }
		.fold(true) { acc, value -> acc && value }

Ivan Brko

03/20/2020, 10:06 AM

Hi all, can anyone reccomend a good site or smaller book or something to get started with kotlin? i have prior programming experience (mostly oop but also hobbyist fp) and I'm looking for something that will help me write some code quickly. i don't want to go through someone explaining me what if/else is and at this point i don't want a book that goes in depth and that would take me a month to read.

gabin

03/20/2020, 4:53 PM

Hi Guys. Could you point me a native kotlin class for working with Date?

Michael Saragih

03/21/2020, 9:22 AM

hello guys, anyone know which library/framework good for back-end of the web for newbie in kotlin

keochris

03/21/2020, 9:25 AM

Hello, I have been coding some Kotlin and ktor programs for some time but I am not a real developer and more importantly, I don’t know Java (at some point, it can be an issue with Kotlin). Then sorry for the following stupid question but if someone can point me out to good doc about those “@” annotations that I find in most advanced codes, it would fantastic! Thank you!

iseki

03/22/2020, 5:17 AM

Hello, I have a problem. How to cast type

<T>

<Base<T>>

open class C<T:C<T>>{
    fun <R> foo(a:R){
        a as T // looks not works...
    }
}

Ray Ryan

03/23/2020, 8:42 PM

I’m probably asking this in the wrong channel, but I’m not sure what the right one is. The question is, how do you extend an abstract inner class in Kotlin? This is legal Java:

class Outer {
  abstract class Inner {
  }
}
class Child extends Outer.Inner {
  Child(Outer outer) {
    // Calls Inner constructor, providing
    // outer as the containing instance
    outer.super();
  }
}

I can’t find an equivalent in Kotlin, if I want

Child

to live in a separate file / module. The auto converter generates this, which doesn’t compile:

class Outer {
  abstract inner class Inner
}

class Child(outer: Outer?) : Inner()

🤯 2

zero_coding

03/24/2020, 1:29 PM

Does it exists immutable list in Kotlin?

Gilberto Diaz

03/24/2020, 2:43 PM

What’s the correct name when you create a

fun Int.add(){}

. I’m trying to find docs about this approach but can’t find it.

Victor Cardona

03/24/2020, 10:45 PM

What is the most idiomatic way of expressing a class hierarchy in Kotlin? I found this blog post which used delegation, but I have heard that delegation is not a feature that many developers choose to use. Though in this case I can’t see the downside to it. My specific use case is a set of immutable classes where sub classes add additional properties only. Thanks!

Gus

03/26/2020, 4:24 PM

I need to create 4 subclasses of an abstract class whose sole constructor has 7 parameters (3 of which are required). Is there a way to avoid having to replicate the parent class' contructor signature so many times?

oday

03/30/2020, 2:51 PM

how can I find an item in a list according to an ID that I have, and then replace the item there with the object i have (whose ID i used to find its occurrence in the list)?

Gus

03/30/2020, 5:03 PM

Hi. If I have a sub-class that sets a different companion object from its parent class, can a method on the parent class access the companion object of the current sub-class? Here's some code:

open class TheCompanion(val foo: String)

open class Parent {
    fun doIt(): String {
        return foo
    }

    companion object : TheCompanion("the-parent")
}

class Child1() : Parent() {}

class Child2() : Parent() {
    companion object : TheCompanion("child2")
}

Child1().doIt()
Child2().doIt()

How can I get

Child2().doIt()

to output

"child2"

instead of

"the-parent"

? I guess this isn't possible because there's no way to enforce that subclasses of

Parent

must have a companion object implementing a given interface, right?

Giovanni Attina

04/01/2020, 1:27 AM

Hi! I'm learning Kotlin, I came from Javascript/Python and I'm focusing now in Kotlin in JVM to backend development. And now at my learning path, I'm learning how to create tests for my projects, should I start to learn JUnit or Spek? Both frameworks are very different. I appreciate Speak, using BDD, but I'm facing a little difficult to setup with IntelliJ. JUnit has a bigger community and the resources to learn look much better. What are you guys are using? Which one do you prefer? PS: I learn Kotlin because I'm moving to a new company and the squad I'm going to work use Kotlin.

Ivan Brko

04/01/2020, 11:15 AM

Hi all, I'm learning Kotlin through koans and I saw this code (not my, part of the exercise). The code is here: https://pastebin.com/A7vwUqiy I don't understand why the extension method set (at the bottom of the snippet) was written as an extension method? why not just write it as a normal method inside the Tag class? Is this just more idiomatic in Kotlin, or do you achieve something here that cannot be achieved with normal methods?

oday

04/01/2020, 1:59 PM

private fun getActiveFilters(categoryId: Int): String? {
        val activeFilters =  mutableFilterItems.value?.mapNotNull {
            it.takeIf { filterItem ->
                filterItem.categoryId == categoryId && filterItem.toggled
            }
        }?.joinToString { it.itemValue.capitalize() }
        
        return activeFilters
    }

oday

04/01/2020, 1:59 PM

activeFilters

is null or empty, I want to return null, otherwise I want to return activeFilters

oday

04/01/2020, 1:59 PM

how do I kotlinize that

harry.singh

04/07/2020, 10:52 PM

Is return type of

plusAssign

operator function always

Unit

:yes: 1

Ben

04/08/2020, 10:54 AM

I have some code that in Java I'd write

if (getFoo() != null && barEnabled()) {
    doSomething();
}

In Kotlin I could write

getFoo().takeIf { barEnabled() }?.let { doSomething() }

but I'm not convinced that's any easier to read, mostly because of the separation between the nullable getFoo() and the let. Any recommendations on how best to write that in Kotlin? Is using scope functions always preferred?

ursus

04/09/2020, 3:26 AM

can there something be done about it? its annoying me in the new kotlin, although I see its point

ursus

04/09/2020, 3:28 AM

I mean I really likes the smart casts, this feels like a step back

vinny2020

04/09/2020, 6:40 PM

Hello All, old javahead here, is there a more elegant way in deal with the classic issue of having to double iterate data struictures like : List<MyPojo> when MyPojo also has collection as a property (ex var myProp: List<String> ) ?

Ilya Pogrebenko

04/10/2020, 8:37 AM

Hi all. How can I get value of const property of object with reflection?

object Consts {
    const val KOTLIN = "kotlin"
}
Consts::class.memberProperties.forEach { println(it.get(???)) }

What should I pass as a receiver?

Ananiya

04/11/2020, 5:34 AM

What is the difference between object and class And what is const

Ananiya

04/11/2020, 11:33 AM

how to make windows app using kotlin

:google: 4

Chris Cordero

04/11/2020, 9:37 PM

I've got a

List<Int>

that I want to map over and do some calculations on. Since I'm learning Kotlin, I thought I'd try to do it two ways. This one works:

val foo: List<Int> = listOf(100, 200, 300)
foo.map { it / 3 - 2 } .forEach { println(it) }

However, I wanted to see what to do if I wanted to make a reusable transform function inside the

map

. So I tried this:

fun myMapFunc(it: Int): Int {
    return it / 3 - 2
}

val foo: List<Int> = listOf(100, 200, 300)
foo.map(myMapFunc).forEach { println(it) } // error

I already know I can fix it by doing this:

foo.map { myMapFunc(it) }

But I feel like the first way should work too... Why doesn't it?

Title

Chris Cordero

04/11/2020, 9:37 PM

I've got a

List<Int>

that I want to map over and do some calculations on. Since I'm learning Kotlin, I thought I'd try to do it two ways. This one works:

val foo: List<Int> = listOf(100, 200, 300)
foo.map { it / 3 - 2 } .forEach { println(it) }

However, I wanted to see what to do if I wanted to make a reusable transform function inside the

map

. So I tried this:

fun myMapFunc(it: Int): Int {
    return it / 3 - 2
}

val foo: List<Int> = listOf(100, 200, 300)
foo.map(myMapFunc).forEach { println(it) } // error

I already know I can fix it by doing this:

foo.map { myMapFunc(it) }

But I feel like the first way should work too... Why doesn't it?

Shawn

04/11/2020, 9:49 PM

it’s because unlike python and similar other languages, function references must be deliberately qualified in order to be passed as an argument or assigned to a variable

try

foo.map(::myMapFunc)

Chris Cordero

04/11/2020, 9:50 PM

That works

Some follow up questions: 1. What do you mean by "deliberately qualified"? 2. What is the

::

doing?

I can see here: https://kotlinlang.org/docs/reference/keyword-reference.html#operators-and-special-symbols That the

::

means that it is "creating a member reference or class reference", but I'm not sure what that means here exactly.

Shawn

04/11/2020, 9:55 PM

So the way symbols work in Kotlin is kinda sorta similar to how it’s done in Java, which doesn’t treat methods or classes as first-class tokens

if you want to pass in data about a class’s type to a function, you can’t just use the class’s name, like

foo(Bar)

you can pass in

foo(Bar::class)

which will generate a

KClass<Bar>

for you and send that to the function

i.e. a class reference

this isn’t just to mirror how Java does things — using a class name without

::

will actually let you reference a class’s

companion object

if one is defined on it

as for the member reference bit, technically speaking, Kotlin allows you to represent a reference to any member of a given class (or top-level declarations in a Kotlin file, which technically compiles to a class on the JVM anyhow)

references to variables aren’t supported yet, but you can reference method/functions as well as properties (variables that are class members and have accessors generated for them) just fine (you really should just have a read of this whole page https://kotlinlang.org/docs/reference/reflection.html)

in a very loose, imperfect sense

::

is kinda like using

to get a pointer to something in C/C++

(this also has some implications for representing references to constructors but I won’t go down that path unless you ask lol)

Chris Cordero

04/11/2020, 10:04 PM

lol that was a great explanation thank you very much for the detailed answer

👍 2

No worries on the constructor thing, I’ve got enough to let marinade in my head

👌 1

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