kotlinlang #getting-started

by Kernighan and Ritchie ? I mean not by lets start with variables etc but more like start with short programs while explaining all Kotlin syntax and then get more complicated functionality.

Joey

11/19/2019, 7:25 AM

How to use kotlinx.seriliazation? My project is completely screwed after importing bunch of libraries and still got unresolved reference

Sebi Sheldin Sebastian

11/20/2019, 6:46 AM

try working with MapString, Model types

:thread-please: 4

Rajiv

11/20/2019, 12:48 PM

Hello guys, should i install java8 or java10 for kotlin to compile from terminal?

11/20/2019, 3:14 PM

is there a better way to write this code? I'm not a big fan of telling the compiler "fuck this null check, just trust me"

get() {
            if (_fulfillmentDetails == null) {
                _fulfillmentDetails = FulfillmentDetails(this)
            }

            return _fulfillmentDetails!!
        }

New111

11/20/2019, 5:32 PM

what are the two exclamation marks

!!

at the end ^^^ ?

Ellen Spertus

11/21/2019, 1:47 AM

I’m trying to provide a default value for a function parameter but have been unable to get the syntax right (and it’s hard to search for function parameters that are themselves functions). Could someone tell me how to correct this?

fun Payload.toString(errorReporter: (String) -> Unit = { (_: String) -> Unit } )String? { ... }

Shawn Witte

11/21/2019, 3:31 AM

Hey everyone, I have kind of a weird thing with generics. I have an

abstract class A<X,Y,Z: A<X,Y,Z>>

which I am subclassing to

class CompositeA<X,Y>: A<X,Y,CompositeA<X,Y>>

Now,

has an

abstract fun copy(): Z

and

CompositeA

has a property

components: List<A<X,Y,*>>

. When I try to iterate through objects from

components

, call

copy()

on them,and put the put the results into a

List<A<X,Y,*>>

the IDE gives me an error saying I'm trying to put something of type of type

A<*,*,*>

into the List, which is incompatible. I can cast the result of

copy()

by using

as A<X,Y,*>

, but then I get an unchecked cast warning. Any ideas what I'm not seeing here?

Hullaballoonatic

11/21/2019, 6:31 PM

is this only a syntactic difference?

sealed class Foo {
    object Bar : Foo()
}

sealed class Foo

object Bar : Foo()

standard class syntax would imply

Bar

has its own

Bar

object in the first one

Hullaballoonatic

11/21/2019, 7:15 PM

Is there a way to enforce an inheriting class provides a static property?

🇳🇴 3

Mani

11/21/2019, 7:22 PM

Folks, need quick help. I am bad with regex. I have a bunch of strings like this

Ivy Gourd- 2 kg fav_variety : Fresh fav_quantity : 2 kg pcp_packaging_type : Loose packing fav_unit_of_measurement : Per kg fav_average_weight : 2 kg is_assortment : false num_items_assortment : 0 length_mm : 250 breadth_mm : 180 height_mm : 120 dead_weight```

Hullaballoonatic

11/21/2019, 7:28 PM

I have a hard time wrapping my head around a difference between

and

Foo<F>

here:

interface Foo<F: Foo<F>> {
    val a: F
    val b: Foo<F>
}

so what is assignable to a and b?

Mark

11/22/2019, 5:02 AM

When converting an old java Utils-style class (never instantiated, only containing statics) to Kotlin, do you typically: 1. remove the Utils class completely, and move those methods up to file-level (converting to use extension functions where possible/appropriate) 2. convert the Utils class to use

object

modifier 3. keep the Utils class and put the methods in a

companion object

2️⃣ 5

1️⃣ 11

Mark

11/22/2019, 7:04 AM

When using a string template like

took: $`it`ms

the compiler gives a warning saying the backticks are redundant. But then removing the backticks gives a compiler error!

Hullaballoonatic

11/22/2019, 10:57 PM

What stdlib function turns

foo

into

bar

val foo = listOf(
  'a' to 1,
  'a' to 2,
  'b' to 3
)

val bar = mapOf(
  'a' to listOf(1, 2),
  'b' to listOf(3)
)

UPDATE: I found the answer

fun <T, K, V> Iterable<T>.groupBy(
    keySelector: (T) -> K,
    valueTransform: (T) -> V
): Map<K, List<V>>

victor-mntl

11/23/2019, 1:13 PM

The correct way of making an enum class with parameters for each value of the enum class is:

enum class Foo1 (val bar:Int) {FIRST(11), SECOND(222)}

so we would be able to call their

bar

value:

println(Foo1.FIRST.bar)

Now, if we look at this one (see that it is missing

val

between the parenthesis):

enum class Foo2 (bar:Int) {FIRST(110), SECOND(2220)}

in this case,

bar

doesn't exist on any call to a value of the enum class:

println(Foo2.FIRST.bar)

error: unresolved reference: bar

So, my question is, does it have any utility to create an enum class with arguments without a val or var expression (= with arguments that are not parameters)? Are the values

and

in this example lost?

🤔 2

Florian

11/23/2019, 6:53 PM

besides

lateinit

, all properties of a class need to be initialized when the object is constructed, right?

Joey

11/25/2019, 4:57 AM

Is it possible to generate random dynamic key like

push

in firebase? If yes what is the best way to do this in kotlin?

Dennis Tel

11/25/2019, 9:28 AM

Is there a simple way to scaffold a new (library) project for kotlin? Just getting started with kotlin and tried the options in intellij but it creates a project without a test runner which makes it somewhat difficult to run/interact with the code (scratch pads and worksheets seem to be broken and don’t output anything). Any recommendations?

rikusen0335

11/25/2019, 11:00 AM

Copy-pasta from #general Hello, I'm new to Kotlin, and I want to use Exposed with it, but I'm getting

java.lang.NoClassDefFoundError: org/jetbrains/exposed/sql/Database

. Does anyone know how this happen and a solution for this? Thanks. This project using Gradle, JVM.

Luismi

11/25/2019, 11:42 AM

Hello, How can I create continuous implementation with Kotlin?

oday

11/25/2019, 11:55 AM

hey so I’m having an issue with Base64 encoding, it seems whenever I encode a certain string in my app, the result is different from when I use something like base64encode.org, always different

New111

11/25/2019, 12:02 PM

Hello, what is the kotlin-native to way to execute another jar (it is not fork !) and then depending on an action restart it or kill it ?

Kstabks

11/25/2019, 12:57 PM

Hello, is there any more further explanation why actually creating a wrapper around a primitive value can be very bad for performance? I am trying to understand that in details. Regarding https://kotlinlang.org/docs/reference/inline-classes.html and the introduction

Jukka Siivonen

11/25/2019, 3:47 PM

I might me missing something obvious here but I'm trying to introduce nullable function parameter like

fun <T : Any?> execute(queryId: (() -> UUID)? = null, operation: Operation<T>): T? {

but I can't invoke this by

execute(Operation(...))

only

execute(null, Operation(...))

works, any pointers?

victor-mntl

11/25/2019, 4:10 PM

Does the pseudo-class

Map.Entry<K,V>

(that appears when you select only one entry of a Map) works exactly the same as

Pair<K,V>

? Are there any differences that should be taken care of?

victor-mntl

11/25/2019, 4:44 PM

I found that IntelliJ asks me to add an only safe call to the value of an entry of a Map although I first verify that it is non-null

val mutableMap<String, mutableSet<String>> myMap = mutableMapOf ("first key" to mutableSetOf("value1 of first entry","value2 of first entry"))

if (myMap["another key"]==null)

myMap["another key"] = mutableSetOf("value1 of second entry")

else

//at this point, we know that myMap["another key"] is NOT NULL myMap["another key"].add("value1 of second entry")
It marks the last line (marked in bold) and says that I should use ?. instead of . because it says the receiver of the call is of type

MutableSet<String>?

(nullable) but it is verified that it is not. Am I missing something?

Matthias R

11/25/2019, 5:07 PM

Hey I am actually a beginner in Kotlin. So my question is: when I have for example an abstract val status of type status which in the end is just an integer how can I use the val status as Integer(for example to compare to an other integer value) ..?

rikusen0335

11/26/2019, 11:08 AM

I want to ask who knows how to disconnect from Datase, using Exposed