Hi there i m trying to understand generics inside kotlin and kotlinlang #announcements

Hi there, i'm trying to understand generics inside kotlin and i'm blocked. Given sample below i wanted to use in base class my FooList as param and extend this class where i'm using BarList. But i'm unable to

open class Foo {

}
class Bar:Foo(){

}
open class FooList {
	open var list:List<Foo>?=null
}
class BarList:FooList(){
	override var list:List<Bar>?=null
}
open class BaseTest {
 open var llist:List<FooList>?=null
}
class BaseTestTwo : BaseTest {
	override var llist:List<BarList>?=null
}

Andreas Sinz

04/18/2018, 1:00 PM

BaseTestTwo

failing?

felislynx

04/18/2018, 1:01 PM

Yes

normally in Java i would use <? extends FooList>

sorry, i've made one mistake rewriting this sample

open class FooList<T:Foo> {
    open var list: List<T>? = null
}
class BarList : FooList<Bar>() {
    override var list: List<Bar>? = null
}

Andreas Sinz

04/18/2018, 1:05 PM

the problem is that

list: List<T>?

is a

var

and thus is reassignable

felislynx

04/18/2018, 1:07 PM

ide error: var property type List<BarList> which is not a type of overriden public var list List<FooList<Foo>>

i've many times overrided default value of var or val in subclass but i didn't changed type of it during process

Andreas Sinz

04/18/2018, 1:09 PM

FooList<T: Foo>

is invariant, that means

FooList<Bar>

is not a subtype of

FooList<Foo>

felislynx

04/18/2018, 1:10 PM

is this achiveable ?

i mean what i meant, not how i did 🙂

Andreas Sinz

04/18/2018, 1:10 PM

if you change it to

val

and use

FooList<out T: Foo>

felislynx

04/18/2018, 1:13 PM

but in doing it as val, i cannot reasign it

there is no other way?

Andreas Sinz

04/18/2018, 1:16 PM

nope, not without losing type-safety

if you can show me more code around it, maybe we can work out a better approach

felislynx

04/18/2018, 1:18 PM

Foo,Bar,FooList,BarList are classes used in retrofit

BaseTest and BaseTestTwo are classes representing ViewModel

where Base is in main module, and Two i is in other flavors

Andreas Sinz

04/18/2018, 1:20 PM

and why do you need to reassign the list?

felislynx

04/18/2018, 1:21 PM

open val lists: MutableLiveData<Pair<String, ArrayList<Foo>>> = MutableLiveData()

after new network call

i have to use lists.value = Pair("Some String",response.body())

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