anybody knows a nicer way to do this ``` val childUnitsById kotlinlang #getting-started

05/29/2017, 12:28 PM

anybody knows a nicer way to do this:

val childUnitsById = units
                .map { if(it.parent != null) it else null }
                .filterNotNull()
                .groupBy {it.parent!!}

Andreas Sinz

05/29/2017, 12:33 PM

jaspersmit: use

mapNotNull { }

05/29/2017, 12:34 PM

great that that exists:)

so then we have

val childUnitsById = units
                .mapNotNull { if(it.parent != null) it else null }
                .groupBy {it.parent!!}

No suggestions? 😛

05/29/2017, 12:38 PM

wasn't a good one haha

05/29/2017, 12:38 PM

thats unfortunate 😞

05/29/2017, 12:38 PM

but why do you need to map first?

Andreas Sinz

05/29/2017, 12:39 PM

@jaspersmit is it correct that you are just filtering by

parent != null

and then group them together? you could do that with

filter { it.parent != null }

05/29/2017, 12:39 PM

exactly what I was thinking

05/29/2017, 12:39 PM

yes you're right but i was looking for something to avoid the !!

but i was not very succesful with that

05/29/2017, 12:42 PM

ooh, yeah, I don't think there's a way to smart cast it

05/29/2017, 12:44 PM

i realize i need a groupByNotNull

05/29/2017, 12:50 PM

However I think this is a pretty-self contained case of the !! operator

I'd go forward with this

05/29/2017, 12:59 PM

yes i will do that 🙂

05/29/2017, 2:27 PM

The only way to get rid of

!!

would be to create a

List<ParentType>

(not nullable), so you'd have to generate that list first. Means you'd iterate twice, but you'd get rid of the

!!

If you know that list is small compared to the original list, probably worth doing. If not, your initial code is fine

Do keep in mind that technically, you can still get an NPE (i.e. if another thread sets one of these parents to

null

by the time you get there)

05/29/2017, 2:40 PM

luckily parent is immutable

05/29/2017, 4:44 PM

this seems to work fine for me, no

!!

val childUnitsById = units
            .filter { it.parent != null }
            .groupBy { it.parent }

05/29/2017, 4:45 PM

What's the type of

it.parent

05/29/2017, 4:46 PM

data class Node(val parent: Node?, val id: String) {
    override fun toString(): String {
        return "Node(id='$id')"
    }
}