``` val newFragment Fragment = if fragment is CarSearchFragm kotlinlang #codereview

val newFragment: Fragment? = if (fragment is CarSearchFragment? && fragment == null) {
            CarSearchFragment()
        } else if (fragment is CarValuationFragment? && fragment == null) {
            CarValuationFragment()
        } else if (fragment is HotDealsFragment? && fragment == null) {
            HotDealsFragment()
        } else if (fragment is FavoritesFragment? && fragment == null) {
            FavoritesFragment()
        } else {
            fragment
        }

louiscad

10/31/2018, 3:06 PM

This code doesn't work. You can't really do

is

check on

null

. Doesn't the IDE warns you about this?

oday

10/31/2018, 3:06 PM

ahh that’s right,

is

is for type not value

yea no I dont get a warning

what Im looking for is how to do && in

when

I guess

dalexander

10/31/2018, 3:14 PM

val newFragment = if(fragment != null) {
  when() {
    ...
  }
} else {
  fragment
}

val newFragment = fragment ?: when(fragment) {
  ...
}

Maybe one of these?

oday

10/31/2018, 3:17 PM

hmm yea

dalexander

10/31/2018, 3:17 PM

There’s probably another way to improve it because it looks like you are just creating an instance of a particular class when it’s null. ie. you could probably add a function on the companion object that would create an instance of the class if it’s null, so it could be written like

val newFragment = fragment.initIfNull()

or something, and that lets you avoid doing all the type checking which generally is good to avoid for OO-design.

oday

10/31/2018, 3:17 PM

let me try that yea

Shawn

10/31/2018, 4:25 PM

this seems like a broader design problem or at least an a XY issue - you can’t really do cascading type checks on

null

and this much type checking is no bueno in general unless you’re working with enum or sealed classes with minimal state

Andreas Sinz

11/01/2018, 10:19 AM

the following ~~could~~ doesn't work:

val newFragment = fragment?.let { when(it) {
    is CarSearchFragment? -> CarSearchFragment()
    is CarValuationFragment? -> CarValuationFragment()
   ...
}

Shawn

11/01/2018, 11:15 AM

I’m pretty sure

null

will always hit the first branch of the when regardless of what type it might’ve had at the call site

Andreas Sinz

11/01/2018, 11:30 AM

nevermind, you are right. Its always the first branch

👍 1

oday

11/01/2018, 12:21 PM

Exactly what was happening to me

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