ahhh

# getting-started

[oday]

ahhh

[Mike]

Yes, ? is part of the type.

[oday]

does

when(fragment){
is MyType, null -> { }
}

mean if fragment is of myType and is null?

[Mike]

would have to be is MyType? although I think you could do

when(fragment) {
  is MyType -> {}
  is MyType? -> {}
}

instead. Although neither ‘feels’ right.

Maybe is MyType? -> fragment?.let { //not null} ?: {null}

[Mike]

I think ‘,’ is correct for AND in when, although I think it changed to &&. Your condition would never be true, though as if it’s a MyType it can’t possibly be null.

[oday]

oh I meant MyType? again sorry

[oday]

issue is that im trying to automate the function of assigning the correct instantiation class to an object, all the classes it can have derive from 1 class, so I have to know which I want exactly

[oday]

so if it’s from this type and it still isnt instantiated, give it ThisClassB(), otherwise just leave it as is

[oday]

and use it after the when

[oday]

let me show code one second

[oday]

private fun openBottomBarFragment(fragment: Fragment?) {
        var newFragment: Fragment? = null

        when (fragment) {
            is CarSearchFragment?, null -> {
                newFragment = CarSearchFragment()
            }
            is CarValuationFragment -> {
                newFragment = CarValuationFragment()
            }
            is HotDealsFragment -> {
                newFragment = HotDealsFragment()
            }
            is FavoritesFragment -> {
                newFragment = FavoritesFragment()
            }
        }

        if (fragment == null) {
            supportFragmentManager.beginTransaction().add(
                R.id.fragment_container, newFragment!!
            ).commit()
        } else {
            supportFragmentManager.beginTransaction()
                .hide(activeFragment).show(fragment)
                .commit()
        }
    }

[Mike]

Hmm. I’ll trust that you do need the code, and I assume the ’CarSearchFragment?, null` is compiling, correct? If so, I can post a fragment that’s a bit cleaner.

Also, if fragment is already the correct type, why is a new one constructed into newFragment and then never used (as fragment != null in order to get into those branches.

[oday]

well, because I got that val cannot be reassigned when I tried to assign it 😄

[oday]

it is used in the end if you notice though

[oday]

in its pristine state where it was passed as is and I didnt reassign it to anything, I use it to show it, otherwise I show the new one which was passed first as null

[Mike]

Are ‘?’ missing for CarValuationFragment branches? Otherwise if it’s a CarValuationFragment (non-null), newFragment gets assigned a new one. BUT fragment is != null, so it will use the original fragment.

Therefore the newFragment assigned variable isn’t used.

And I suspect you’ve forgotten that things in Kotlin are expressions.

val newFragment = when (fragment) {
        is CarSearchFragment?, null -> {
            CarSearchFragment()
        }
        is CarValuationFragment -> {
            CarValuationFragment()
        }
        is HotDealsFragment -> {
            HotDealsFragment()
        }
        is FavoritesFragment -> {
            FavoritesFragment()
        }
        else -> fragment
    }

[Mike]

or something like that. Wasn’t 100% sure what to put in the else

[oday]

they are not ? yet cause i hadnt gotten to them yet

[oday]

so I didnt edit those parts yet, still working on the first branch

[Mike]

Ok, wasn’t sure. That makes more sense.

[oday]

ok so yea yours makes more sense

[Mike]

Given that, I think the ‘else’ would return null rather than what I have right now

[oday]

ah there must be an else yea

[Mike]

If you don’t assign a when to a variable, it doesn’t do the branch completion check. As soon as you assign it to something, it then enforces the branch check.

I think there’s a KEEP requesting that when always be complete, but not yet.

[Mike]

Hmmm, and right now, if a new subtype of Fragment is added, you won’t have any notice, and may try and use a null ‘newFragment’ in your assignment.

If possible, I’d consider a sealed class here so the when will ensure all types are handled correctly.

[oday]

well I control what fragments are added and how they’re passed into this function

[oday]

my knowledge of those classes is the limiting factor of how many classes this function can accept 😄

[Mike]

sealed class gets the compiler to help you, though. But obviously it’s up to you.

[oday]

I see here that those duplicate null conditions are coming up as just that, duplicate labels in when

[oday]

I think an if is better here..

[oday]

that , is not behaving as I expect

[Mike]

I think the comma is only good if the items being compared are the same type. Not sure as I haven't used it yet.