Nullable Type handling in RxJava with Kotlin

Question

When using RxJava2 in Java I have the advantage of map() filtering emitted null values automatically. However with nullable types in Kotlin I end up doing something like this:

val loadConferencesSingle = service.getConferences()
        .map { it.conferences ?: listOf<Conference>() }

The call service.getConferences() in this case is a Single emitting a ConferecesResponse which looks like

data class ConferencesResponse(val conferences: List<Conference?>? = null)

So in case the complete emitted list of conferences is null, I ended up using the Elvis operator to emit an empty list instead. Emitting a null in map is not possible.

Has anybody a clue, how to handle nullable types better in RxJava with Kotlin? I already like this solution and its terseness, but I've got the feeling that there are even better ways to handle this.

Hi, I think you need to see en.m.wikipedia.org/wiki/Null_Object_pattern — holi-java
– holi-java, Commented Jul 18, 2017 at 8:45

nhaarman · Accepted Answer · 2017-07-18 08:36:15Z

8

Optional is a completely valid type for this in Kotlin. The absence of a value is also a result. You could create a simple data class and function like so:

data class Optional<T>(val value: T?)
fun <T> T?.carry() = Optional(this)

Optionally, you could create some simple extension functions to make subsequent operators easier:

fun <T, R> Observable<Optional<T>>.mapNullable(mapper: (T?) -> R?) : Observable<Optional<R>>
  = map { mapper(it.value).carry() }

fun <T, R> Observable<Optional<T>>.mapFromNullable(mapper: (T?) -> R) : Observable<R>
  = map { mapper(it.value).carry() }

..and so on.

answered Jul 18, 2017 at 8:36

nhaarman

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3 Comments

Robin Over a year ago

Why would you prefer introducing a new custom Optional type if you already have nullable types in the type system that perfectly cover this case?

nhaarman Over a year ago

Because the reactive streams don't allow null values.

Robin Over a year ago

Ah, wasn't aware of that. Sorry!

Kiskae · Accepted Answer · 2017-07-18 08:11:17Z

1

It entirely depends on what is considered a valid result. Does a lack of conferences mean there are none? Then the empty list is an appropriate substitute. If it is not meant to happen then throwing an exception might be the appropriate response.

In addition throwing an exception also works with the elvis operator: it.conferences ?: throw IllegalStateException("Why is it null?")

answered Jul 18, 2017 at 8:11

Kiskae

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1 Comment

Stefan Medack Over a year ago

Thanks for the reply. So what I am doing right now is the desired behaviour: if the list of Conferences is null, I do not consider this being an error but as the absence of conferences. So what I am doing right now is actually working, but I felt like there is a better way of handling this.

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