Unexpected NullPointException with RxKotlin when mapping optionals

Question

To start I have the following Moshi json.

@JsonClass(generateAdapter = true)
data class OrderDetails(
    @Json(name = "_id") val id: Int,
    @Json(name = "status") val status: String,
    @Json(name = "tableNo") val tableNo: Int,
    @Json(name = "serverId") val serverId: Int?,
    @Json(name = "items") val orderItems: List<OrderDetailsItem>
)

All these fields are expected to have data except for serverId. This data is fetched from the server where I can allow the user to select order.

onSeletedOrder
   .map { it.orderDetails.serverId } //blows up here apparently.
   .filterNotNull() //have tried this but it doesn't matter.
   .flatMap { findServerBy(it) }
   .map { "${it.firstname} ${it.lastname}" }

When I map to the serverId above I blow up with an NPE. It's interesting that the map (even though it is optional) does an unsafe cast afterwards. I'd expect it to maintain the optional-ness after the map. I'm assuming this is because of the bridging backwards to RxJava. Curious if anyone has a further explanation on why this is.

Null valuess are not allowed in RxJava so map to null is forbidden and crashes with NPE. You could prefilter with filter { it.orderDetails.serverId != null } before the map. As for why map doesn't result in type Optional<T> in Kotlin, I have no idea. — akarnokd
– akarnokd, Commented Nov 28, 2019 at 17:06
What is the type of onSeletedOrder: Observable<OrderDetails>? Single? Something else? — Alexey Romanov
– Alexey Romanov, Commented Nov 29, 2019 at 7:24

Damon Baker · Accepted Answer · 2019-11-29 06:52:26Z

1

RxJava does not allow nulls inside the stream. Ideally you would perform this filter before the items enter the stream, but if you can't do that one workaround you could get away with is to use an empty string in place of null.

onSeletedOrder
   .map { it.orderDetails.serverId.orEmpty() }
   .filter { it.isNotEmpty() }
   .flatMap { findServerBy(it) }
   .map { "${it.firstname} ${it.lastname}" }

answered Nov 29, 2019 at 6:52

Damon Baker

9077 silver badges9 bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

Evan Anger Over a year ago

I'm not happy with the feedback given with the extra defensive-ness need but this works. I suppose I can do the filter even earlier than the map. So filter { it.orderDetails.serverId != null } then map { it.orderDetails.serverId }

Alexey Romanov · Accepted Answer · 2019-11-29 07:39:40Z

0

For "map, but exclude some elements", RxJava has flatMapMaybe (other types than Observable can also have it with corresponding return type):

// helper
fun <T> T?.toMaybe(): Maybe<T> = if (this != null) Maybe.just(this) else Maybe.empty<T>

onSeletedOrder
   .flatMapMaybe { it.orderDetails.serverId.toMaybe() }
   .flatMap { findServerBy(it) }
   .map { "${it.firstname} ${it.lastname}" }

answered Nov 29, 2019 at 7:39

Alexey Romanov

172k38 gold badges329 silver badges509 bronze badges

Comments

Pulkit · Accepted Answer · 2019-11-29 07:44:37Z

0

The best approach is to format your Data Class with optional data.

@JsonClass(generateAdapter = true)
data class OrderDetails(
    @Json(name = "_id") val id: Int? = 0,
    @Json(name = "status") val status: String? = "Not active",
    @Json(name = "tableNo") val tableNo: Int? = 0,
    @Json(name = "serverId") val serverId: Int? = 0,
    @Json(name = "items") val orderItems: List<OrderDetailsItem>? = listOf()
)

You won't receive any NPE

answered Nov 29, 2019 at 7:44

Pulkit

1,0503 gold badges13 silver badges27 bronze badges

3 Comments

Evan Anger Over a year ago

Thats actually not the case when I start putting these particular optional-fields into RxJava.

Pulkit Over a year ago

Interesting, I found that moshi doesn't support this, based on this github.com/square/moshi/issues/807 Can you experiment a bit and use, Kotlinx Serialization. I have been using it in my project and this use case is coverd. Just wanted to see, if it would work with RxJava map filter

Evan Anger Over a year ago

I'll take a look at it, but fundamentally as I've researched this, there is no null safety in RxJava streams. It would take a rewrite in order to accommodate from what I've gathered of RxJava. This was a good thread which discussed some of the issues/differences (github.com/reactor/reactor-core/issues/979)

Collectives™ on Stack Overflow

Unexpected NullPointException with RxKotlin when mapping optionals

3 Answers 3

1 Comment

Comments

3 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

3 Answers 3

1 Comment

Comments

3 Comments

Your Answer

Sign up or log in

Post as a guest

Related