Observable#take(Long) not returning desired size of items in RxJava

Question

I am using RxJava/Kotlin Observable#take() to get first 50 items from the list. But #take() operator is not behaving as it should as per Rx docs.

In Rx docs, #take() is defined as:

"Emit only the first n items emitted by an Observable"

I have a function like this:

As we can see the pageSize argument is 50

And initial size of the list is 300

After that #take(50) is applied to that Observable and at next breakpoint I still get the full size list i.e. size = 300

But just for the check, if something is wrong with the debugger or observable, I tried to take only items whose displayName contains "9", but this time I get the expected result of smaller list with 9 in each of their #displayName field.

I believe RxJava/Kotlin's #take() operator is not that crazy and it's just me.

I am not sure, but I think you confuse two different things here `Observable.take()` operator considers emissions of the observable, not the size of the list emitted. So if your `remoteFollowersService.getFollowers()` would emit more than 50 items it would return only first 50 of them. — iluu, Nov 17 '18 at 16:11

score 5 · Accepted Answer · answered Nov 17 '18 at 16:13

take behaves correctly as it will give you only 50 List<FollowersEntry> "marbles". Based on your screenshots and wording, I guess you wanted 50 FollowersEntry. There is a fundamental logical difference between a container of objects and the objects themselves. RxJava sees only an object sequence of type List<> but it can't know about the nested objects you intended to work with.

Therefore, you either have to use it.take(50) inside map (or whatever the Kotlin collections function is) or unroll the sequence of lists into sequence of entries via flatMapIterable:

getFollowers()
.flatMapIterable(entry -> entry)
.take(50 /* entries this time, not lists */)

I was trying to sleep and same thing hit my mind, I opened laptop and I get this answer, amazing.. thanks :) Mind always finds a create solution after it tries to ask a good question.. — erluxman, Nov 17 '18 at 16:17

score 3 · Answer 2 · answered Nov 17 '18 at 16:12

Take a good look at the return type of your method - Single<List<FollowersEntity>>. The Observable returned from remoteFollowersService.getFollowers() is not an Observable that emits 300 FollowersEntity items - it is an Observable that emits a single item, and that single item is a List containing 300 FollowersEntity items. In other words you need to call take on the list, not on the observable.

    return remoteFollowersService.getFollowers()
        .map { val size = it.size; it } // for debugging
        .map { it.take(pageSize) }
        .map { val size = it.size; it } // for debugging
        .map { it.filter { item -> item.displayName.contains("9") } }
        .single(emptyList())

Observable#take(Long) not returning desired size of items in RxJava

2 Answers2