Grouping and Combining Observables in RxJava

Question

I would like to do the following with RxJava

class Invoice(val dayOfMonth:Int,val amount:Int)

below is the sample monthInvoices:List< Invoice > to process

Invoice(3,100) Invoice(3,150) Invoice(3,50) Invoice(4,350) Invoice(8,400) Invoice(8,100)

First, I would like to group it by the day of the month like the following

Invoice(3,300) Invoice(4,350) Invoice(8,500)

Then I would like to create a list containing all the days of the month. Say, we are having 30 days for this month, then the output list must contain inserting a empty Invoice object with 0 amount for the days where there is no invoice

Desired output List

Invoice(1,0) //Since day 1 is not in the group summed list Invoice(2,0) //day 2 is also not there Invoice(3,300) Invoice(4,350) Invoice(5,0) Invoice(6,0) Invoice(7,0) Invoice(8,500) ….. Invoice(30,0)

Hope I have explained the need clearly. Can anyone please answer me a solution to do it entirely using RxJava?

why rx ? I do not see anything reactive in this ? why not juste Java Stream ? — bubbles
– bubbles, Commented Oct 31, 2019 at 10:53

MinseongPark · Accepted Answer · 2019-10-31 08:04:40Z

Try this

fun task(invoices: List<Invoice>) = Observable.fromIterable(invoices) .groupBy { it.dayOfMonth } .flatMapSingle { group -> group.reduce(0) { t1, t2 -> t1 + t2.amount } .map { group.key to it }} .toMap({ it.first }, { it.second }) .flatMapObservable { map -> Observable.range(1, 30) .map { Invoice(it, map[it] ?: 0) } }

Damon Baker · Accepted Answer · 2019-10-31 07:57:46Z

This can be achieved much more easily using the collection operators inside Kotlin's standard library, but in pure RxJava you can do this by using groupBy and reduce.

 val invoices = listOf( Invoice(3, 100), Invoice(3, 150), Invoice(3, 50), Invoice(4, 350), Invoice(8, 400), Invoice(8, 100) ) Observable.range(1, 30) .map { Invoice(it, 0) } // Create an Observable of Invoice([day], 0) .mergeWith(Observable.fromIterable(invoices)) .groupBy { it.dayOfMonth } // Merge the sources and groupBy day .flatMapMaybe { group -> group.reduce { t1: Invoice, t2: Invoice -> Invoice(t1.dayOfMonth, t1.amount + t2.amount) // Reduce each group into a single Invoice } } .subscribe { // Optionally you can call toList before this if you want to aggregate the emissions into a single list println(it) }

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