RxJava How to return more than 2 ArrayLists within Observable in a function

Question

I'm able to pass two lists via observable as it is mentioned below

fun loadAll(contact: String?): Single<Pair<ArrayList<Contact>, ArrayList<Contact>>> {
    return packageDB.loadAll()
        .map { table ->
            val one = ArrayList< Contact >()
            val two = ArrayList< Contact >()
            val three = ArrayList< Contact >()
            
            one to two
        }
}

And in Fragment, I'm trying to return as:

  disposables.add(
        viewModel.loadAll(contact)
            .subscribe({
                val firstList = it.first
                val secondList = it.second
                val third = ArrayList<Contact>()
 }))

I'm new to reactive programming so I'm unable to understand how I can pass or return more than two lists as Pair object can have two child objects only. I would be thankful if you can let me know the solution.

Can you not just create a class to hold whatever you want and return that instead? — Ivan Wooll
– Ivan Wooll, Commented Sep 29, 2021 at 10:29
Agree with @IvanWooll. Create a class that holds whichever data you want and use it in Single<YourClass>. — gioravered
– gioravered, Commented Sep 29, 2021 at 11:12

Fred · Accepted Answer · 2021-09-30 13:45:22Z

There is more than one way of doing this, I can list some. You simply have to think of the return object. If you want to return 3 things only, I wouldn't even bother with creating a specific class if it's clear what you're doing. Kotlin has already a class for this - Triple. It's like a pair, but holds 3 values:

fun loadAll(contact: String?): Single<Triple<ArrayList<Contact>, ArrayList<Contact>, ArrayList<Contact>>> {
    return packageDB.loadAll()
        .map { table ->
            val one = ArrayList< Contact >()
            val two = ArrayList< Contact >()
            val three = ArrayList< Contact >()
            
            Triple(one, two, three)
        }
}

And then access them as:

disposables.add(
        viewModel.loadAll(contact)
            .subscribe({
                val firstList = it.first
                val secondList = it.second
                val third = it.third
 }))

If you have more than 3 values, I'm not sure if there's already a class for this, but there's always the option of using arrays of arrays:

fun loadAll(contact: String?): Single<Array<ArrayList<Contact>> {
    return packageDB.loadAll()
        .map { table ->
            val one = ArrayList< Contact >()
            val two = ArrayList< Contact >()
            val three = ArrayList< Contact >()
            
            arrayListOf(one, two, three)
        }
} 


disposables.add(
        viewModel.loadAll(contact)
            .subscribe({ (firList, secondList, thirdList) ->
                  // the above destructures the list into 3 variables
                  // if the list has less than 3 elements you'll get an index out of bounds exception
                  // otherwise you can just use the variables
 }))

As pointed out in the comments, this might create some problems understanding what each value is, so sometimes it's better to create your own class:

data class Values(
   val primaryContacts: ArrayList< Contact >(),
   val secondaryContacts: ArrayList< Contact >(),
   val emergencyContacts: ArrayList< Contact >(),
)

fun loadAll(contact: String?): Single<Values> {
    return packageDB.loadAll()
        .map { table ->
            val one = ArrayList< Contact >()
            val two = ArrayList< Contact >()
            val three = ArrayList< Contact >()
            
            Values(one, two, three)
        }
} 

disposables.add(
        viewModel.loadAll(contact)
            .subscribe({
                val firstList = it.primaryContacts
                val secondList = it.secondaryContacts
                val third = it.emergencyContacts
 }))

please excuse the naming. It's just an example.

Thanks man. I also solved it like this yesterday. Sorry I'm new in Rx so taking time to understand.
No need for apologies. We've all been there and some of us before StackOverflow :D

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