Rx Java map functions in parallel

Question

New to RxJava and Reactive Programming so to speak.

I'm trying to map two functions in parallel as part of a single Observable pipeline, but doesn't seem to work this way. Here is my code.

Observable.fromCallable(thatReturnsNumberOne())
                .observeOn(newThread())
                .map(doubleIt())
                .observeOn(newThread())
                .map(doubleIt())
                .subscribe(testSubscriber);

I'd like the 2 doubleIt() calls to be spawned at the same time. But it always appears to be that once the first doubleIt() finishes, only then the second one starts. ie blocking/sequential.

What am I missing?

Jon G Stødle · Accepted Answer · 2017-08-17 05:45:58Z

1

I'm assuming thatReturnsNumberOne() only returns a single value. The value that is returned is passed to each of the operators in sequence. By using observeOn(newThread()) your only changing to a new thread when the value gets to that point in the chain.

If you want to do calculations in parallel, you have to use multiple observables:

Observable.fromCallable(thatReturnsNumberOne())
    .flatMap(number -> Observable.fromCallable(doubleIt(number)).subscribeOn(newThread())
        .combineLatest(Observable.fromCallable(doubleIt(number)).subscribeOn(newThread()),
        doubles -> doubles[0] + doubles[1]))
    .subscribe(testSubscriber);

answered Aug 17, 2017 at 5:45

Jon G Stødle

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