Combining Observables
This section explains operators you can use to combine multiple Observables.
Outline
startWith
Available in: 
Flowable, Observable, 
Maybe, Single, Completable
ReactiveX documentation: http://reactivex.io/documentation/operators/startwith.html
Emit a specified sequence of items before beginning to emit the items from the Observable.
startWith Example
Observable<String> names = Observable.just("Spock", "McCoy");
names.startWith("Kirk").subscribe(item -> System.out.println(item));
// prints Kirk, Spock, McCoymerge
Combines multiple Observables into one.
merge
Available in: Flowable, Observable, Maybe, Single, Completable
ReactiveX documentation: http://reactivex.io/documentation/operators/merge.html
Combines multiple Observables into one. Any onError notifications passed from any of the source observables will immediately be passed through to through to the observers and will terminate the merged Observable.
merge Example
Observable.just(1, 2, 3)
.mergeWith(Observable.just(4, 5, 6))
.subscribe(item -> System.out.println(item));
// prints 1, 2, 3, 4, 5, 6mergeDelayError
Available in: Flowable, Observable, Maybe, Single, Completable
ReactiveX documentation: http://reactivex.io/documentation/operators/merge.html
Combines multiple Observables into one. Any onError notifications passed from any of the source observables will be withheld until all merged Observables complete, and only then will be passed along to the observers.
mergeDelayError Example
Observable<String> observable1 = Observable.error(new IllegalArgumentException(""));
Observable<String> observable2 = Observable.just("Four", "Five", "Six");
Observable.mergeDelayError(observable1, observable2)
.subscribe(item -> System.out.println(item));
// emits 4, 5, 6 and then the IllegalArgumentException (in this specific
// example, this throws an `OnErrorNotImplementedException`).zip
Available in: Flowable, Observable, Maybe, Single, Completable
ReactiveX documentation: http://reactivex.io/documentation/operators/zip.html
Combines sets of items emitted by two or more Observables together via a specified function and emit items based on the results of this function.
zip Example
Observable<String> firstNames = Observable.just("James", "Jean-Luc", "Benjamin");
Observable<String> lastNames = Observable.just("Kirk", "Picard", "Sisko");
firstNames.zipWith(lastNames, (first, last) -> first + " " + last)
.subscribe(item -> System.out.println(item));
// prints James Kirk, Jean-Luc Picard, Benjamin SiskocombineLatest
Available in: Flowable, Observable, Maybe, Single, Completable
ReactiveX documentation: http://reactivex.io/documentation/operators/combinelatest.html
When an item is emitted by either of two Observables, combine the latest item emitted by each Observable via a specified function and emit items based on the results of this function.
combineLatest Example
Observable<Long> newsRefreshes = Observable.interval(100, TimeUnit.MILLISECONDS);
Observable<Long> weatherRefreshes = Observable.interval(50, TimeUnit.MILLISECONDS);
Observable.combineLatest(newsRefreshes, weatherRefreshes,
(newsRefreshTimes, weatherRefreshTimes) ->
"Refreshed news " + newsRefreshTimes + " times and weather " + weatherRefreshTimes)
.subscribe(item -> System.out.println(item));
// prints:
// Refreshed news 0 times and weather 0
// Refreshed news 0 times and weather 1
// Refreshed news 0 times and weather 2
// Refreshed news 1 times and weather 2
// Refreshed news 1 times and weather 3
// Refreshed news 1 times and weather 4
// Refreshed news 2 times and weather 4
// Refreshed news 2 times and weather 5
// ...switchOnNext
Available in: Flowable, Observable, Maybe, Single, Completable
ReactiveX documentation: http://reactivex.io/documentation/operators/switch.html
Convert an Observable that emits Observables into a single Observable that emits the items emitted by the most-recently emitted of those Observables.
switchOnNext Example
Observable<Observable<String>> timeIntervals =
Observable.interval(1, TimeUnit.SECONDS)
.map(ticks -> Observable.interval(100, TimeUnit.MILLISECONDS)
.map(innerInterval -> "outer: " + ticks + " - inner: " + innerInterval));
Observable.switchOnNext(timeIntervals)
.subscribe(item -> System.out.println(item));
// prints:
// outer: 0 - inner: 0
// outer: 0 - inner: 1
// outer: 0 - inner: 2
// outer: 0 - inner: 3
// outer: 0 - inner: 4
// outer: 0 - inner: 5
// outer: 0 - inner: 6
// outer: 0 - inner: 7
// outer: 0 - inner: 8
// outer: 1 - inner: 0
// outer: 1 - inner: 1
// outer: 1 - inner: 2
// outer: 1 - inner: 3
// ...joins
-
join( )andgroupJoin( )— combine the items emitted by two Observables whenever one item from one Observable falls within a window of duration specified by an item emitted by the other Observable
rxjava-joins
- (
rxjava-joins)and( ),then( ), andwhen( )— combine sets of items emitted by two or more Observables by means ofPatternandPlanintermediaries
(
rxjava-joins) — indicates that this operator is currently part of the optionalrxjava-joinspackage underrxjava-contriband is not included with the standard RxJava set of operators