Combining Observables
This section explains operators you can use to combine multiple Observables.
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.
Observable<String> names = Observable.just("Spock", "McCoy");
names.startWith("Kirk").subscribe(item -> System.out.println(item));
// prints Kirk, Spock, McCoyCombines multiple Observables into one.
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.
Observable.just(1, 2, 3)
.mergeWith(Observable.just(4, 5, 6))
.subscribe(item -> System.out.println(item));
// prints 1, 2, 3, 4, 5, 6Available 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.
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`).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.
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 SiskoAvailable 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.
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
// ...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.
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
// ...-
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)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