Filtering Observables
This page shows operators you can use to filter and select items emitted by reactive sources, such as Observables.
debouncedistinctdistinctUntilChangedelementAtelementAtOrErrorfilterfirstfirstElementfirstOrErrorignoreElementignoreElementslastlastElementlastOrErrorofTypesampleskipskipLasttaketakeLastthrottleFirstthrottleLastthrottleLatestthrottleWithTimeouttimeout
Available in: 
Flowable, Observable, 
Maybe, Single, Completable
ReactiveX documentation: http://reactivex.io/documentation/operators/debounce.html
Drops items emitted by a reactive source that are followed by newer items before the given timeout value expires. The timer resets on each emission.
This operator keeps track of the most recent emitted item, and emits this item only when enough time has passed without the source emitting any other items.
// Diagram:
// -A--------------B----C-D-------------------E-|---->
// a---------1s
// b---------1s
// c---------1s
// d---------1s
// e-|---->
// -----------A---------------------D-----------E-|-->
Observable<String> source = Observable.create(emitter -> {
emitter.onNext("A");
Thread.sleep(1_500);
emitter.onNext("B");
Thread.sleep(500);
emitter.onNext("C");
Thread.sleep(250);
emitter.onNext("D");
Thread.sleep(2_000);
emitter.onNext("E");
emitter.onComplete();
});
source.subscribeOn(Schedulers.io())
.debounce(1, TimeUnit.SECONDS)
.blockingSubscribe(
item -> System.out.println("onNext: " + item),
Throwable::printStackTrace,
() -> System.out.println("onComplete"));
// prints:
// onNext: A
// onNext: D
// onNext: E
// onComplete
Available in: Flowable, Observable, Maybe, Single, Completable
ReactiveX documentation: http://reactivex.io/documentation/operators/distinct.html
Filters a reactive source by only emitting items that are distinct by comparison from previous items. A io.reactivex.functions.Function can be specified that projects each item emitted by the source into a new value that will be used for comparison with previous projected values.
Observable.just(2, 3, 4, 4, 2, 1)
.distinct()
.subscribe(System.out::println);
// prints:
// 2
// 3
// 4
// 1
Available in: Flowable, Observable, Maybe, Single, Completable
ReactiveX documentation: http://reactivex.io/documentation/operators/distinct.html
Filters a reactive source by only emitting items that are distinct by comparison from their immediate predecessors. A io.reactivex.functions.Function can be specified that projects each item emitted by the source into a new value that will be used for comparison with previous projected values. Alternatively, a io.reactivex.functions.BiPredicate can be specified that is used as the comparator function to compare immediate predecessors with each other.
Observable.just(1, 1, 2, 1, 2, 3, 3, 4)
.distinctUntilChanged()
.subscribe(System.out::println);
// prints:
// 1
// 2
// 1
// 2
// 3
// 4
Available in: Flowable, Observable, Maybe, Single, Completable
ReactiveX documentation: http://reactivex.io/documentation/operators/elementat.html
Emits the single item at the specified zero-based index in a sequence of emissions from a reactive source. A default item can be specified that will be emitted if the specified index is not within the sequence.
Observable<Long> source = Observable.<Long, Long>generate(() -> 1L, (state, emitter) -> {
emitter.onNext(state);
return state + 1L;
}).scan((product, x) -> product * x);
Maybe<Long> element = source.elementAt(5);
element.subscribe(System.out::println);
// prints 720
Available in: Flowable, Observable, Maybe, Single, Completable
ReactiveX documentation: http://reactivex.io/documentation/operators/elementat.html
Emits the single item at the specified zero-based index in a sequence of emissions from a reactive source, or signals a java.util.NoSuchElementException if the specified index is not within the sequence.
Observable<String> source = Observable.just("Kirk", "Spock", "Chekov", "Sulu");
Single<String> element = source.elementAtOrError(4);
element.subscribe(
name -> System.out.println("onSuccess will not be printed!"),
error -> System.out.println("onError: " + error));
// prints:
// onError: java.util.NoSuchElementException
Available in: Flowable, Observable, Maybe, Single, Completable
ReactiveX documentation: http://reactivex.io/documentation/operators/filter.html
Filters items emitted by a reactive source by only emitting those that satisfy a specified predicate.
Observable.just(1, 2, 3, 4, 5, 6)
.filter(x -> x % 2 == 0)
.subscribe(System.out::println);
// prints:
// 2
// 4
// 6
Available in: Flowable, Observable, Maybe, Single, Completable
ReactiveX documentation: http://reactivex.io/documentation/operators/first.html
Emits only the first item emitted by a reactive source, or emits the given default item if the source completes without emitting an item. This differs from firstElement in that this operator returns a Single whereas firstElement returns a Maybe.
Observable<String> source = Observable.just("A", "B", "C");
Single<String> firstOrDefault = source.first("D");
firstOrDefault.subscribe(System.out::println);
// prints A
Available in: Flowable, Observable, Maybe, Single, Completable
ReactiveX documentation: http://reactivex.io/documentation/operators/first.html
Emits only the first item emitted by a reactive source, or just completes if the source completes without emitting an item. This differs from first in that this operator returns a Maybe whereas first returns a Single.
Observable<String> source = Observable.just("A", "B", "C");
Maybe<String> first = source.firstElement();
first.subscribe(System.out::println);
// prints A
Available in: Flowable, Observable, Maybe, Single, Completable
ReactiveX documentation: http://reactivex.io/documentation/operators/first.html
Emits only the first item emitted by a reactive source, or signals a java.util.NoSuchElementException if the source completes without emitting an item.
Observable<String> emptySource = Observable.empty();
Single<String> firstOrError = emptySource.firstOrError();
firstOrError.subscribe(
element -> System.out.println("onSuccess will not be printed!"),
error -> System.out.println("onError: " + error));
// prints:
// onError: java.util.NoSuchElementException
Available in: Flowable, Observable, Maybe, Single, Completable
ReactiveX documentation: http://reactivex.io/documentation/operators/ignoreelements.html
Ignores the single item emitted by a Single or Maybe source, and returns a Completable that signals only the error or completion event from the the source.
Single<Long> source = Single.timer(1, TimeUnit.SECONDS);
Completable completable = source.ignoreElement();
completable.doOnComplete(() -> System.out.println("Done!"))
.blockingAwait();
// prints (after 1 second):
// Done!
Available in: Flowable, Observable, Maybe, Single, Completable
ReactiveX documentation: http://reactivex.io/documentation/operators/ignoreelements.html
Ignores all items from the Observable or Flowable source, and returns a Completable that signals only the error or completion event from the source.
Observable<Long> source = Observable.intervalRange(1, 5, 1, 1, TimeUnit.SECONDS);
Completable completable = source.ignoreElements();
completable.doOnComplete(() -> System.out.println("Done!"))
.blockingAwait();
// prints (after 5 seconds):
// Done!
Available in: Flowable, Observable, Maybe, Single, Completable
ReactiveX documentation: http://reactivex.io/documentation/operators/last.html
Emits only the last item emitted by a reactive source, or emits the given default item if the source completes without emitting an item. This differs from lastElement in that this operator returns a Single whereas lastElement returns a Maybe.
Observable<String> source = Observable.just("A", "B", "C");
Single<String> lastOrDefault = source.last("D");
lastOrDefault.subscribe(System.out::println);
// prints C
Available in: Flowable, Observable, Maybe, Single, Completable
ReactiveX documentation: http://reactivex.io/documentation/operators/last.html
Emits only the last item emitted by a reactive source, or just completes if the source completes without emitting an item. This differs from last in that this operator returns a Maybe whereas last returns a Single.
Observable<String> source = Observable.just("A", "B", "C");
Maybe<String> last = source.lastElement();
last.subscribe(System.out::println);
// prints C
Available in: Flowable, Observable, Maybe, Single, Completable
ReactiveX documentation: http://reactivex.io/documentation/operators/last.html
Emits only the last item emitted by a reactive source, or signals a java.util.NoSuchElementException if the source completes without emitting an item.
Observable<String> emptySource = Observable.empty();
Single<String> lastOrError = emptySource.lastOrError();
lastOrError.subscribe(
element -> System.out.println("onSuccess will not be printed!"),
error -> System.out.println("onError: " + error));
// prints:
// onError: java.util.NoSuchElementException
Available in: Flowable, Observable, Maybe, Single, Completable
ReactiveX documentation: http://reactivex.io/documentation/operators/filter.html
Filters items emitted by a reactive source by only emitting those of the specified type.
Observable<Number> numbers = Observable.just(1, 4.0, 3, 2.71, 2f, 7);
Observable<Integer> integers = numbers.ofType(Integer.class);
integers.subscribe((Integer x) -> System.out.println(x));
// prints:
// 1
// 3
// 7
Available in: Flowable, Observable, Maybe, Single, Completable
ReactiveX documentation: http://reactivex.io/documentation/operators/sample.html
Filters items emitted by a reactive source by only emitting the most recently emitted item within periodic time intervals.
// Diagram:
// -A----B-C-------D-----E-|-->
// -0s-----c--1s---d----2s-|-->
// -----------C---------D--|-->
Observable<String> source = Observable.create(emitter -> {
emitter.onNext("A");
Thread.sleep(500);
emitter.onNext("B");
Thread.sleep(200);
emitter.onNext("C");
Thread.sleep(800);
emitter.onNext("D");
Thread.sleep(600);
emitter.onNext("E");
emitter.onComplete();
});
source.subscribeOn(Schedulers.io())
.sample(1, TimeUnit.SECONDS)
.blockingSubscribe(
item -> System.out.println("onNext: " + item),
Throwable::printStackTrace,
() -> System.out.println("onComplete"));
// prints:
// onNext: C
// onNext: D
// onComplete
Available in: Flowable, Observable, Maybe, Single, Completable
ReactiveX documentation: http://reactivex.io/documentation/operators/skip.html
Drops the first n items emitted by a reactive source, and emits the remaining items.
Observable<Integer> source = Observable.just(1, 2, 3, 4, 5, 6, 7, 8, 9, 10);
source.skip(4)
.subscribe(System.out::println);
// prints:
// 5
// 6
// 7
// 8
// 9
// 10
Available in: Flowable, Observable, Maybe, Single, Completable
ReactiveX documentation: http://reactivex.io/documentation/operators/skiplast.html
Drops the last n items emitted by a reactive source, and emits the remaining items.
Observable<Integer> source = Observable.just(1, 2, 3, 4, 5, 6, 7, 8, 9, 10);
source.skipLast(4)
.subscribe(System.out::println);
// prints:
// 1
// 2
// 3
// 4
// 5
// 6
Available in: Flowable, Observable, Maybe, Single, Completable
ReactiveX documentation: http://reactivex.io/documentation/operators/take.html
Emits only the first n items emitted by a reactive source.
Observable<Integer> source = Observable.just(1, 2, 3, 4, 5, 6, 7, 8, 9, 10);
source.take(4)
.subscribe(System.out::println);
// prints:
// 1
// 2
// 3
// 4
Available in: Flowable, Observable, Maybe, Single, Completable
ReactiveX documentation: http://reactivex.io/documentation/operators/takelast.html
Emits only the last n items emitted by a reactive source.
Observable<Integer> source = Observable.just(1, 2, 3, 4, 5, 6, 7, 8, 9, 10);
source.takeLast(4)
.subscribe(System.out::println);
// prints:
// 7
// 8
// 9
// 10
Available in: Flowable, Observable, Maybe, Single, Completable
ReactiveX documentation: http://reactivex.io/documentation/operators/sample.html
Emits only the first item emitted by a reactive source during sequential time windows of a specified duration.
// Diagram:
// -A----B-C-------D-----E-|-->
// a---------1s
// d-------|-->
// -A--------------D-------|-->
Observable<String> source = Observable.create(emitter -> {
emitter.onNext("A");
Thread.sleep(500);
emitter.onNext("B");
Thread.sleep(200);
emitter.onNext("C");
Thread.sleep(800);
emitter.onNext("D");
Thread.sleep(600);
emitter.onNext("E");
emitter.onComplete();
});
source.subscribeOn(Schedulers.io())
.throttleFirst(1, TimeUnit.SECONDS)
.blockingSubscribe(
item -> System.out.println("onNext: " + item),
Throwable::printStackTrace,
() -> System.out.println("onComplete"));
// prints:
// onNext: A
// onNext: D
// onComplete
Available in: Flowable, Observable, Maybe, Single, Completable
ReactiveX documentation: http://reactivex.io/documentation/operators/sample.html
Emits only the last item emitted by a reactive source during sequential time windows of a specified duration.
// Diagram:
// -A----B-C-------D-----E-|-->
// -0s-----c--1s---d----2s-|-->
// -----------C---------D--|-->
Observable<String> source = Observable.create(emitter -> {
emitter.onNext("A");
Thread.sleep(500);
emitter.onNext("B");
Thread.sleep(200);
emitter.onNext("C");
Thread.sleep(800);
emitter.onNext("D");
Thread.sleep(600);
emitter.onNext("E");
emitter.onComplete();
});
source.subscribeOn(Schedulers.io())
.throttleLast(1, TimeUnit.SECONDS)
.blockingSubscribe(
item -> System.out.println("onNext: " + item),
Throwable::printStackTrace,
() -> System.out.println("onComplete"));
// prints:
// onNext: C
// onNext: D
// onComplete
Available in: Flowable, Observable, Maybe, Single, Completable
ReactiveX documentation: http://reactivex.io/documentation/operators/sample.html
Emits the next item emitted by a reactive source, then periodically emits the latest item (if any) when the specified timeout elapses between them.
// Diagram:
// -A----B-C-------D-----E-|-->
// -a------c--1s
// -----d----1s
// -e-|-->
// -A---------C---------D--|-->
Observable<String> source = Observable.create(emitter -> {
emitter.onNext("A");
Thread.sleep(500);
emitter.onNext("B");
Thread.sleep(200);
emitter.onNext("C");
Thread.sleep(800);
emitter.onNext("D");
Thread.sleep(600);
emitter.onNext("E");
emitter.onComplete();
});
source.subscribeOn(Schedulers.io())
.throttleLatest(1, TimeUnit.SECONDS)
.blockingSubscribe(
item -> System.out.println("onNext: " + item),
Throwable::printStackTrace,
() -> System.out.println("onComplete"));
// prints:
// onNext: A
// onNext: C
// onNext: D
// onComplete
Available in: Flowable, Observable, Maybe, Single, Completable
ReactiveX documentation: http://reactivex.io/documentation/operators/debounce.html
Alias to debounce
Drops items emitted by a reactive source that are followed by newer items before the given timeout value expires. The timer resets on each emission.
// Diagram:
// -A--------------B----C-D-------------------E-|---->
// a---------1s
// b---------1s
// c---------1s
// d---------1s
// e-|---->
// -----------A---------------------D-----------E-|-->
Observable<String> source = Observable.create(emitter -> {
emitter.onNext("A");
Thread.sleep(1_500);
emitter.onNext("B");
Thread.sleep(500);
emitter.onNext("C");
Thread.sleep(250);
emitter.onNext("D");
Thread.sleep(2_000);
emitter.onNext("E");
emitter.onComplete();
});
source.subscribeOn(Schedulers.io())
.throttleWithTimeout(1, TimeUnit.SECONDS)
.blockingSubscribe(
item -> System.out.println("onNext: " + item),
Throwable::printStackTrace,
() -> System.out.println("onComplete"));
// prints:
// onNext: A
// onNext: D
// onNext: E
// onComplete
Available in: Flowable, Observable, Maybe, Single, Completable
ReactiveX documentation: http://reactivex.io/documentation/operators/timeout.html
Emits the items from the Observable or Flowable source, but terminates with a java.util.concurrent.TimeoutException if the next item is not emitted within the specified timeout duration starting from the previous item. For Maybe, Single and Completable the specified timeout duration specifies the maximum time to wait for a success or completion event to arrive. If the Maybe, Single or Completable does not complete within the given time a java.util.concurrent.TimeoutException will be emitted.
// Diagram:
// -A-------B---C-----------D-|-->
// a---------1s
// b---------1s
// c---------1s
// -A-------B---C---------X------>
Observable<String> source = Observable.create(emitter -> {
emitter.onNext("A");
Thread.sleep(800);
emitter.onNext("B");
Thread.sleep(400);
emitter.onNext("C");
Thread.sleep(1200);
emitter.onNext("D");
emitter.onComplete();
});
source.timeout(1, TimeUnit.SECONDS)
.subscribe(
item -> System.out.println("onNext: " + item),
error -> System.out.println("onError: " + error),
() -> System.out.println("onComplete will not be printed!"));
// prints:
// onNext: A
// onNext: B
// onNext: C
// onError: java.util.concurrent.TimeoutException: The source did not signal an event for 1 seconds and has been terminated.