Creating Observables

This page shows methods that create reactive sources, such as Observables.

Outline

• create
• defer
• empty
• error
• from
• generate
• interval
• just
• never
• range
• timer

just

Available in: Flowable, Observable, Maybe, Single, Completable

ReactiveX documentation: http://reactivex.io/documentation/operators/just.html

Constructs a reactive type by taking a pre-existing object and emitting that specific object to the downstream consumer upon subscription.

just example:

String greeting = "Hello world!";

Observable<String> observable = Observable.just(greeting);

observable.subscribe(item -> System.out.println(item));

There exist overloads with 2 to 9 arguments for convenience, which objects (with the same common type) will be emitted in the order they are specified.

Observable<Object> observable = Observable.just("1", "A", "3.2", "def");

  observable.subscribe(item -> System.out.print(item), error -> error.printStackTrace(),
                () -> System.out.println());

From

Constructs a sequence from a pre-existing source or generator type.

Note: These static methods use the postfix naming convention (i.e., the argument type is repeated in the method name) to avoid overload resolution ambiguities.

ReactiveX documentation: http://reactivex.io/documentation/operators/from.html

fromIterable

Available in: Flowable, Observable, Maybe, Single, Completable

Signals the items from a java.lang.Iterable source (such as Lists, Sets or Collections or custom Iterables) and then completes the sequence.

fromIterable example:

List<Integer> list = new ArrayList<>(Arrays.asList(1, 2, 3, 4, 5, 6, 7, 8));

Observable<Integer> observable = Observable.fromIterable(list);

observable.subscribe(item -> System.out.println(item), error -> error.printStackTrace(), 
     () -> System.out.println("Done"));

fromArray

Available in: Flowable, Observable, Maybe, Single, Completable

Signals the elements of the given array and then completes the sequence.

fromArray example:

Integer[] array = new Integer[10];
for (int i = 0; i < array.length; i++) {
    array[i] = i;
}

Observable<Integer> observable = Observable.fromArray(array);

observable.subscribe(item -> System.out.println(item), error -> error.printStackTrace(), 
     () -> System.out.println("Done"));

Note: RxJava does not support primitive arrays, only (generic) reference arrays.

fromCallable

Available in: Flowable, Observable, Maybe, Single, Completable

When a consumer subscribes, the given java.util.concurrent.Callable is invoked and its returned value (or thrown exception) is relayed to that consumer.

fromCallable example:

Callable<String> callable = () -> {
    System.out.println("Hello World!");
    return "Hello World!");
}

Observable<String> observable = Observable.fromCallable(callable);

observable.subscribe(item -> System.out.println(item), error -> error.printStackTrace(), 
    () -> System.out.println("Done"));

Remark: In Completable, the actual returned value is ignored and the Completable simply completes.

fromAction

Available in: Flowable, Observable, Maybe, Single, Completable

When a consumer subscribes, the given io.reactivex.function.Action is invoked and the consumer completes or receives the exception the Action threw.

fromAction example:

Action action = () -> System.out.println("Hello World!");

Completable completable = Completable.fromAction(action);

completable.subscribe(() -> System.out.println("Done"), error -> error.printStackTrace());

Note: the difference between fromAction and fromRunnable is that the Action interface allows throwing a checked exception while the java.lang.Runnable does not.

fromRunnable

Available in: Flowable, Observable, Maybe, Single, Completable

When a consumer subscribes, the given io.reactivex.function.Action is invoked and the consumer completes or receives the exception the Action threw.

fromRunnable example:

Runnable runnable = () -> System.out.println("Hello World!");

Completable completable = Completable.fromRunnable(runnable);

completable.subscribe(() -> System.out.println("Done"), error -> error.printStackTrace());

Note: the difference between fromAction and fromRunnable is that the Action interface allows throwing a checked exception while the java.lang.Runnable does not.

fromFuture

Available in: Flowable, Observable, Maybe, Single, Completable

Given a pre-existing, already running or already completed java.util.concurrent.Future, wait for the Future to complete normally or with an exception in a blocking fashion and relay the produced value or exception to the consumers.

fromFuture example:

ScheduledExecutorService executor = Executors.newSingleThreadScheduledExecutor();

Future<String> future = executor.schedule(() -> "Hello world!", 1, TimeUnit.SECONDS);

Observable<String> observable = Observable.fromFuture(future);

observable.subscribe(
    item -> System.out.println(item), 
    error -> error.printStackTrace(),
    () -> System.out.println("Done"));

executor.shutdown();

from{reactive type}

Wraps or converts another reactive type to the target reactive type.

The following combinations are available in the various reactive types with the following signature pattern: targetType.from{sourceType}()

Available in:

targetType \ sourceType	Publisher	Observable	Maybe	Single	Completable
Flowable
Observable
Maybe
Single
Completable

*Note: not all possible conversion is implemented via the from{reactive type} method families. Check out the to{reactive type} method families for further conversion possibilities.

from{reactive type} example:

Flux<Integer> reactorFlux = Flux.fromCompletionStage(CompletableFuture.<Integer>completedFuture(1));

Observable<Integer> observable = Observable.fromPublisher(reactorFlux);

observable.subscribe(
    item -> System.out.println(item), 
    error -> error.printStackTrace(),
    () -> System.out.println("Done"));

generate

Available in: Flowable, Observable, Maybe, Single, Completable

ReactiveX documentation: http://reactivex.io/documentation/operators/create.html

Creates a cold, synchronous and stateful generator of values.

generate example:

int startValue = 1;
int incrementValue = 1;
Flowable<Integer> flowable = Flowable.generate(() -> startValue, (s, emitter) -> {
	int nextValue = s + incrementValue;
	emitter.onNext(nextValue);
	return nextValue;
});
flowable.subscribe(value -> System.out.println(value));

create

Available in: Flowable, Observable, Maybe, Single, Completable

ReactiveX documentation: http://reactivex.io/documentation/operators/create.html

Construct a safe reactive type instance which when subscribed to by a consumer, runs an user-provided function and provides a type-specific Emitter for this function to generate the signal(s) the designated business logic requires. This method allows bridging the non-reactive, usually listener/callback-style world, with the reactive world.

create example:

ScheduledExecutorService executor = Executors.newSingleThreadedScheduledExecutor();

ObservableOnSubscribe<String> handler = emitter -> {

     Future<Object> future = executor.schedule(() -> {
          emitter.onNext("Hello");
          emitter.onNext("World");
          emitter.onComplete();
          return null;
     }, 1, TimeUnit.SECONDS);

     emitter.setCancellable(() -> future.cancel(false));
};

Observable<String> observable = Observable.create(handler);

observable.subscribe(item -> System.out.println(item), error -> error.printStackTrace(),
     () -> System.out.println("Done"));

Thread.sleep(2000);
executor.shutdown();

Note: Flowable.create() must also specify the backpressure behavior to be applied when the user-provided function generates more items than the downstream consumer has requested.

defer

Available in: Flowable, Observable, Maybe, Single, Completable

ReactiveX documentation: http://reactivex.io/documentation/operators/defer.html

Calls an user-provided java.util.concurrent.Callable when a consumer subscribes to the reactive type so that the Callable can generate the actual reactive instance to relay signals from towards the consumer. defer allows:

• associating a per-consumer state with such generated reactive instances,
• allows executing side-effects before an actual/generated reactive instance gets subscribed to,
• turn hot sources (i.e., Subjects and Processors) into cold sources by basically making those hot sources not exist until a consumer subscribes.

defer example:

Observable<Long> observable = Observable.defer(() -> {
    long time = System.currentTimeMillis();
    return Observable.just(time);
});

observable.subscribe(time -> System.out.println(time));

Thread.sleep(1000);

observable.subscribe(time -> System.out.println(time));

range

Available in: Flowable, Observable, Maybe, Single, Completable

ReactiveX documentation: http://reactivex.io/documentation/operators/range.html

Generates a sequence of values to each individual consumer. The range() method generates Integers, the rangeLong() generates Longs.

range example:

String greeting = "Hello World!";

Observable<Integer> indexes = Observable.range(0, greeting.length());

Observable<Character> characters = indexes
    .map(index -> greeting.charAt(index));

characters.subscribe(character -> System.out.print(character), error -> error.printStackTrace(),
        () -> System.out.println());

interval

Available in: Flowable, Observable, Maybe, Single, Completable

ReactiveX documentation: http://reactivex.io/documentation/operators/interval.html

Periodically generates an infinite, ever increasing numbers (of type Long). The intervalRange variant generates a limited amount of such numbers.

interval example:

Observable<Long> clock = Observable.interval(1, TimeUnit.SECONDS);

clock.subscribe(time -> {
    if (time % 2 == 0) {
        System.out.println("Tick");
    } else {
        System.out.println("Tock");
    }
});

timer

Available in: Flowable, Observable, Maybe, Single, Completable

ReactiveX documentation: http://reactivex.io/documentation/operators/timer.html

After the specified time, this reactive source signals a single 0L (then completes for Flowable and Observable).

timer example:

Observable<Long> eggTimer = Observable.timer(5, TimeUnit.MINUTES);

eggTimer.blockingSubscribe(v -> System.out.println("Egg is ready!"));

empty

Available in: Flowable, Observable, Maybe, Single, Completable

ReactiveX documentation: http://reactivex.io/documentation/operators/empty-never-throw.html

This type of source signals completion immediately upon subscription.

empty example:

Observable<String> empty = Observable.empty();

empty.subscribe(
    v -> System.out.println("This should never be printed!"), 
    error -> System.out.println("Or this!"),
    () -> System.out.println("Done will be printed."));

never

Available in: Flowable, Observable, Maybe, Single, Completable

ReactiveX documentation: http://reactivex.io/documentation/operators/empty-never-throw.html

This type of source does not signal any onNext, onSuccess, onError or onComplete. This type of reactive source is useful in testing or "disabling" certain sources in combinator operators.

never example:

Observable<String> never = Observable.never();

never.subscribe(
    v -> System.out.println("This should never be printed!"), 
    error -> System.out.println("Or this!"),
    () -> System.out.println("This neither!"));

error

Available in: Flowable, Observable, Maybe, Single, Completable

ReactiveX documentation: http://reactivex.io/documentation/operators/empty-never-throw.html

Signal an error, either pre-existing or generated via a java.util.concurrent.Callable, to the consumer.

error example:

Observable<String> error = Observable.error(new IOException());

error.subscribe(
    v -> System.out.println("This should never be printed!"), 
    e -> e.printStackTrace(),
    () -> System.out.println("This neither!"));

A typical use case is to conditionally map or suppress an exception in a chain utilizing onErrorResumeNext:

Observable<String> observable = Observable.fromCallable(() -> {
    if (Math.random() < 0.5) {
        throw new IOException();
    }
    throw new IllegalArgumentException();
});

Observable<String> result = observable.onErrorResumeNext(error -> {
    if (error instanceof IllegalArgumentException) {
        return Observable.empty();
    }
    return Observable.error(error);
});

for (int i = 0; i < 10; i++) {
    result.subscribe(
        v -> System.out.println("This should never be printed!"), 
        error -> error.printStackTrace(),
        () -> System.out.println("Done"));
}