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This page shows methods that create reactive sources, such as `Observable`s.
### Outline
- [`create`](#create)
- [`defer`](#defer)
- [`empty`](#empty)
- [`error`](#error)
- [`from`](#from)
- [`generate`](#generate)
- [`interval`](#interval)
- [`just`](#just)
- [`never`](#never)
- [`range`](#range)
- [`timer`](#timer)
## just
**Available in:**  `Flowable`,  `Observable`,  `Maybe`,  `Single`,  `Completable`
**ReactiveX documentation:** [http://reactivex.io/documentation/operators/just.html](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:
```java
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.
```java
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](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 `List`s, `Set`s or `Collection`s or custom `Iterable`s) and then completes the sequence.
#### fromIterable example:
```java
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:
```java
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:
```java
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:
```java
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:
```java
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:
```java
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:
```java
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](http://reactivex.io/documentation/operators/create.html)
Creates a cold, synchronous and stateful generator of values.
#### generate example:
```java
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](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:
```java
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](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., `Subject`s and `Processor`s) into cold sources by basically making those hot sources not exist until a consumer subscribes.
#### defer example:
```java
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](http://reactivex.io/documentation/operators/range.html)
Generates a sequence of values to each individual consumer. The `range()` method generates `Integer`s, the `rangeLong()` generates `Long`s.
#### range example:
```java
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](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:
```java
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](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:
```java
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](http://reactivex.io/documentation/operators/empty-never-throw.html)
This type of source signals completion immediately upon subscription.
#### empty example:
```java
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](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:
```java
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](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:
```java
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`:
```java
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"));
}
```
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