rx-java Observable Create an Observable

Example

There are several ways to create an Observable in RxJava. The most powerful way is to use the Observable.create method. But it's also the most complicated way. So you must avoid using it, as much as possible.

Emitting an exiting value

If you already have a value, you can use Observable.just to emit your value.

 Observable.just("Hello World").subscribe(System.out::println);

Emitting a value that should be computed

If you want to emit a value that is not already computed, or that can take long to be computed, you can use Observable.fromCallable to emit your next value.

Observable.fromCallable(() -> longComputation()).subscribe(System.out::println);

longComputation() will only be called when you subscribe to your Observable. This way, the computation will be lazy.

Alternative way to Emitting a value that should be computed

Observable.defer builds an Observable just like Observable.fromCallable but it is used when you need to return an Observable instead of a value. It is useful when you want to manage the errors in your call.

Observable.defer(() -> {
          try {
                return Observable.just(longComputation());
          } catch(SpecificException e) {
                return Observable.error(e);
          }).subscribe(System.out::println);