Following are the base classes to create observables.
Flowable − 0..N flows, Emits 0 or n items. Supports Reactive-Streams and back-pressure.
Observable − 0..N flows ,but no back-pressure.
Single − 1 item or error. Can be treated as a reactive version of method call.
Completable − No item emitted. Used as a signal for completion or error. Can be treated as a reactive version of Runnable.
MayBe − Either No item or 1 item emitted. Can be treated as a reactive version of Optional.
Following are the convenient methods to create observables in Observable class.
just(T item) − Returns an Observable that signals the given (constant reference) item and then completes.
fromIterable(Iterable source) − Converts an Iterable sequence into an ObservableSource that emits the items in the sequence.
fromArray(T... items) − Converts an Array into an ObservableSource that emits the items in the Array.
fromCallable(Callable supplier) − Returns an Observable that, when an observer subscribes to it, invokes a function you specify and then emits the value returned from that function.
fromFuture(Future future) − Converts a Future into an ObservableSource.
interval(long initialDelay, long period, TimeUnit unit) − Returns an Observable that emits a 0L after the initialDelay and ever increasing numbers after each period of time thereafter.