Learning RxJava

Observable.fromCallable()

If you need to perform a calculation or action and then emit it, you can use Observable.just() (or Single.just() or Maybe.just(), which we will learn about later). But sometimes, we want to do this in a lazy or deferred manner. Also, if that procedure throws an error, we want it to be emitted up the Observable chain through onError() rather than throw the error at that location in traditional Java fashion. For instance, if you try to wrap Observable.just() around an expression that divides  1 by 0, the exception will be thrown, not emitted up to Observer:

    import io.reactivex.Observable;    public class Launcher {      public static void main(String[] args) {        Observable.just(1 / 0)        .subscribe(i -> System.out.println("RECEIVED: ...