code: public static <R> Observable<R> fromCallable(Callable<? extends R> callable) {
          return fromCallable(callable, Schedulers.computation());
      }
docs: Return an Observable that calls the given Callable and emits its result or Exception when an Observer
  subscribes.
  <p>
  <img width="640" src="https://raw.github.com/wiki/ReactiveX/RxJava/images/rx-operators/fromCallable.png" alt="">
  <p>
  The Callable is called on the default thread pool for computation.
  
  @param <R> the return type
  @param callable the callable to call on each subscription
  @return an Observable that calls the given Callable and emits its result or Exception when an Observer
  subscribes
  @see #start(rx.functions.Func0)
  @see <a href="https://github.com/ReactiveX/RxJava/wiki/Async-Operators#wiki-fromcallable">RxJava Wiki: fromCallable()</a>