src/main/java/io/reactivex/Completable.java
@CheckReturnValue
    @NonNull
    @SchedulerSupport(SchedulerSupport.NONE)
    public static Completable fromCallable(final Callable<?> callable) {
        ObjectHelper.requireNonNull(callable, "callable is null");
        return RxJavaPlugins.onAssembly(new CompletableFromCallable(callable));
    }
@CheckReturnValue
    @NonNull
    @SchedulerSupport(SchedulerSupport.NONE)
    public static Completable fromCallable(final Callable<?> callable) {
        ObjectHelper.requireNonNull(callable, "callable is null");
        return RxJavaPlugins.onAssembly(new CompletableFromCallable(callable));
    }
Returns a Completable which when subscribed, executes the callable function, ignores its
normal result and emits onError or onComplete only.
<p>
<img width="640" height="286" src="https://raw.github.com/wiki/ReactiveX/RxJava/images/rx-operators/Completable.fromCallable.png" alt="">
<dl>
<dt><b>Scheduler:</b></dt>
<dd>{@code fromCallable} does not operate by default on a particular {@link Scheduler}.</dd>
<dt><b>Error handling:</b></dt>
<dd> If the {@link Callable} throws an exception, the respective {@link Throwable} is
delivered to the downstream via {@link CompletableObserver#onError(Throwable)},
except when the downstream has disposed this {@code Completable} source.
In this latter case, the {@code Throwable} is delivered to the global error handler via
{@link RxJavaPlugins#onError(Throwable)} as an {@link io.reactivex.exceptions.UndeliverableException UndeliverableException}.
</dd>
</dl>
@param callable the callable instance to execute for each subscriber
@return the new Completable instance