src/main/java/io/reactivex/Maybe.java
@CheckReturnValue
    @SchedulerSupport(SchedulerSupport.NONE)
    @SuppressWarnings("unchecked")
    public static <T> Maybe<T> empty() {
        return RxJavaPlugins.onAssembly((Maybe<T>)MaybeEmpty.INSTANCE);
    }
@CheckReturnValue
    @SchedulerSupport(SchedulerSupport.NONE)
    @SuppressWarnings("unchecked")
    public static <T> Maybe<T> empty() {
        return RxJavaPlugins.onAssembly((Maybe<T>)MaybeEmpty.INSTANCE);
    }
Returns a (singleton) Maybe instance that calls {@link MaybeObserver#onComplete onComplete}
immediately.
<p>
<img width="640" height="190" src="https://raw.github.com/wiki/ReactiveX/RxJava/images/rx-operators/empty.png" alt="">
<dl>
<dt><b>Scheduler:</b></dt>
<dd>{@code empty} does not operate by default on a particular {@link Scheduler}.</dd>
</dl>
@param <T> the value type
@return the new Maybe instance