src/main/java/io/reactivex/Observable.java
@CheckReturnValue
    @SchedulerSupport(SchedulerSupport.NONE)
    public final Observable<T> switchIfEmpty(ObservableSource<? extends T> other) {
        ObjectHelper.requireNonNull(other, "other is null");
        return RxJavaPlugins.onAssembly(new ObservableSwitchIfEmpty<T>(this, other));
    }
@CheckReturnValue
    @SchedulerSupport(SchedulerSupport.NONE)
    public final Observable<T> switchIfEmpty(ObservableSource<? extends T> other) {
        ObjectHelper.requireNonNull(other, "other is null");
        return RxJavaPlugins.onAssembly(new ObservableSwitchIfEmpty<T>(this, other));
    }
Returns an Observable that emits the items emitted by the source ObservableSource or the items of an alternate
ObservableSource if the source ObservableSource is empty.
<p>
<img width="640" height="255" src="https://raw.github.com/wiki/ReactiveX/RxJava/images/rx-operators/switchifempty.png" alt="">
<dl>
<dt><b>Scheduler:</b></dt>
<dd>{@code switchIfEmpty} does not operate by default on a particular {@link Scheduler}.</dd>
</dl>

@param other
the alternate ObservableSource to subscribe to if the source does not emit any items
@return  an ObservableSource that emits the items emitted by the source ObservableSource or the items of an
alternate ObservableSource if the source ObservableSource is empty.
@since 1.1.0