src/main/java/io/reactivex/Completable.java
@CheckReturnValue
    @NonNull
    @SchedulerSupport(SchedulerSupport.NONE)
    public final <T> Observable<T> startWith(Observable<T> other) {
        ObjectHelper.requireNonNull(other, "other is null");
        return other.concatWith(this.<T>toObservable());
    }
@CheckReturnValue
    @NonNull
    @SchedulerSupport(SchedulerSupport.NONE)
    public final <T> Observable<T> startWith(Observable<T> other) {
        ObjectHelper.requireNonNull(other, "other is null");
        return other.concatWith(this.<T>toObservable());
    }
Returns an Observable which first delivers the events
of the other Observable then runs this CompletableConsumable.
<p>
<img width="640" height="289" src="https://raw.github.com/wiki/ReactiveX/RxJava/images/rx-operators/Completable.startWith.o.png" alt="">
<dl>
<dt><b>Scheduler:</b></dt>
<dd>{@code startWith} does not operate by default on a particular {@link Scheduler}.</dd>
</dl>
@param <T> the value type
@param other the other Observable to run first
@return the new Observable instance
@throws NullPointerException if other is null