src/main/java/io/reactivex/Observable.java
@SuppressWarnings("unchecked")
    @CheckReturnValue
    @SchedulerSupport(SchedulerSupport.NONE)
    public final Observable<T> startWith(ObservableSource<? extends T> other) {
        ObjectHelper.requireNonNull(other, "other is null");
        return concatArray(other, this);
    }
@SuppressWarnings("unchecked")
    @CheckReturnValue
    @SchedulerSupport(SchedulerSupport.NONE)
    public final Observable<T> startWith(ObservableSource<? extends T> other) {
        ObjectHelper.requireNonNull(other, "other is null");
        return concatArray(other, this);
    }
Returns an Observable that emits the items in a specified {@link ObservableSource} before it begins to emit
items emitted by the source ObservableSource.
<p>
<img width="640" height="315" src="https://raw.github.com/wiki/ReactiveX/RxJava/images/rx-operators/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 other
an ObservableSource that contains the items you want the modified ObservableSource to emit first
@return an Observable that emits the items in the specified {@link ObservableSource} and then emits the items
emitted by the source ObservableSource
@see <a href="http://reactivex.io/documentation/operators/startwith.html">ReactiveX operators documentation: StartWith</a>