src/main/java/io/reactivex/Observable.java
@SuppressWarnings("unchecked")
    @CheckReturnValue
    @SchedulerSupport(SchedulerSupport.NONE)
    public final Observable<T> startWith(T item) {
        ObjectHelper.requireNonNull(item, "item is null");
        return concatArray(just(item), this);
    }
@SuppressWarnings("unchecked")
    @CheckReturnValue
    @SchedulerSupport(SchedulerSupport.NONE)
    public final Observable<T> startWith(T item) {
        ObjectHelper.requireNonNull(item, "item is null");
        return concatArray(just(item), this);
    }
Returns an Observable that emits a specified item 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.item.png" alt="">
<dl>
<dt><b>Scheduler:</b></dt>
<dd>{@code startWith} does not operate by default on a particular {@link Scheduler}.</dd>
</dl>

@param item
the item to emit first
@return an Observable that emits the specified item before it begins to emit items emitted by the source
ObservableSource
@see <a href="http://reactivex.io/documentation/operators/startwith.html">ReactiveX operators documentation: StartWith</a>