src/main/java/io/reactivex/Observable.java
@CheckReturnValue
    @SchedulerSupport(SchedulerSupport.NONE)
    public final Observable<T> defaultIfEmpty(T defaultItem) {
        ObjectHelper.requireNonNull(defaultItem, "defaultItem is null");
        return switchIfEmpty(just(defaultItem));
    }
@CheckReturnValue
    @SchedulerSupport(SchedulerSupport.NONE)
    public final Observable<T> defaultIfEmpty(T defaultItem) {
        ObjectHelper.requireNonNull(defaultItem, "defaultItem is null");
        return switchIfEmpty(just(defaultItem));
    }
Returns an Observable that emits the items emitted by the source ObservableSource or a specified default item
if the source ObservableSource is empty.
<p>
<img width="640" height="305" src="https://raw.github.com/wiki/ReactiveX/RxJava/images/rx-operators/defaultIfEmpty.png" alt="">
<dl>
<dt><b>Scheduler:</b></dt>
<dd>{@code defaultIfEmpty} does not operate by default on a particular {@link Scheduler}.</dd>
</dl>

@param defaultItem
the item to emit if the source ObservableSource emits no items
@return an Observable that emits either the specified default item if the source ObservableSource emits no
items, or the items emitted by the source ObservableSource
@see <a href="http://reactivex.io/documentation/operators/defaultifempty.html">ReactiveX operators documentation: DefaultIfEmpty</a>