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));
    }
@ 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>
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 >