src/main/java/io/reactivex/Observable.java
@CheckReturnValue
    @SchedulerSupport(SchedulerSupport.NONE)
    public final Single<T> last(T defaultItem) {
        ObjectHelper.requireNonNull(defaultItem, "defaultItem is null");
        return RxJavaPlugins.onAssembly(new ObservableLastSingle<T>(this, defaultItem));
    }
@CheckReturnValue
    @SchedulerSupport(SchedulerSupport.NONE)
    public final Single<T> last(T defaultItem) {
        ObjectHelper.requireNonNull(defaultItem, "defaultItem is null");
        return RxJavaPlugins.onAssembly(new ObservableLastSingle<T>(this, defaultItem));
    }
Returns a Single that emits only the last item emitted by this Observable, or a default item
if this Observable completes without emitting any items.
<p>
<img width="640" height="305" src="https://raw.github.com/wiki/ReactiveX/RxJava/images/rx-operators/last.2.png" alt="">
<dl>
<dt><b>Scheduler:</b></dt>
<dd>{@code last} does not operate by default on a particular {@link Scheduler}.</dd>
</dl>

@param defaultItem
the default item to emit if the source ObservableSource is empty
@return a Single that emits only the last item emitted by the source ObservableSource, or a default item
if the source ObservableSource is empty
@see <a href="http://reactivex.io/documentation/operators/last.html">ReactiveX operators documentation: Last</a>