src/main/java/io/reactivex/Observable.java
@CheckReturnValue
    @SchedulerSupport(SchedulerSupport.NONE)
    public final Single<T> single(T defaultItem) {
        ObjectHelper.requireNonNull(defaultItem, "defaultItem is null");
        return RxJavaPlugins.onAssembly(new ObservableSingleSingle<T>(this, defaultItem));
    }
@CheckReturnValue
    @SchedulerSupport(SchedulerSupport.NONE)
    public final Single<T> single(T defaultItem) {
        ObjectHelper.requireNonNull(defaultItem, "defaultItem is null");
        return RxJavaPlugins.onAssembly(new ObservableSingleSingle<T>(this, defaultItem));
    }
Returns a Single that emits the single item emitted by this Observable, if this Observable
emits only a single item, or a default item if the source ObservableSource emits no items. If the source
ObservableSource emits more than one item, an {@code IllegalArgumentException} is signalled instead.
<p>
<img width="640" height="315" src="https://raw.github.com/wiki/ReactiveX/RxJava/images/rx-operators/single.2.png" alt="">
<dl>
<dt><b>Scheduler:</b></dt>
<dd>{@code single} does not operate by default on a particular {@link Scheduler}.</dd>
</dl>

@param defaultItem
a default value to emit if the source ObservableSource emits no item
@return the new Single instance
@see <a href="http://reactivex.io/documentation/operators/first.html">ReactiveX operators documentation: First</a>