src/main/java/io/reactivex/Observable.java
@CheckReturnValue
    @SchedulerSupport(SchedulerSupport.NONE)
    public final Single<T> singleOrError() {
        return RxJavaPlugins.onAssembly(new ObservableSingleSingle<T>(this, null));
    }
@CheckReturnValue
    @SchedulerSupport(SchedulerSupport.NONE)
    public final Single<T> singleOrError() {
        return RxJavaPlugins.onAssembly(new ObservableSingleSingle<T>(this, null));
    }
Returns a Single that emits the single item emitted by this Observable if this Observable
emits only a single item, otherwise
if this Observable completes without emitting any items or emits more than one item a
{@link NoSuchElementException} or {@code IllegalArgumentException} will be signalled respectively.
<p>
<img width="640" height="228" src="https://raw.github.com/wiki/ReactiveX/RxJava/images/rx-operators/singleOrError.png" alt="">
<dl>
<dt><b>Scheduler:</b></dt>
<dd>{@code singleOrError} does not operate by default on a particular {@link Scheduler}.</dd>
</dl>

@return the new Single instance
@see <a href="http://reactivex.io/documentation/operators/first.html">ReactiveX operators documentation: First</a>