src/main/java/io/reactivex/Observable.java
@CheckReturnValue
    @SchedulerSupport(SchedulerSupport.NONE)
    public final Maybe<T> singleElement() {
        return RxJavaPlugins.onAssembly(new ObservableSingleMaybe<T>(this));
    }
@CheckReturnValue
    @SchedulerSupport(SchedulerSupport.NONE)
    public final Maybe<T> singleElement() {
        return RxJavaPlugins.onAssembly(new ObservableSingleMaybe<T>(this));
    }
Returns a Maybe that completes if this Observable is empty or emits the single item emitted by this Observable,
or signals an {@code IllegalArgumentException} if this Observable emits more than one item.
<p>
<img width="640" height="217" src="https://raw.githubusercontent.com/wiki/ReactiveX/RxJava/images/rx-operators/singleElement.o.png" alt="">
<dl>
<dt><b>Scheduler:</b></dt>
<dd>{@code singleElement} does not operate by default on a particular {@link Scheduler}.</dd>
</dl>

@return a {@link Maybe} that emits the single item emitted by the source ObservableSource
@see <a href="http://reactivex.io/documentation/operators/first.html">ReactiveX operators documentation: First</a>