src/main/java/io/reactivex/Flowable.java
@CheckReturnValue
    @BackpressureSupport(BackpressureKind.SPECIAL) // take may trigger UNBOUNDED_IN
    @SchedulerSupport(SchedulerSupport.NONE)
    public final Single<T> firstOrError() {
        return elementAtOrError(0);
    }
@CheckReturnValue
    @BackpressureSupport(BackpressureKind.SPECIAL) // take may trigger UNBOUNDED_IN
    @SchedulerSupport(SchedulerSupport.NONE)
    public final Single<T> firstOrError() {
        return elementAtOrError(0);
    }
Returns a Single that emits only the very first item emitted by this Flowable or
signals a {@link NoSuchElementException} if this Flowable is empty.
<p>
<img width="640" height="237" src="https://raw.github.com/wiki/ReactiveX/RxJava/images/rx-operators/firstOrError.png" alt="">
<dl>
<dt><b>Backpressure:</b></dt>
<dd>The operator honors backpressure from downstream and consumes the source {@code Publisher} in an
unbounded manner (i.e., without applying backpressure).</dd>
<dt><b>Scheduler:</b></dt>
<dd>{@code firstOrError} 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>