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);
    }
@ 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>
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 >