src/main/java/io/reactivex/Flowable.java
@CheckReturnValue
    @BackpressureSupport(BackpressureKind.UNBOUNDED_IN)
    @SchedulerSupport(SchedulerSupport.NONE)
    public final Maybe<T> singleElement() {
        return RxJavaPlugins.onAssembly(new FlowableSingleMaybe<T>(this));
    }
@CheckReturnValue
    @BackpressureSupport(BackpressureKind.UNBOUNDED_IN)
    @SchedulerSupport(SchedulerSupport.NONE)
    public final Maybe<T> singleElement() {
        return RxJavaPlugins.onAssembly(new FlowableSingleMaybe<T>(this));
    }
@ CheckReturnValue @ BackpressureSupport ( BackpressureKind . UNBOUNDED_IN ) @ SchedulerSupport ( SchedulerSupport . NONE ) public final Maybe < T > singleElement ( ) { return RxJavaPlugins . onAssembly ( new FlowableSingleMaybe < T > ( this ) ) ; }
Returns a Maybe that completes if this Flowable is empty, signals one item if this Flowable
signals exactly one item or signals an {@code IllegalArgumentException} if this Flowable signals
more than one item.
<p>
<img width="640" height="315" src="https://raw.github.com/wiki/ReactiveX/RxJava/images/rx-operators/single.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 singleElement} does not operate by default on a particular {@link Scheduler}.</dd>
</dl>

@return a Maybe that emits the single item emitted by the source Publisher
@see <a href="http://reactivex.io/documentation/operators/first.html">ReactiveX operators documentation: First</a>
Returns a Maybe that completes if this Flowable is empty signals one item if this Flowable signals exactly one item or signals an { @code IllegalArgumentException } if this Flowable signals more than one item . <p > <img width = 640 height = 315 src = https : // raw . github . com / wiki / ReactiveX / RxJava / images / rx - operators / single . 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 singleElement } does not operate by default on a particular { @link Scheduler } . < / dd > < / dl >