src/main/java/io/reactivex/Flowable.java
@CheckReturnValue
    @BackpressureSupport(BackpressureKind.UNBOUNDED_IN)
    @SchedulerSupport(SchedulerSupport.NONE)
    public final T blockingFirst() {
        BlockingFirstSubscriber<T> s = new BlockingFirstSubscriber<T>();
        subscribe(s);
        T v = s.blockingGet();
        if (v != null) {
            return v;
        }
        throw new NoSuchElementException();
    }
@CheckReturnValue
    @BackpressureSupport(BackpressureKind.UNBOUNDED_IN)
    @SchedulerSupport(SchedulerSupport.NONE)
    public final T blockingFirst() {
        BlockingFirstSubscriber<T> s = new BlockingFirstSubscriber<T>();
        subscribe(s);
        T v = s.blockingGet();
        if (v != null) {
            return v;
        }
        throw new NoSuchElementException();
    }
Returns the first item emitted by this {@code Flowable}, or throws
{@code NoSuchElementException} if it emits no items.
<dl>
<dt><b>Backpressure:</b></dt>
<dd>The operator consumes the source {@code Flowable} in an unbounded manner
(i.e., no backpressure applied to it).</dd>
<dt><b>Scheduler:</b></dt>
<dd>{@code blockingFirst} does not operate by default on a particular {@link Scheduler}.</dd>
<dt><b>Error handling:</b></dt>
<dd>If the source signals an error, the operator wraps a checked {@link Exception}
into {@link RuntimeException} and throws that. Otherwise, {@code RuntimeException}s and
{@link Error}s are rethrown as they are.</dd>
</dl>

@return the first item emitted by this {@code Flowable}
@throws NoSuchElementException
if this {@code Flowable} emits no items
@see <a href="http://reactivex.io/documentation/operators/first.html">ReactiveX documentation: First</a>