src/main/java/io/reactivex/Observable.java
@CheckReturnValue
    @SchedulerSupport(SchedulerSupport.NONE)
    public final T blockingFirst() {
        BlockingFirstObserver<T> observer = new BlockingFirstObserver<T>();
        subscribe(observer);
        T v = observer.blockingGet();
        if (v != null) {
            return v;
        }
        throw new NoSuchElementException();
    }
@CheckReturnValue
    @SchedulerSupport(SchedulerSupport.NONE)
    public final T blockingFirst() {
        BlockingFirstObserver<T> observer = new BlockingFirstObserver<T>();
        subscribe(observer);
        T v = observer.blockingGet();
        if (v != null) {
            return v;
        }
        throw new NoSuchElementException();
    }
Returns the first item emitted by this {@code Observable}, or throws
{@code NoSuchElementException} if it emits no items.
<p>
<img width="640" height="412" src="https://github.com/ReactiveX/RxJava/wiki/images/rx-operators/blockingFirst.o.png" alt="">
<dl>
<dt><b>Scheduler:</b></dt>
<dd>{@code blockingFirst} does not operate by default on a particular {@link Scheduler}.</dd>
</dl>

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