src/main/java/io/reactivex/Observable.java
@CheckReturnValue
    @SchedulerSupport(SchedulerSupport.NONE)
    public final T blockingSingle() {
        T v = singleElement().blockingGet();
        if (v == null) {
            throw new NoSuchElementException();
        }
        return v;
    }
@CheckReturnValue
    @SchedulerSupport(SchedulerSupport.NONE)
    public final T blockingSingle() {
        T v = singleElement().blockingGet();
        if (v == null) {
            throw new NoSuchElementException();
        }
        return v;
    }
If this {@code Observable} completes after emitting a single item, return that item, otherwise
throw a {@code NoSuchElementException}.
<p>
<img width="640" height="315" src="https://github.com/ReactiveX/RxJava/wiki/images/rx-operators/blockingSingle.o.png" alt="">
<dl>
<dt><b>Scheduler:</b></dt>
<dd>{@code blockingSingle} 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 single item emitted by this {@code Observable}
@see <a href="http://reactivex.io/documentation/operators/first.html">ReactiveX documentation: First</a>