src/main/java/io/reactivex/Observable.java
@CheckReturnValue
    @SchedulerSupport(SchedulerSupport.NONE)
    public final Observable<T> serialize() {
        return RxJavaPlugins.onAssembly(new ObservableSerialized<T>(this));
    }
@CheckReturnValue
    @SchedulerSupport(SchedulerSupport.NONE)
    public final Observable<T> serialize() {
        return RxJavaPlugins.onAssembly(new ObservableSerialized<T>(this));
    }
Forces an ObservableSource's emissions and notifications to be serialized and for it to obey
<a href="http://reactivex.io/documentation/contract.html">the ObservableSource contract</a> in other ways.
<p>
It is possible for an ObservableSource to invoke its Observers' methods asynchronously, perhaps from
different threads. This could make such an ObservableSource poorly-behaved, in that it might try to invoke
{@code onComplete} or {@code onError} before one of its {@code onNext} invocations, or it might call
{@code onNext} from two different threads concurrently. You can force such an ObservableSource to be
well-behaved and sequential by applying the {@code serialize} method to it.
<p>
<img width="640" height="400" src="https://raw.github.com/wiki/ReactiveX/RxJava/images/rx-operators/synchronize.png" alt="">
<dl>
<dt><b>Scheduler:</b></dt>
<dd>{@code serialize} does not operate by default on a particular {@link Scheduler}.</dd>
</dl>

@return an {@link ObservableSource} that is guaranteed to be well-behaved and to make only serialized calls to
its observers
@see <a href="http://reactivex.io/documentation/operators/serialize.html">ReactiveX operators documentation: Serialize</a>