src/main/java/io/reactivex/Completable.java
@CheckReturnValue
    @NonNull
    @SchedulerSupport(SchedulerSupport.NONE)
    public final <T> Observable<T> andThen(ObservableSource<T> next) {
        ObjectHelper.requireNonNull(next, "next is null");
        return RxJavaPlugins.onAssembly(new CompletableAndThenObservable<T>(this, next));
    }
@CheckReturnValue
    @NonNull
    @SchedulerSupport(SchedulerSupport.NONE)
    public final <T> Observable<T> andThen(ObservableSource<T> next) {
        ObjectHelper.requireNonNull(next, "next is null");
        return RxJavaPlugins.onAssembly(new CompletableAndThenObservable<T>(this, next));
    }
Returns an Observable which will subscribe to this Completable and once that is completed then
will subscribe to the {@code next} ObservableSource. An error event from this Completable will be
propagated to the downstream subscriber and will result in skipping the subscription of the
Observable.
<p>
<img width="640" height="278" src="https://raw.github.com/wiki/ReactiveX/RxJava/images/rx-operators/Completable.andThen.o.png" alt="">
<dl>
<dt><b>Scheduler:</b></dt>
<dd>{@code andThen} does not operate by default on a particular {@link Scheduler}.</dd>
</dl>
@param <T> the value type of the next ObservableSource
@param next the Observable to subscribe after this Completable is completed, not null
@return Observable that composes this Completable and next
@throws NullPointerException if next is null