src/main/java/io/reactivex/Completable.java
@CheckReturnValue
    @NonNull
    @SchedulerSupport(SchedulerSupport.NONE)
    public final <T> Single<T> andThen(SingleSource<T> next) {
        ObjectHelper.requireNonNull(next, "next is null");
        return RxJavaPlugins.onAssembly(new SingleDelayWithCompletable<T>(next, this));
    }
@CheckReturnValue
    @NonNull
    @SchedulerSupport(SchedulerSupport.NONE)
    public final <T> Single<T> andThen(SingleSource<T> next) {
        ObjectHelper.requireNonNull(next, "next is null");
        return RxJavaPlugins.onAssembly(new SingleDelayWithCompletable<T>(next, this));
    }
Returns a Single which will subscribe to this Completable and once that is completed then
will subscribe to the {@code next} SingleSource. An error event from this Completable will be
propagated to the downstream subscriber and will result in skipping the subscription of the
Single.
<p>
<img width="640" height="437" src="https://raw.github.com/wiki/ReactiveX/RxJava/images/rx-operators/Completable.andThen.s.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 SingleSource
@param next the Single to subscribe after this Completable is completed, not null
@return Single that composes this Completable and next