src/main/java/io/reactivex/Completable.java
@CheckReturnValue
    @SchedulerSupport(SchedulerSupport.NONE)
    public final Completable andThen(CompletableSource next) {
        ObjectHelper.requireNonNull(next, "next is null");
        return RxJavaPlugins.onAssembly(new CompletableAndThenCompletable(this, next));
    }
@CheckReturnValue
    @SchedulerSupport(SchedulerSupport.NONE)
    public final Completable andThen(CompletableSource next) {
        ObjectHelper.requireNonNull(next, "next is null");
        return RxJavaPlugins.onAssembly(new CompletableAndThenCompletable(this, next));
    }
Returns a Completable that first runs this Completable
and then the other completable.
<p>
<img width="640" height="437" src="https://raw.github.com/wiki/ReactiveX/RxJava/images/rx-operators/Completable.andThen.c.png" alt="">
<p>
This is an alias for {@link #concatWith(CompletableSource)}.
<dl>
<dt><b>Scheduler:</b></dt>
<dd>{@code andThen} does not operate by default on a particular {@link Scheduler}.</dd>
</dl>
@param next the other Completable, not null
@return the new Completable instance
@throws NullPointerException if other is null