src/main/java/io/reactivex/Completable.java
@CheckReturnValue
    @NonNull
    @SchedulerSupport(SchedulerSupport.NONE)
    public final Completable concatWith(CompletableSource other) {
        ObjectHelper.requireNonNull(other, "other is null");
        return RxJavaPlugins.onAssembly(new CompletableAndThenCompletable(this, other));
    }
@CheckReturnValue
    @NonNull
    @SchedulerSupport(SchedulerSupport.NONE)
    public final Completable concatWith(CompletableSource other) {
        ObjectHelper.requireNonNull(other, "other is null");
        return RxJavaPlugins.onAssembly(new CompletableAndThenCompletable(this, other));
    }
Concatenates this Completable with another Completable.
<p>
<img width="640" height="317" src="https://raw.github.com/wiki/ReactiveX/RxJava/images/rx-operators/Completable.concatWith.png" alt="">
<dl>
<dt><b>Scheduler:</b></dt>
<dd>{@code concatWith} does not operate by default on a particular {@link Scheduler}.</dd>
</dl>
@param other the other Completable, not null
@return the new Completable which subscribes to this and then the other Completable
@throws NullPointerException if other is null
@see #andThen(MaybeSource)
@see #andThen(ObservableSource)
@see #andThen(SingleSource)
@see #andThen(Publisher)