src/main/java/io/reactivex/Completable.java
@CheckReturnValue
    @NonNull
    @SchedulerSupport(SchedulerSupport.NONE)
    public static Completable concatArray(CompletableSource... sources) {
        ObjectHelper.requireNonNull(sources, "sources is null");
        if (sources.length == 0) {
            return complete();
        } else
        if (sources.length == 1) {
            return wrap(sources[0]);
        }
        return RxJavaPlugins.onAssembly(new CompletableConcatArray(sources));
    }
@CheckReturnValue
    @NonNull
    @SchedulerSupport(SchedulerSupport.NONE)
    public static Completable concatArray(CompletableSource... sources) {
        ObjectHelper.requireNonNull(sources, "sources is null");
        if (sources.length == 0) {
            return complete();
        } else
        if (sources.length == 1) {
            return wrap(sources[0]);
        }
        return RxJavaPlugins.onAssembly(new CompletableConcatArray(sources));
    }
Returns a Completable which completes only when all sources complete, one after another.
<p>
<img width="640" height="283" src="https://raw.github.com/wiki/ReactiveX/RxJava/images/rx-operators/Completable.concatArray.png" alt="">
<dl>
<dt><b>Scheduler:</b></dt>
<dd>{@code concatArray} does not operate by default on a particular {@link Scheduler}.</dd>
</dl>
@param sources the sources to concatenate
@return the Completable instance which completes only when all sources complete
@throws NullPointerException if sources is null