src/main/java/io/reactivex/Observable.java
@SuppressWarnings({ "unchecked", "rawtypes" })
    @CheckReturnValue
    @NonNull
    @SchedulerSupport(SchedulerSupport.NONE)
    public static <T> Observable<T> concat(ObservableSource<? extends ObservableSource<? extends T>> sources, int prefetch) {
        ObjectHelper.requireNonNull(sources, "sources is null");
        ObjectHelper.verifyPositive(prefetch, "prefetch");
        return RxJavaPlugins.onAssembly(new ObservableConcatMap(sources, Functions.identity(), prefetch, ErrorMode.IMMEDIATE));
    }
@SuppressWarnings({ "unchecked", "rawtypes" })
    @CheckReturnValue
    @NonNull
    @SchedulerSupport(SchedulerSupport.NONE)
    public static <T> Observable<T> concat(ObservableSource<? extends ObservableSource<? extends T>> sources, int prefetch) {
        ObjectHelper.requireNonNull(sources, "sources is null");
        ObjectHelper.verifyPositive(prefetch, "prefetch");
        return RxJavaPlugins.onAssembly(new ObservableConcatMap(sources, Functions.identity(), prefetch, ErrorMode.IMMEDIATE));
    }
@ SuppressWarnings ( { "unchecked" , "rawtypes" } ) @ CheckReturnValue @ NonNull @ SchedulerSupport ( SchedulerSupport . NONE ) public static < T > Observable < T > concat ( ObservableSource < ? extends ObservableSource < ? extends T > > sources , int prefetch ) { ObjectHelper . requireNonNull ( sources , "sources is null" ) ; ObjectHelper . verifyPositive ( prefetch , "prefetch" ) ; return RxJavaPlugins . onAssembly ( new ObservableConcatMap ( sources , Functions . identity ( ) , prefetch , ErrorMode . IMMEDIATE ) ) ; }
Returns an Observable that emits the items emitted by each of the ObservableSources emitted by the source
ObservableSource, one after the other, without interleaving them.
<p>
<img width="640" height="380" src="https://raw.github.com/wiki/ReactiveX/RxJava/images/rx-operators/concat.png" alt="">
<dl>
<dt><b>Scheduler:</b></dt>
<dd>{@code concat} does not operate by default on a particular {@link Scheduler}.</dd>
</dl>

@param <T> the common element base type
@param sources
an ObservableSource that emits ObservableSources
@param prefetch
the number of ObservableSources to prefetch from the sources sequence.
@return an Observable that emits items all of the items emitted by the ObservableSources emitted by
{@code ObservableSources}, one after the other, without interleaving them
@see <a href="http://reactivex.io/documentation/operators/concat.html">ReactiveX operators documentation: Concat</a>
Returns an Observable that emits the items emitted by each of the ObservableSources emitted by the source ObservableSource one after the other without interleaving them . <p > <img width = 640 height = 380 src = https : // raw . github . com / wiki / ReactiveX / RxJava / images / rx - operators / concat . png alt = > <dl > <dt > <b > Scheduler : < / b > < / dt > <dd > { @code concat } does not operate by default on a particular { @link Scheduler } . < / dd > < / dl >