Method concat

src/main/java/io/reactivex/rxjava3/core/Flowable.java:1182–1191

Concatenates elements of each Publisher provided via an Iterable sequence into a single sequence of elements without interleaving them. Backpres

(@NonNull Iterable<@NonNull ? extends Publisher<? extends T>> sources)

     * @throws NullPointerException if {@code sources} is {@code null}
     */
    @SuppressWarnings({ "unchecked", "rawtypes" })
    @CheckReturnValue
    @NonNull
    @BackpressureSupport(BackpressureKind.FULL)
    @SchedulerSupport(SchedulerSupport.NONE)
    public static <@NonNull T> Flowable<T> concat(@NonNull Iterable<@NonNull ? extends Publisher<? extends T>> sources) {
        Objects.requireNonNull(sources, "sources is null");
        // unlike general sources, fromIterable can only throw on a boundary because it is consumed only there
        return fromIterable(sources).concatMapDelayError((Function)Functions.identity(), false, 2);
    }

    /**
     * Returns a {@code Flowable} that emits the items emitted by each of the {@link Publisher}s emitted by the source