src/main/java/io/reactivex/parallel/ParallelFlowable.java
@CheckReturnValue
    @NonNull
    public final <R> ParallelFlowable<R> flatMap(
            @NonNull Function<? super T, ? extends Publisher<? extends R>> mapper,
            boolean delayError, int maxConcurrency, int prefetch) {
        ObjectHelper.requireNonNull(mapper, "mapper is null");
        ObjectHelper.verifyPositive(maxConcurrency, "maxConcurrency");
        ObjectHelper.verifyPositive(prefetch, "prefetch");
        return RxJavaPlugins.onAssembly(new ParallelFlatMap<T, R>(this, mapper, delayError, maxConcurrency, prefetch));
    }
@CheckReturnValue
    @NonNull
    public final <R> ParallelFlowable<R> flatMap(
            @NonNull Function<? super T, ? extends Publisher<? extends R>> mapper,
            boolean delayError, int maxConcurrency, int prefetch) {
        ObjectHelper.requireNonNull(mapper, "mapper is null");
        ObjectHelper.verifyPositive(maxConcurrency, "maxConcurrency");
        ObjectHelper.verifyPositive(prefetch, "prefetch");
        return RxJavaPlugins.onAssembly(new ParallelFlatMap<T, R>(this, mapper, delayError, maxConcurrency, prefetch));
    }
@ CheckReturnValue @ NonNull public final < R > ParallelFlowable < R > flatMap ( @ NonNull Function < ? super T , ? extends Publisher < ? extends R > > mapper , boolean delayError , int maxConcurrency , int prefetch ) { ObjectHelper . requireNonNull ( mapper , "mapper is null" ) ; ObjectHelper . verifyPositive ( maxConcurrency , "maxConcurrency" ) ; ObjectHelper . verifyPositive ( prefetch , "prefetch" ) ; return RxJavaPlugins . onAssembly ( new ParallelFlatMap < T , R > ( this , mapper , delayError , maxConcurrency , prefetch ) ) ; }
Generates and flattens Publishers on each 'rail', optionally delaying errors,
having a total number of simultaneous subscriptions to the inner Publishers
and using the given prefetch amount for the inner Publishers.

@param <R> the result type
@param mapper the function to map each rail's value into a Publisher
@param delayError should the errors from the main and the inner sources delayed till everybody terminates?
@param maxConcurrency the maximum number of simultaneous subscriptions to the generated inner Publishers
@param prefetch the number of items to prefetch from each inner Publisher
@return the new ParallelFlowable instance
Generates and flattens Publishers on each rail optionally delaying errors having a total number of simultaneous subscriptions to the inner Publishers and using the given prefetch amount for the inner Publishers .