src/main/java/io/reactivex/parallel/ParallelFlowable.java
@CheckReturnValue
    @NonNull
    public final <R> ParallelFlowable<R> map(@NonNull Function<? super T, ? extends R> mapper) {
        ObjectHelper.requireNonNull(mapper, "mapper");
        return RxJavaPlugins.onAssembly(new ParallelMap<T, R>(this, mapper));
    }
@CheckReturnValue
    @NonNull
    public final <R> ParallelFlowable<R> map(@NonNull Function<? super T, ? extends R> mapper) {
        ObjectHelper.requireNonNull(mapper, "mapper");
        return RxJavaPlugins.onAssembly(new ParallelMap<T, R>(this, mapper));
    }
Maps the source values on each 'rail' to another value.
<p>
Note that the same mapper function may be called from multiple threads concurrently.
@param <R> the output value type
@param mapper the mapper function turning Ts into Us.
@return the new ParallelFlowable instance