code: @CheckReturnValue
      @BackpressureSupport(BackpressureKind.PASS_THROUGH)
      @SchedulerSupport(SchedulerSupport.NONE)
      public final <R> Flowable<R> map(Function<? super T, ? extends R> mapper) {
          ObjectHelper.requireNonNull(mapper, "mapper is null");
          return RxJavaPlugins.onAssembly(new FlowableMap<T, R>(this, mapper));
      }
docs: Returns a Flowable that applies a specified function to each item emitted by the source Publisher and
  emits the results of these function applications.
  <p>
  <img width="640" height="305" src="https://raw.github.com/wiki/ReactiveX/RxJava/images/rx-operators/map.png" alt="">
  <dl>
  <dt><b>Backpressure:</b></dt>
  <dd>The operator doesn't interfere with backpressure which is determined by the source {@code Publisher}'s backpressure
  behavior.</dd>
  <dt><b>Scheduler:</b></dt>
  <dd>{@code map} does not operate by default on a particular {@link Scheduler}.</dd>
  </dl>
  
  @param <R> the output type
  @param mapper
  a function to apply to each item emitted by the Publisher
  @return a Flowable that emits the items from the source Publisher, transformed by the specified
  function
  @see <a href="http://reactivex.io/documentation/operators/map.html">ReactiveX operators documentation: Map</a>