code: @NotNull
      public LongStream mapToLong(@NotNull final ToLongFunction<? super T> mapper) {
          return new LongStream(params, new ObjMapToLong<T>(iterator, mapper));
      }
docs: Returns {@code LongStream} with elements that obtained by applying the given function.
  
  <p>This is an intermediate operation.
  
  @param mapper  the mapper function used to apply to each element
  @return the new {@code LongStream}
  @since 1.1.4
  @see #map(com.annimon.stream.function.Function)