code: @NotNull
      public DoubleStream mapToDouble(@NotNull final ToDoubleFunction<? super T> mapper) {
          return new DoubleStream(params, new ObjMapToDouble<T>(iterator, mapper));
      }
docs: Returns {@code DoubleStream} 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 DoubleStream}
  @since 1.1.4
  @see #map(com.annimon.stream.function.Function)