code: @NotNull
      public DoubleStream mapToDouble(@NotNull final IntToDoubleFunction mapper) {
          return new DoubleStream(params, new IntMapToDouble(iterator, mapper));
      }
docs: Returns a {@code DoubleStream} consisting of the results of applying the given
  function to the elements of this stream.
  
  <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 #flatMap(com.annimon.stream.function.IntFunction)