code: @NotNull
      public IntStream mapToInt(@NotNull final DoubleToIntFunction mapper) {
          return new IntStream(params, new DoubleMapToInt(iterator, mapper));
      }
docs: Returns an {@code IntStream} 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 IntStream}