code: @NotNull
      public IntStream mapToInt(@NotNull final ToIntFunction<? super T> mapper) {
          return new IntStream(params, new ObjMapToInt<T>(iterator, mapper));
      }
docs: Returns {@code IntStream} 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 IntStream}
  @see #map(com.annimon.stream.function.Function)