code: @NotNull
      public OptionalInt mapToInt(@NotNull ToIntFunction<? super T> mapper) {
          if (!isPresent()) return OptionalInt.empty();
          return OptionalInt.of(mapper.applyAsInt(value));
      }
docs: Invokes the given mapping function on inner value if present.
  
  @param mapper  mapping function
  @return an {@code OptionalInt} with transformed value if present,
  otherwise an empty {@code OptionalInt}
  @throws NullPointerException if value is present and
  {@code mapper} is {@code null}
  @since 1.1.3