code: public static @NonNull OptionalDouble map(final @NonNull OptionalDouble optional, final @NonNull DoubleUnaryOperator operator) {
      requireNonNull(operator, "operator");
      return optional.isPresent() ? OptionalDouble.of(operator.applyAsDouble(optional.getAsDouble())) : OptionalDouble.empty();
    }
docs: Apply the provided mapping operator to the {@code optional} if a value is present,
  and return an {@code OptionalDouble} describing the result.
  
  @param optional the optional
  @param operator the operator
  @return an optional