在旨在容纳单个字符串的类层次结构中存储多个字符串

Question

I have a hierarchy of classes. Each class represent different kind of question in a survey.

Up until now, users can give a single answer to a question (via input box, ratio button or from a dropdown). The answer to each asked question is stored as a string in the answer field inherited from Question .

I have been tasked with adding a MultiChoice question to the hierarchy. A multi choise question enable users to provide multiple answers (such as by selecting multiple options in a group of checkboxes, or via multiple text boxes).

In the diagram below, the green class is the one I need to add. My problem is that the current structure assumes a single answer only.

I've thought of two possible modifications:

• Encoding multiple answers as a csv and store them in the answer field.
• Introducing a new field list<String> answers and adding an getter getAnswers() to MultiChoice .

I see problems with both approaches. The first one adds the responsability of parsing a possible csv of answers to the caller. The second one violates the Liskov substitution principle.

My question is: How can I support multiple answers in MultiChoice without introducing such problems?

Answer 1

The simple answer is of course to change the interface to

List<String> getAnswers();
void addAnswer(String answer);

But that's still a struct rather than object oriented design. To me this rather feels like a case for a visitor pattern with just having a interface like

void accept(Visitor v);

and a vistor interface like

interface Visitor<T> {
  T visit(T arg, DropDown answer);
  T visit(T arg, MultiChoice answer);
  T visit(T arg, SingleChoice answer);
  ...
}

with a visit method per concrete answer type. And then implement each statistics on the answers as a implementation of Visitor if statistics is what you wanna do. However, always think in what you wanna do with the data, not in dump fields.