/**
	 * A {@link Comparator} for {@link QuestionChoice} objects which orders them
	 * according to their corresponding choiceIndex values in the
	 * {@link ChoiceQuestionAssociation} objects of a parent {@link Question}.
	 * This is necssary because {@link QuestionChoice} objects themselves have
	 * no sense of order; they are ordered by their association to
	 * {@link Question}s.
	 *
	 * @author Ben
	 */
	private class ChoiceComparator implements Comparator {

		// Maps a {@link QuestionChoice} to its choiceIndex
		Map<QuestionChoice, Integer> choiceIndexMap;

		/**
		 * The constructor builds the map from {@link QuestionChoice}s to their
		 * indicies based on the {@link ChoiceQuestionAssociation} couplings in
		 * the {@link Question}.
		 *
		 * @param question
		 *            the {@link Question} whose
		 *            {@link ChoiceQuestionAssociation}s define the indicies
		 */
		public ChoiceComparator(Question question) {
			choiceIndexMap = new HashMap<QuestionChoice, Integer>();
			for (ChoiceQuestionAssociation coupling : question.getChoiceCouplings()) {
				choiceIndexMap.put(coupling.getChoice(), coupling.getChoiceIndex());
			}
		}

		public int compare(Object obj1, Object obj2) {
			if (!(obj1 instanceof QuestionChoice) || (!(obj2 instanceof QuestionChoice))) {
				throw new IllegalArgumentException("Can only sort QuestionChoice objects.");
			}
			QuestionChoice c1 = (QuestionChoice) obj1;
			QuestionChoice c2 = (QuestionChoice) obj2;
			return choiceIndexMap.get(c1) - choiceIndexMap.get(c2);
		}
	}