Edit entity on the web tier

[integral]

Hi all,

I'm working on a small portal project for handling questionnaires and now I'm facing a curious problem.

I have one abstract entity "Answer", two extending entities "TextAnswer" and "MarkAnswer". Finally there is the AnswerSheet entity that contains Answers.

public abstract class Answer {
 long id;
 Question question;//answer also remembers its question
 AnswerSheet answerSheet; +getters/setter
}

public class TextAnswer extends Answer {
 String answer; + getters/setters
}
public class MarkAnswer extends Answer {
 int answerValue; + getters/setters
}

public class AnswerSheet {
 List<Answer> answers; 
}

Now I want to edit AnswerSheets using JSP and controller.
In the controller I put the answersheet I want to edit to the "model".
Now in JSP:

<form:form action="${saveSheetAction}" method="post" modelAttribute="answerSheet">
<form:hidden path="id"/> //the id of answerSheet
<c:forEach var = "answer" items = "${answerSheet.answers}" varStatus="answersLoop">
                <form:hidden path="answers[${answersLoop.index}].id"/>
	<br/>
	<c:if test="${answer.question.type == 'TEXT'}">
		<form:input path="answers[${answersLoop.index}].text"/>
	</c:if>
	<c:if test="${answer.question.type == 'MARK'}">
		<form:input path="answer.answerValue"/>
	</c:if>
</c:forEach>
</form:form>

Well, now when I try to submit the form, spring complains about not being able to instantiate the Answer class (which is quite obvious, stacktrace is at the end of the post). Now I ask whether it is possible to tell spring to instantiate the TextQuestion (or MarkQuestion) class in jsp for editing.

One solution would be using some kind of more intelligent DTO in the form and then later (in the controller) instantiate the correct class. But I was expecting something simpler

I'll be happy for any constructive response.
Thank you

Stacktrace:

 Invalid property 'answers[0]' of bean class [eu.ibacz.k3.gadgets.questionnaire.entities.AnswerSheet]: Invalid index in property path 'answers[0]'; nested exception is org.springframework.beans.NullValueInNestedPathException: Invalid property 'answers' of bean class [eu.ibacz.k3.gadgets.questionnaire.entities.AnswerSheet]: Could not instantiate property type [eu.ibacz.k3.gadgets.questionnaire.entities.Answer] to auto-grow nested property path: java.lang.InstantiationException