How to map Interfaces and their implementing classes?

elophant · **Posted:** Wed Dec 10, 2003 5:31 am

Hello!

I have several JAVA-Interfaces: a QUESTION and an ANSWER. The QUESTION-Interface is extended by two more interfaces: NumericQUESTION and TextQUESTION. The ANSWER-Interface is extended by the interfaces NumericANSWER and TextANSWER.

Each Interface is implemented by a Class with Name [InterfaceName]+DTO, i. e. QUESTION is implemented by QuestionDTO. The "DTO" stands for "Data Transfer Object".

Now I want to express that each subclass of AnswerDTO, that is NumericAnswerDTO and TextAnswerDTO, knows about its corresponding question. So NumericAnswerDTO has an attribute "question". The type of the attribute is the one of the interface! Here is a code example:

Code:

public class TextAnswerDTO extends AnswerDTO implements TextAnswer {

//... some inherited attributes

// Now the linkage to the corresponding question
TextQuestion question;

// ... Constructors

public void setQuestion(TextQuestion q) throws AnException {
this.question = q;
}

public TextQuestion getQuestion() throws AnException {
return question;
}

}

The relation between answers and questions is many-to-one, which means that many answers are related to one question.

How can I map this scenario? The classes that are obviously persistet to the database are the DTOs, I use the interfaces to add a level of abstraction, so I can later change the underlying implementation. But it is very important, that the attributes of the DTO-classes holding the reference to a corresponding question are of the interface type!

Thanks for all help in advance!

Philipp Hinnah (Germany)

emmanuel · **Posted:** Wed Dec 10, 2003 11:20 am

chapter 5.1.3 helps ?

elophant · **Posted:** Wed Dec 10, 2003 5:13 pm

... yeah, it helps. I've read over this several times. Maybe I should try this with the mapping of implementing classes by the <subclass>-element.

Thx!

elophant · **Posted:** Thu Dec 11, 2003 6:41 am

Here is my first attempt to map a part of the scenario I described above.

Code:

<!-- Interface Question -->
    <class name="org.openuss.group.specification.business.Question" table="QUESTION" discriminator-value="Question">
        <id name="id" column="ID">
            <generator class="uuid.string" />
        </id>
        <discriminator column="CLASS_NAME" />
        <property name="text" column="TEXT" not-null="true" />
        <many-to-one name="parentQuestion" column="PARENT_ID" class="org.openuss.group.specification.business.Question" />
        
        <!-- Implementing Class QuestionDTO -->
        <subclass name="org.openuss.group.business.entity.QuestionDTO" discriminator-value="QuestionDTO" />
        
        <!-- Subclass (Subinterface) TextQuestion -->
        <subclass name="org.openuss.group.specification.business.TextQuestion" discriminator-value="TextQuestion">
            <property ... />
            
            <!-- Implementing Class TextQuestionDTO -->
            <subclass name="org.openuss.group.business.entity.TextQuestionDTO" discriminator-value="TextQuestionDTO" />
            
        </subclass>
    </class>

Is this correct? The Hibernate Reference says to use the <subclass>-Tag to declare implementing classes of the interface. But I also need to use the <subclass>-Tag to declare a subinterface ("TextQuestion") of the interface "Question". And then I use this tag once again to declare the implementing class of the "TextQuestion" interface.

I am really wondering, if this solution works?

gavin · **Posted:** Thu Dec 11, 2003 6:42 am

sure, why not. did you try it??

elophant · **Posted:** Thu Dec 11, 2003 7:42 am

Not yet... I haven't installed Hibernate yet. I am working in a 6-member-team on a 3-tier software development. We use Hibernate instead of Entity Beans for the data layer.

But I am quite a newbie... so I first need to understand how to set up the whole Hibernate-Environment and how to access the database (we use HyperSonicSQL) via sessions... but I will learn... :-)

I think future posts of mine will follow when the first errors occur?