Hi,
I would like to know when to use Joined Subclass.Pls give some examples to understand it clearly.
I read it in doc but it's not clear.

Hope someone can help in this one.

Joined sub class allows polymorphic behaviour where the derived class properties are contained in an additional table rather then the one table hence the join is required. The relationship is determined from the key attribute, if the join was successful then the derived class will be instanciated instead of the base class. For example,

<class name="eg.Cat" table="CATS">
<id name="id" column="uid" type="long">
<generator class="hilo"/>
</id>
<property name="birthdate" type="date"/>
<property name="color" not-null="true"/>
<property name="sex" not-null="true"/>
<property name="weight"/>
<many-to-one name="mate"/>
<set name="kittens">
<key column="MOTHER"/>
<one-to-many class="eg.Cat"/>
</set>
<joined-subclass name="eg.DomesticCat" table="DOMESTIC_CATS">
<key column="CAT"/>
<property name="name" type="string"/>
</joined-subclass>
</class>

Mapping from the documentation. There are two domain objects where class "Cat" is the base class to the second domain object "DomesticCat", eg,

public class Cat {
..
}
and
public class DomesticCat extends Cat {
...
}

If table DOMESTIC_CATS has en entry which matches the Key in table CATS then the DomesticCat domain object will be created.

You use it when:
1) Your database exists and the model matches.
2) You have many and possibly differing attributes for the derived class(es) and/or there are few entries for the derived class(es). Separates the data and saves storage.
3) You don't consider using a single table to map derived classes as an appropriate practice.

Balance the tradeoffs.

Hope this helps.

So the bottom line is Joind subclass is similar to (One-to-one) Association .The difference is if the objects needs to be extended then use Joined Subclass else use Association.Is it right or i am confusing the concept.

I have the same question as "ashoknan".
Q: What's the difference between "one-to-one" and "joined-subclass"?

Same difference between inheritance and attribute in OO.

One practical thing : you can lazy load a one-to-one

Did you mean that there are only theoretical differenc and no other practical differences except a one-to-one can be lazy loaded?

inheritance vs attribute in OO are very practical differences to me.

In the post "Mapping of inheritance hierarchy" (http://forum.hibernate.org/viewtopic.php?t=925297&start=0&postdays=0&postorder=asc&highlight=joinedsubclass), Gavin advised that if one wanted to find something to replace joined-subclass (since joined-subclass and subclass can't be used together), he/she can use one-to-one as a subsititution.

God!
Is there really some huge difference between one-to-one and joined-subclass ??

Already answered

This solution is an acceptable substitute for the lacking of subclass / joined-subclass mix. Have a concrete look at the way you *use* your objects to find the *real* difference.