11. Joined Subclass

Definition: Joined subclasses are those that are mapped to a table-per-subclass design rather than a table-per-hierarchy.

Scenario: We have one first-rank class, Foo, and another first-rank class, Bar, which is a subclass of Foo.

public class Bar extends Foo // Bar is a subclass of Foo

Hibernate Mapping:
In Hibernate, this could be mapped as follows:

<class name="Foo" table="foo">     ...     <property name="name" column="name" type="string"     <joined-subclass name="subclass.Bar" table="bar">          <key column="foo_id"/>          <property name="age" column="age" type="string"/>     </joined-subclass> </class>

Table Schema:

Foo
id	name

Bar
foo_id	age

Here Bar inherits from Foo and so is joined using the PK foo_id and adds the extra data column age.

Bidirectionality:
Inheritance relationships are only unidirectional in Java. A child can determine its parent class but the reverse has no meaning.