public interface IComponent {

//setters and getters for the common properties
}

public class Leaf implements IComponent{
//setters and getters for the common properties

}

public class CompositeElement implements IComponent{
//setter and getter for the common properties

//access and modifiers for the IComponent List

}

<hibernate-mapping>
    <class name="domainlayer.Leaf" table="Leaf">
        <id name="oid" type="string" unsaved-value="null" >
            <column name="OID" sql-type="char(32)" not-null="true"/>
            <generator class="uuid.hex"/>
        </id>
        <property name="String1">
            <column name="String1" sql-type="varchar(255)" not-null="false"/>
        </property>
<!--
    The remaining properties
-->
       </class>
</hibernate-mapping>
-------------
<hibernate-mapping>
    <class name="domainlayer.CompositeElement" table="CompositeElement">
        <id name="oid" type="string" unsaved-value="null" >
            <column name="OID" sql-type="char(32)" not-null="true"/>
            <generator class="uuid.hex"/>
        </id>
        <property name="String1">
            <column name="String1" sql-type="varchar(255)" not-null="false"/>
        </property>
<!--
    The remaining properties
-->
[color=red]<!--
    I guess my problem is here
-->[/color]
        <bag name="childs" table="CompElement_Composite" cascade="all">
            <key>
                <column name="OID_CompElemet" not-null="true"/>
            </key>
            <many-to-many class="domainlayer.[color=red]??[/color]">
                <column name="OID_Component" not-null="true"/>
            </many-to-many>
        </bag>
    </class>
</hibernate-mapping>