Kind time of day!

There was a question if is there is in bean a field extentions type Map and in it some objects will be added and it is necessary, that they were accessible on the known key certain beforehand, for example, that bean UserGroupBean were accessible on a key "UserGroup", etc.

In mapping I specify:

<map name="extentions" table="UserGroup" lazy="false" inverse="false">
<key column="UserId" />
<map-key type="java.lang.String" <!-??? How to let know Hib., what object it is necessary to lay down on a key "UserGroup"???->/>
<many-to-many class="com.bsu.entity.user.UserGroupBean" column="GroupId" />
</map>


How to let know Hibernate, what object it is necessary to lay down on a key "UserGroup"?