Map<String,String[]>

class User {
     private long userId;
     private Map<String, String[]> preferences;
}

Map<String, UserPreference>

<class name="test.User" table="User">
...
<map name="preferences" table="USER_PR" >
       <key column="USER_ID"/>
       <index column="PR_KEY" type="string"/>
       <many-to-many class="test.UserPreference" column="PR_ID"  />
</map>

</class>

class UserPreference {
       Long id;
       List<String> values;
}