Create auto sort in Listbox

<?xml version="1.0" encoding="UTF-8"?>
<?init class="org.zkoss.zkplus.databind.AnnotateDataBinderInit" root="./winDepartemen" ?>
<?variable-resolver class="org.zkoss.zkplus.spring.DelegatingVariableResolver"?>
<zk xmlns="http://www.zkoss.org/2005/zul"                                     xmlns:h="http://www.w3.org/1999/xhtml"
       xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"  xsi:schemaLocation="http://www.zkoss.org/2005/zul
      http://www.zkoss.org/2005/zul/zul.xsd">

<zscript>
<![CDATA[
   zk.DepartmentController;
   ]]>
</zscript>

<window id="winDepartemen" width="650px"  border="normal" apply="${DepartmentController}" >
                              <listbox id="listDept" multiple="true" rows="10"
                                   model="@{winDepartemen$composer.allDepartment}"
                                   mold="paging" pageSize="10">

                               <listhead>
                                   <listheader label="Id" width="0px"/>
                                   <listheader label="Code"           sort="auto"/>
                                   <listheader label="Description" sort="auto"/>
                                   <listheader label="Update/Delete"/>
                                </listhead>

                               <listitem self="@{each='department'}" value="@{department}">
                                  <listcell label="@{department.id}"/>
                                  <listcell label="@{department.code}"/>
                                  <listcell label="@{department.description}"/>
                               </listitem>
                             </listbox>

</window>
</zk>

public class DepartmentController extends GenericForwardComposer {
//show data dept
public List<Department> getAllDepartment(){
   try {
        List<Department> list=new ArrayList<Department>();
        list=departmentService.getList();
         return list;
     } catch (Exception ex) {
         ex.printStackTrace();
         return null;
   }
}

//auto refresh data after crud
public void refreshList(){
 List<Department> list=new ArrayList<Department>();
 list=departmentService.getList();
 ListModel listModel = new SimpleListModel(list);
 listDept.setModel(listModel);
}
}

SEVERE: >>java.lang.ClassCastException: model.Department cannot be cast to java.lang.Comparable
>>      at org.zkoss.zul.ListitemComparator.compare(ListitemComparator.java:250)
>>      at java.util.TimSort.countRunAndMakeAscending(TimSort.java:324)
>>      at java.util.TimSort.sort(TimSort.java:189)
>>      at java.util.TimSort.sort(TimSort.java:173)
>>      at java.util.Arrays.sort(Arrays.java:659)