Castor - Generation of xml using castor

The requirement in Castor to develop a XML with the Complex Type as shown below

<SimplDataEntry>
   <Envlp>
      <FirstName>XXX</FirstName>
   </Envlp>

</SimplDataEntry>
<SimplDataEntry>
    <Envlp>
        <LastName>XXX</LastName>
     </Envlp>
</SimplDataEntry>

What would be the JAVA representation for the above Requirement. I tried with arrayList but the problem here is After ENVLP the subtype is not constant its getting changed.

I am getting like below structure when I designed it as an ArrayList.

Castor.xml

---

<field name="firstName" collection="arraylist" type="java.lang.String">
       <bind-xml name="FirstName" location="SplmtryData/Envlp"/>
 </field>

 <field name="LastName" collection="arraylist" type="java.lang.String">
       <bind-xml name="LastName" location="SplmtryData/Envlp"/>
 </field>

 OutPut
 --------                               

 <SimplDataEntry>
       <Envlp>
            <FirstName>XXX</FirstName>
            <LastName>XXX</LastName>
       </Envlp>
 </SimplDataEntry>