jsp tradition add-on library (unable setter slight attribute)

i'm carrying problem tradition tag:-

org.apache.jasper.jasperexception: /custom_tags.jsp(1,0) amateurish setter slight charge : firstname

this taghandler class:

package com.cg.tags;

import javax.servlet.jsp.jspexception;
import javax.servlet.jsp.jspwriter;
import javax.servlet.jsp.tagext.tagsupport;

public category nametag extends tagsupport{

    open twine firstname;
    open twine lastname;

    open vacant setfirstname(string firstname){

        this.firstname=firstname;
        }
    open vacant setlastname(string lastname){

        this.lastname=lastname;
        }

    open int dostarttag() throws jspexception {
        try {
            jspwriter out=pagecontext.getout();
            out.println( "first name:  "+firstname+ "last name: "+lastname);

        } locate (exception ex) {
            pitch new jspexception("io problems");
        }
        relapse skip_body;
    }


}

this tld file:

?xml version="1.0" encoding="utf-8"?>
<taglib>
     <tlibversion>1.1</tlibversion>
     <jspversion>1.1</jspversion>
     <shortname>utility</shortname>
     <uri>/web-inf/nametagdesc.tld</uri>
     <info>
       rudimentary add-on library examples
     </info>
   <tag>
       <name>name</name>
       <tagclass>com.cg.tags.nametag</tagclass>
       <bodycontent>empty</bodycontent>
      <attribute>
      <name>firstname</name>
      <required>true</required>
      <rtexprvalue>true</rtexprvalue>
      </attribute>
      <attribute>
      <name>lastname</name>
      <required>true</required>
      <rtexprvalue>true</rtexprvalue>
    </attribute>
 </tag>
</taglib>

and jsp page:

<%@ taglib uri="/web-inf/nametagdesc.tld" prefix="cg"  %>

<cg:name firstname="fname" lastname="lname"/>

i have checked formula recompiled deployed justly etc etc....

so, doubt , since can't setter method???