mapping exception with generator - sequence

[indu iyengar]

Hi

am trying to generate sequence on studentID

<class name="RegisterStudent" table="details_student">
     <id name="stid" column="studID">
       <generator class="sequence">
         <param name="sequence">st_seq</param>
       </generator>
     </id>
     <property name="stpw" column="studPW"></property>
     <property name="stname" column="studName"></property>
     <property name="dob" column="DOB"></property>
    </class>

but this exception is coming..

java.lang.ExceptionInInitializerError: exception in session org.hibernate.MappingException: could not instantiate id generator [entity-name=student.RegisterStudent]

[indu iyengar]

please give me an idea..

studentId is of type "String" how to generate sequence for this ???