what is getIdentifier method ?

POJO contains some identifier to identify the record and this method returns that value.
if we are checking the HBM file then we can tell what is the identifier column,suppose if we are not checking the HBM file then how to know what is the identifier  value....
    Student student = session.get(Student.class,"1");
    Object identifierValue = session.getIdentifier(student);
    System.out.println(".IdentifierValue."+ identifierValue);