Clarification for Use of ?. operator

How does the Use of ?. operator in principal id help in security? or How are the below two lines different?

 @PostAuthorize("hasRole('ROLE_SUPERVISOR') or ( isAuthenticated() and returnObject.employee.id == principal.id )")

 @PostAuthorize("hasRole('ROLE_SUPERVISOR') or returnObject.employee.id == principal?.id")