@Entity
@Table(name="TBL_USERS")
@Inheritance(strategy=InheritanceType.SINGLE_TABLE)
@DiscriminatorColumn(
    name="CLASS",
    discriminatorType=DiscriminatorType.STRING
)
@DiscriminatorValue("User")
public class User extends BaseObject implements Comparable<User> {
   
   private static final long serialVersionUID = 1013602284129249520L;
   public static final String FLAG_ACTIVE = "Y";
   public static final String FLAG_INACTIVE = "N";
   
   private long id;
   private String password;
   private String name;
.....
...

@Entity
@DiscriminatorValue(value="VolunteerUser")
public class VolunteersUser extends User {
    private static final long serialVersionUID = 4805522435037583872L;
   
    @Column(name="FIRST_NAME")
    private String firstName;
   
    @Column(name="SURNAME")
    private String surname;
    ....
    .....