SQL & PL/SQL :: Constraints On Objects?

Ive created a scenario to what im trying to achieve here so ignore how its set-up attribute wise.

CREATE TYPE object_obj AS OBJECT (
obj_id NUMBER,
NAME VARCHAR2(20),
age NUMBER) NOT FINAL;
/

CREATE TYPE object_ext UNDER object_obj (
course_name VARCHAR2(20));

/
CREATE TYPE object_ext2 UNDER object_obj (
location VARCHAR2(20);
/

CREATE TABLE object_tab OF object_obj(obj_id PRIMARY KEY);
/

Now for the course_name i need to make sure it can only be on of these three values ('Science','Math','English'). I can't find a way to apply this constraint without making a table. But then that creates another issue of how to insert the data from the main supertype (object_obj) and i only want the object_tab table and view the subtypes via a select query.