SQL & PL/SQL :: How To Modify Null Column To Not Null

when i follow this steps mention on this website

[URL].........

to modify column from null to not null i got this error and on this website its show successful

my steps are

first i create a table

SQL> create table Stu_Table(Stu_Id varchar(2), Stu_Name varchar(10),
2 Stu_Class varchar(10));

Table created.

Then insert some rows into Stu_Table

SQL> insert into Stu_Table (Stu_Id, Stu_Name) values(1,'Komal');

1 row created.

SQL> insert into Stu_Table (Stu_Id, Stu_Name) values(2,'Ajay');

1 row created.

SQL> insert into Stu_Table (Stu_Id, Stu_Name) values(3,'Rakesh');

1 row created.

SQL> insert into Stu_Table (Stu_Id, Stu_Name) values(4,'Bhanu');

1 row created.

SQL> insert into Stu_Table (Stu_Id, Stu_Name) values(5,'Santosh');

1 row created.

SQL> select * from Stu_Table;

ST STU_NAME STU_CLASS
-- ---------- ----------
1 Komal
2 Ajay
3 Rakesh
4 Bhanu
5 Santosh

Table Structure is like this

SQL> Describe Stu_Table
Name Null? Type
----------------------------------------- -------- ----------------------------
STU_ID VARCHAR2(2)
STU_NAME VARCHAR2(10)
STU_CLASS VARCHAR2(10)

now when i try to modify this Stu_id column to not null its give me error.

SQL>ALTER TABLE Stu_Table MODIFY Stu_Id int(3)not null;
ALTER TABLE Stu_Table MODIFY Stu_Id int(3)not null
*
ERROR at line 1:
ORA-01735: invalid ALTER TABLE option

and when i try to add new column with not null its also gives me error

SQL> ALTER TABLE Stu_Table add C1_TEMP integer NOT NULL;
ALTER TABLE Stu_Table add C1_TEMP integer NOT NULL
*
ERROR at line 1:
ORA-01758: table must be empty to add mandatory (NOT NULL) column