Oracle: What are the pre-requisites to (i) modify data type of a column (ii) add a column with NOT NULL constraint?

Question

I am using below statement but i got error.
ALTER TABLE XXXtable
ADD xxx_name varchar2(50) not null;

error message:
ORA-01758: table must be empty to add mandatory (NOT NULL) column

Answer 1

To Modify the data type of a column the column must be empty.
To add a column with NOT NULL constrain, the table must be empty