Create NotNull Field Programmatically

Hello,

Is it possible to create a table having fields

NOT NULL ?

I know to create mandatory fields having default values. But I need a mandatory fields that has no default value. It will help me in validation.

Basically I've a Combobox on my SAP Screen where validation required. So, I'm thinking to create a not null field by DIAPI programming.

Is it possible?

If not how will I do the validation on Combobox?

Rgds

Subrata