So what happens if:
1. the user enters a character that is not a number? Now you need to add validation code
2. the user enters a couple of numbers and then uses the Escape key to cancel cell editing? How to you reset the value in the Total column?
How to detect that the user has typed a character in a JTable column?"
1. You would need to create a DefaultCellEditor editor and add the editor to your Quantity column of your table.
2. You can then get the JTextField from the editor.
3. Once you have the text field you can add a DocumentListener to the text field.
4. Whenever a character is added or removed the DocumentListener is notified and you can get the text from the text field, convert is to a number and then get the Price from the TableModel and calculate the Total and then update the TableModel to reflect the new Total.
You would need to repeat that above steps for the Price column as well.
Read the
Swing Tutorial. There are section on "How to Use Tables" and "How to Write a DocumentListener" that might help.