I have solved it!
The prob in not getting the default selected item in the JComboBox is due to the fact I was using ItemStateChange(). Since when I select "Taxable"(the default selected item in the JComboBox), there is no change in selected item, it won't trigger or register anything in the ItemStateChange() for the
part. Therefore, I changed it to ActionPerformed() and added that code snippet in the event handling for the button.
Here is the final code:
Thank you for all the replies