Add idea files to gitignore.
Fix spelling mistakes. == "USA" changed to .equals("USA").
Remove unneeded comments.
Make whitespace match previous hole.
Align methods order.
Add one more Hole.
Move creating discounts to ProductCategory.
Reformat all code to have consistent whitespaces.
Remove unnecessary comment from OrderItem. Change field access to private.
Update Hole 6. with 5th changes.