Item.java is your easiest program. Create it first
Then you want to test it before you go any further. So next would be ItemTests.java
Since you have the .class file for Inventory, you can edit the InventoryTests.java to test my code (which might fail you test, which is okay since that it the purpose of an test driver), or you can write Inventory.java
Once you have Inventory.java written, I recommend testing it with my InventoryTests first. Then test it with your version of it adapted.