Program plan:
• Create a class named GroceryBill to keeps track of a list of items.
• Create a class named Item. This class is used to store the item details.
• Create a class named Employee. This class is used to store the Employee details
• Create a class named DiscountBill which is extends from the class named GroceryBill to compute discounts for preferred customers.
• Create a tester class named DiscountBillTester. This class is used to test the GroceryBill class.