Design and implement a Java program that simulates a basic online shopping system. The system should allow customers to browse products, add items to their cart, and checkout. The program should also provide functionality for managing the inventory of products and keeping track of sales.

The program should consist of the following classes:

$1.$ Product: A class representing a product with fields for name, description, price, and quantity.

$2.$ Customer: A class representing a customer with fields for name, address, and a shopping cart.

$3.$ ShoppingCart: A class representing a shopping cart with methods for adding items, removing items, and calculating the total cost.

$4.$ Inventory: A class representing the inventory of products with methods for adding products, removing products, and updating quantities.

$5.$ SalesReport: A class representing a report of sales with methods for generating a report of total sales and individual product sales.

Your program should also implement the following features:

$1.$ A menu system for navigating the program and performing actions such as browsing products, adding items to the cart, and checking out.

$2.$ A database or file system for storing product and sales data.

$3.$ Exception handling to handle errors such as invalid inputs and out$-$of$-$stock products.

$4.$ Threading to allow multiple customers to shop simultaneously.

$5.$ An admin interface for managing the inventory and generating sales reports.

Your program should be well$-$structured and use appropriate design patterns. You should also provide JUnit tests to verify the functionality of your program