i need explanation with the code for this problem

(Core Java)

# Problem Statement

Surabi is a chain of restaurants. Till this time there billing system was manual but due to COVID 19 they wanted to have a Billing System to provide better service to their customers.

## Use Stories

### Users

1. As a user I should be able to log in and log out and register in the application. 2.

As a user I should be able to see all the items available along with the price.

3. As a user I should be able to select the item I want to order.

4. As a user I should be able to order n number of plates per item,

5. As a user I should be able to order more than one item.

6. As a user I should be able to see my final bill on the screen.

### Admin

1. As an admin I should be able to login and logout in the application

2. As an admin I should be able to see all the bills getting generated today.

3. As an admin I should be able to see the total sale from this month.

4. As an admin I should be able to perform CRUD on menu and menu items.

## Instructions: - 1.

In order to make your application efficient please use new Thread as soon as a user logged in and the threads properly closed as soon as a user logout.

2. A user should get the proper see the error messages if some input is not correct. Use custom Exceptions to show the error messages

3. Use Proper design patters to make the application more effective

4. Please use Proper table structure to Manage the data.

5. Share the default data using .sql files

6. Please use proper testing cases to test the application written

7. Use codding standards and comments in the application.

8. Use proper interfaces and classes combo to create the application