F$.$ Write code for the services package that includes each of the following:

$$ a purchase data class with a customer cart and a set of cart items

$$ a purchase response data class that contains an order tracking number

$$ a checkout service interface

$$ a checkout service implementation class

G$.$ Write code to include validation to enforce the inputs needed by the Angular front$-$end.

H$.$ Write code for the controllers package that includes a REST controller checkout controller class with a post mapping to place orders.

Note: You do not need to duplicate REST functionality for each repository by creating methods in Java.