Let’s go over the main concepts we covered during this example project:
First, we modeled the products in the ProductDetail class, and made it immutable by making the class and the attributes final, and not including any setter methods. In this way, the state of the instances couldn’t be modified after construction.
We created a ProductCatalog interface that is the contract that separates the concerns of the vending machine from those of the specific implementation of the catalog. Then we implemented this interface in the InMemoryProductCatalog class that stored the products in a HashSet. This was made possible by overriding the equals and hashCode methods in the ProductDetail class so they only depend on the productCode, and thus preventing us from adding two products with the same code into the Set.
The VendingMachine class contains the business logic of the operation. It holds the current balance and the maximum price for any item. It also receives the ProductCatalog as an argument at construction, and throws an IllegalArgumentException if the catalog is null, empty, or any of its products exceeds the maximum allowed price.
For user interaction, we defined a Command Line Interface, or CLI that receives a VendingMachine to run and manages its interaction with the user.
It is modeled separately from the business logic of the VendingMachine to facilitate possible changes in the user interface in the future (like adding a web interface for the vending machine). It uses a try/catch/finally block to handle the possible InputMismatchException that could be generated by an unexpected input from the Scanner object.
And finally, we tested our program using a VendingMachineApp with a HashSet of dummy products in order to create an InMemoryProductCatalog, a VendingMachine, and a CLI. And finally, it called the method on the CLI class to run the vending machine and allow the user to enter commands and interact with the vending machine.