This first milestone assumes the implementation of a simple backend for a streaming platform similar with Netflix or HBO GO. The actions that a user will be able to perform are: register, login, logout, search, filter, buy movie, view movie, like movie, rate movie, buy premium account, buy tokens and switch between different pages where all these activities are available. Those steps are received at the input, which is a .json file whose content will be deserialized into a Java object that represents the platform. Also, each user is identified by his/her credentials and will have a different perspective of the platform depending on the country from which they connect to it.

I started the implementation by creating a class Platform where I will store all the information about it: the registered users, current available movies, current user that is logged in, the page that the current user is on and a list of all the actions that will be performed. The actions will be separately processed with the help of the ActionsWrapper class. For now, we have two types of activities that can be completed:

• change page will change the current page to the specified destination page
• on page will perform an action on the current page

To be able to have an easy to extend design, I used the Visitor pattern in the implementation of these two functionalities. Thus, I created an interface Visitable which will be implemented by the Platform class and will contain all the methods that accept the actions performed by the user. On the other hand, I defined the Visitor interface that is implemented by PlatformVisitor class where we have the definitions for all the actions that the current user can do. This thing enables a modular setup, where functionality can be easily added in the future without affecting the Platform object structure. Also, each action is encapsulated together with its own data and depends only on the Platform instance that is being processed. The ActionsWrapper class defines the performActions() method, which is technically a parser that decides which command should be executed according to the input received from the user. After that, it writes at the output the generated result if the action performs successfully or a predefined standard error output in case of error. You can take a look at the diagram below to better understand the project's structure described earlier.

It was an interesting challenge to develop this first milestone. I learned how to apply the Visitor design pattern in a practical situation and also improved my understanding of some important OOP concepts (for example working with inner classes or lambda expressions). Some important resources that I found useful in making this project are: Visitor design pattern, Laboratorul 5: Abstractizare, Laboratorul 6: Clase interne and Laboratorul 7: Overriding, overloading & Visitor pattern.