Features Required :

1. User Authentication:

   • Users should be able to create accounts, log in, and log out.

2. Video Catalog:

   • The system should have a catalog of movies and TV shows with details like title, genre, release date, and duration.

3. User Profile Management:

   • Users should be able to view and edit their profiles, including preferences, watch history, etc.

4. Watchlist:

   • Users should be able to add/remove videos to/from their watchlist.

5. Video Streaming:

   • Users should be able to stream videos.

6. Recommendation System:

   • The system should provide personalized recommendations based on user preferences and watch history.

7. User Reviews and Ratings:

   • Users should be able to rate and review videos.

8. Subscription Management:

   • Users should be able to subscribe, and the system should enforce subscription rules.

Design Patterns and SOLID Principles:

1. Singleton Pattern:

   • For the VideoCatalog to ensure a single point of access.

2. Factory Method Pattern:

   • For creating different types of videos like movies and TV shows.

3. Observer Pattern:

   • For notifying users about new releases, recommendations, etc.

4. Strategy Pattern:

   • For different recommendation algorithms.

5. Command Pattern:

   • For executing commands like adding/removing videos from the watchlist.

6. Decorator Pattern:

   • For enhancing basic video objects with additional features.

7. Composite Pattern:

   • For creating playlists that can contain both individual videos and other playlists.

8. State Pattern:

   • For managing the state of a video (e.g., playing, paused, stopped).

9. Proxy Pattern:

   • For controlling access to streaming videos based on user subscription.

10. Dependency Inversion Principle (SOLID):

    • High-level modules (e.g., User Interface) should not depend on low-level modules but both should depend on abstractions.

11. Open/Closed Principle (SOLID):

    • Classes should be open for extension but closed for modification.

Algorithm Involved:

1. Recommendation Algorithm:

   • Algorithms for providing personalized recommendations based on user behavior and preferences.

Diagrams

Code:

class VideoCatalog {
    private static VideoCatalog instance;
    private List<Video> videos;

    private VideoCatalog() {
        videos = new ArrayList<>();
    }

    public static VideoCatalog getInstance() {
        if (instance == null) {
            instance = new VideoCatalog();
        }
        return instance;
    }

    public void addVideo(Video video) {
        videos.add(video);
    }

    public Video getVideo(String videoId) {
        for (Video video : videos) {
            if (video.getId().equals(videoId)) {
                return video;
            }
        }
        return null;
    }
}

interface VideoFactory {
    Video createVideo(String type, String title);
}

class MovieFactory implements VideoFactory {
    public Video createVideo(String title) {
        return new Movie(title);
    }
}

class TVShowFactory implements VideoFactory {
    public Video createVideo(String title) {
        return new TVShow(title);
    }
}

interface Observer {
    void update(String message);
}

class User implements Observer {
    private String userId;

    public User(String userId) {
        this.userId = userId;
    }

    public void update(String message) {
        System.out.println("User " + userId + " received update: " + message);
    }
}

class VideoNotifier {
    private List<Observer> observers;

    public VideoNotifier() {
        observers = new ArrayList<>();
    }

    public void addObserver(Observer observer) {
        observers.add(observer);
    }

    public void notifyObservers(String message) {
        for (Observer observer : observers) {
            observer.update(message);
        }
    }
}

interface RecommendationStrategy {
    List<Video> recommendVideos(User user);
}

class PopularVideosStrategy implements RecommendationStrategy {
    public List<Video> recommendVideos(User user) {
        // Implementation details...
        return null;
    }
}

class PersonalizedVideosStrategy implements RecommendationStrategy {
    public List<Video> recommendVideos(User user) {
        // Implementation details...
        return null;
    }
}

interface VideoCommand {
    void execute();
}

class AddToWatchlistCommand implements VideoCommand {
    private Video video;

    public AddToWatchlistCommand(Video video) {
        this.video = video;
    }

    public void execute() {
        video.addToWatchlist();
    }
}

class RemoveFromWatchlistCommand implements VideoCommand {
    private Video video;

    public RemoveFromWatchlistCommand(Video video) {
        this.video = video;
    }

    public void execute() {
        video.removeFromWatchlist();
    }
}

abstract class VideoDecorator implements Video {
    protected Video decoratedVideo;

    public VideoDecorator(Video decoratedVideo) {
        this.decoratedVideo = decoratedVideo;
    }

    public void play() {
        decoratedVideo.play();
    }
}

class SubtitledVideo extends VideoDecorator {
    public SubtitledVideo(Video decoratedVideo) {
        super(decoratedVideo);
    }

    public void play() {
        // Add subtitle logic...
        System.out.println("Playing subtitled video...");
        super.play();
    }
}

class HDVideo extends VideoDecorator {
    public HDVideo(Video decoratedVideo) {
        super(decoratedVideo);
    }

    public void play() {
        // Add HD quality logic...
        System.out.println("Playing HD video...");
        super.play();
    }
}

interface Playlist extends Video {
    void addVideo(Video video);

    void removeVideo(Video video);
}

class BasicPlaylist implements Playlist {
    private List<Video> videos;

    public BasicPlaylist() {
        videos = new ArrayList<>();
    }

    public void addVideo(Video video) {
        videos.add(video);
    }

    public void removeVideo(Video video) {
        videos.remove(video);
    }

    public void play() {
        System.out.println("Playing all videos in the playlist...");
        for (Video video : videos) {
            video.play();
        }
    }
}

interface VideoState {
    void play();

    void pause();

    void stop();
}

class PlayingState implements VideoState {
    public void play() {
        System.out.println("Video is already playing.");
    }

    public void pause() {
        System.out.println("Pausing video...");
    }

    public void stop() {
        System.out.println("Stopping video...");
    }
}

class PausedState implements VideoState {
    public void play() {
        System.out.println("Resuming video...");
    }

    public void pause() {
        System.out.println("Video is already paused.");
    }

    public void stop() {
        System.out.println("Stopping video...");
    }
}

class StoppedState implements VideoState {
    public void play() {
        System.out.println("Starting to play video...");
    }

    public void pause() {
        System.out.println("Cannot pause. Video is stopped.");
    }

    public void stop() {
        System.out.println("Video is already stopped.");
    }
}

interface Video {
    String getId();

    void play();
}

class StreamingVideo implements Video {
    private String videoId;

    public StreamingVideo(String videoId) {
        this.videoId = videoId;
    }

    public String getId() {
        return videoId;
    }

    public void play() {
        System.out.println("Playing streaming video " + videoId + "...");
    }
}

class VideoProxy implements Video {
    private Video video;
    private User user;

    public VideoProxy(Video video, User user) {
        this.video = video;
        this.user = user;
    }

    public String getId() {
        return video.getId();
    }

    public void play() {
        if (userHasSubscription()) {
            video.play();
        } else {
            System.out.println("User " + user.getId() + " does not have a subscription.");
        }
    }

    private boolean userHasSubscription() {
        // Check user subscription status...
        return true;
    }
}

class Movie implements Video {
    private String title;

    public Movie(String title) {
        this.title = title;
    }

    public String getId() {
        return title.replaceAll("\\s", "_");
    }

    public void play() {
        System.out.println("Playing movie: " + title);
    }

    public void addToWatchlist() {
        System.out.println("Added movie " + title + " to watchlist.");
    }

    public void removeFromWatchlist() {
        System.out.println("Removed movie " + title + " from watchlist.");
    }
}

class TVShow implements Video {
    private String title

;

    public TVShow(String title) {
        this.title = title;
    }

    public String getId() {
        return title.replaceAll("\\s", "_");
    }

    public void play() {
        System.out.println("Playing TV show: " + title);
    }

    public void addToWatchlist() {
        System.out.println("Added TV show " + title + " to watchlist.");
    }

    public void removeFromWatchlist() {
        System.out.println("Removed TV show " + title + " from watchlist.");
    }
}

class AmazonPrimeVideo {
    public static void main(String[] args) {
        // Implementation details...
        User user1 = new User("user1");
        User user2 = new User("user2");

        VideoCatalog videoCatalog = VideoCatalog.getInstance();

        Video movie = new Movie("Inception");
        Video tvShow = new TVShow("Breaking Bad");

        videoCatalog.addVideo(movie);
        videoCatalog.addVideo(tvShow);

        VideoNotifier videoNotifier = new VideoNotifier();
        videoNotifier.addObserver(user1);
        videoNotifier.addObserver(user2);

        videoNotifier.notifyObservers("New video available!");

        RecommendationStrategy strategy = new PopularVideosStrategy();
        List<Video> recommendations = strategy.recommendVideos(user1);
        System.out.println("Recommendations for user1: " + recommendations);

        VideoCommand addToWatchlistCommand = new AddToWatchlistCommand(movie);
        addToWatchlistCommand.execute();

        VideoCommand removeFromWatchlistCommand = new RemoveFromWatchlistCommand(tvShow);
        removeFromWatchlistCommand.execute();

        VideoDecorator subtitledMovie = new SubtitledVideo(movie);
        subtitledMovie.play();

        VideoDecorator hdTvShow = new HDVideo(tvShow);
        hdTvShow.play();

        Playlist playlist = new BasicPlaylist();
        playlist.addVideo(movie);
        playlist.addVideo(tvShow);
        playlist.play();

        VideoState playingState = new PlayingState();
        playingState.play();
        playingState.pause();
        playingState.stop();

        Video video = new StreamingVideo("xyz123");
        VideoProxy videoProxy = new VideoProxy(video, user1);
        videoProxy.play();
    }
}

This contains a basic implementation of the Amazon Prime Video system with explanations of design patterns and principles used. Adjustments and additional implementations can be made based on specific requirements and features.