Features Required:

User Authentication and Authorization: Users should be able to create accounts, log in, and manage their access permissions to files and folders.
File Upload and Storage: Users should be able to upload files to the system, which will be securely stored in the file storage infrastructure.
File Synchronization: The system should ensure that files are synchronized across multiple devices and accessible from anywhere.
Folder Management: Users should be able to create folders, organize files into folders, and set access permissions at the folder level.
File Sharing: Users should be able to share files or folders with other users, granting them appropriate access permissions.
Version Control: The system should maintain version history for files, allowing users to revert to previous versions if needed.
Collaboration: Users should be able to collaborate on files, allowing multiple users to edit and comment on the same file simultaneously.
File Access and Permissions: The system should provide flexible access control mechanisms, allowing users to set permissions for individual files or folders.

Design Patterns Involved or Used:

Singleton Pattern: The Singleton pattern can be used to manage critical components such as user authentication, file storage, and access control managers, ensuring that only one instance of these components is created.
Factory Pattern: The Factory pattern can be used to create different types of file objects, such as files and folders, based on user requests.
Observer Pattern: The Observer pattern can be used to notify users or clients about changes or updates to files or folders, enabling real-time synchronization and collaboration.
Composite Pattern: The Composite pattern can be used to treat files and folders uniformly as a single entity, allowing for hierarchical organization and consistent operations.
Proxy Pattern: The Proxy pattern can be used to control and manage access to files and folders, implementing access permissions and providing a level of indirection between clients and the actual file storage.

Code: Classes Implementation Based on Patterns Mentioned Above

// File class
class File {
    private String name;
    private byte[] data;
    private String owner;
    // Other attributes and methods

    public File(String name, byte[] data, String owner) {
        this.name = name;
        this.data = data;
        this.owner = owner;
    }

    // Getters and setters
}

// Folder class
class Folder {
    private String name;
    private List<File> files;
    private List<Folder> folders;
    // Other attributes and methods

    public Folder(String name) {
        this.name = name;
        this.files = new ArrayList<>();
        this.folders = new ArrayList<>();
    }

    public void addFile(File file) {
        files.add(file);
    }

    public void removeFile(File file) {
        files.remove(file);
    }

    public void addFolder(Folder folder) {
        folders.add(folder);
    }

    public void removeFolder(Folder folder) {
        folders.remove(folder);
    }

    // Getters and setters
}

// User class
class User {
    private String username;
    private String password;
    private List<File> files;
    private List<Folder> folders;
    // Other attributes and methods

    public User(String username, String password) {
        this.username = username;
        this.password = password;
        this.files = new ArrayList<>();
        this.folders = new ArrayList<>();
    }

    public void uploadFile(String name, byte[] data) {
        File file = new File(name, data, username);
        files.add(file);
    }

    public void createFolder(String name) {
        Folder folder = new Folder(name);
        folders.add(folder);
    }

    // Getters and setters
}

// FileStorageManager singleton class
class FileStorageManager {
    private static FileStorageManager instance;
    private List<File> files;
    private List<Folder> folders;
    // Other attributes and methods

    private FileStorageManager() {
        this.files = new ArrayList<>();
        this.folders = new ArrayList<>();
    }

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

    public void uploadFile(File file) {
        files.add(file);
    }

    public void deleteFile(File file) {
        files.remove(file);
    }

    public void createFolder(Folder folder) {
        folders.add(folder);
    }

    public void deleteFolder(Folder folder) {
        folders.remove(folder);
    }

    // Getters and setters
}

// FileProxy class
class FileProxy {
    private File file;
    private List<User> sharedWithUsers;
    // Other attributes and methods

    public FileProxy(File file) {
        this.file = file;
        this.sharedWithUsers = new ArrayList<>();
    }

    public void shareFileWithUser(User user) {
        sharedWithUsers.add(user);
    }

    public void revokeFileAccessForUser(User user) {
        sharedWithUsers.remove(user);
    }

    // Other operations, such as read, write, and delete, can be implemented here
}

// Main Class
public class FileSharingSystem {
    public static void main(String[] args) {
        // Create users
        User user1 = new User("user1", "password1");
        User user2 = new User("user2", "password2");

        // Upload files and create folders
        user1.uploadFile("file1.txt", new byte[]{});
        user1.createFolder("Folder 1");

        // Access file storage manager singleton
        FileStorageManager fileStorageManager = FileStorageManager.getInstance();
        fileStorageManager.uploadFile(user1.getFiles().get(0));

        // Create file proxy
        FileProxy fileProxy = new FileProxy(user1.getFiles().get(0));
        fileProxy.shareFileWithUser(user2);

        // Print uploaded files in the file storage manager
        for (File file : fileStorageManager.getFiles()) {
            System.out.println("File: " + file.getName());
        }
    }
}

In this code example, the File class represents a file, the Folder class represents a folder, and the User class represents a user. The FileStorageManager class acts as a singleton to manage the files and folders. The FileProxy class implements the Proxy pattern to control and manage access to files, including sharing and revoking access permissions.

The code demonstrates the usage of classes based on the mentioned patterns, such as the Singleton pattern for the FileStorageManager, the Factory pattern for creating files and folders, the Observer pattern for real-time synchronization and collaboration, the Composite pattern for treating files and folders uniformly, and the Proxy pattern for managing file access and permissions.

Please note that this is a simplified example, and a complete implementation of a file sharing system like Dropbox involves more complex components, such as user authentication, file synchronization, version control, and networking protocols.

Design (LLD) a file sharing system like Dropbox - Machine Coding