Features Required:

1. Real-time Collaboration: Multiple users should be able to collaborate and edit a document simultaneously in real-time.

2. Document Synchronization: Changes made by one user should be immediately reflected in the document for all other users.

3. User Presence: Users should be able to see the presence of other users currently viewing or editing the document.

4. Cursor Position Tracking: Users should be able to see the cursor positions of other users in real-time.

5. Collaborative Text Editing: Users should be able to add, delete, and modify text in the document collaboratively.

6. Version History: The system should maintain a version history of the document, allowing users to revert to previous versions if needed.

7. Document Sharing: Users should be able to share documents with other users, granting them appropriate access permissions.

8. Access Controls: The system should provide access control mechanisms to manage user permissions for viewing and editing documents.

Design Patterns Involved or Used:

1. Observer Pattern: The Observer pattern can be used to notify users about changes or updates in the document, such as new edits or cursor positions.

2. State Pattern: The State pattern can be used to manage the different states of a document, such as viewing, editing, or locked states, and handle state transitions.

3. Proxy Pattern: The Proxy pattern can be used to manage access controls and permissions for documents, providing a level of indirection and encapsulation for user operations.

4. Command Pattern: The Command pattern can be used to encapsulate document editing operations as commands, allowing for easy undo/redo functionality and collaboration tracking.

Code: Classes Implementation Based on Patterns Mentioned Above

// Observer interface
interface DocumentObserver {
    void update(Document document);
}

// CursorPositionObserver class
class CursorPositionObserver implements DocumentObserver {
    private User user;

    public CursorPositionObserver(User user) {
        this.user = user;
    }

    @Override
    public void update(Document document) {
        // Update cursor position for the user
        System.out.println("Cursor position updated for user " + user.getUsername() + ": " + document.getCursorPosition(user));
    }
}

// Document class
class Document {
    private String content;
    private List<User> collaborators;
    private List<String> versionHistory;
    private Map<User, Integer> cursorPositions;
    private List<DocumentObserver> observers;

    public Document() {
        this.content = "";
        this.collaborators = new ArrayList<>();
        this.versionHistory = new ArrayList<>();
        this.cursorPositions = new HashMap<>();
        this.observers = new ArrayList<>();
    }

    public void editDocument(String newText) {
        // Perform document editing operations
        content = newText;

        // Notify observers about the document update
        notifyObservers();
    }

    public void addCollaborator(User user) {
        collaborators.add(user);
        cursorPositions.put(user, 0);

        // Add cursor position observer for the new collaborator
        CursorPositionObserver observer = new CursorPositionObserver(user);
        observers.add(observer);
    }

    public void removeCollaborator(User user) {
        collaborators.remove(user);
        cursorPositions.remove(user);

        // Remove cursor position observer for the collaborator
        DocumentObserver observer = new CursorPositionObserver(user);
        observers.remove(observer);
    }

    public void updateCursorPosition(User user, int newPosition) {
        cursorPositions.put(user, newPosition);
    }

    public int getCursorPosition(User user) {
        return cursorPositions.getOrDefault(user, 0);
    }

    public void subscribeObserver(DocumentObserver observer) {
        observers.add(observer);
    }

    public void unsubscribeObserver(DocumentObserver observer) {
        observers.remove(observer);
    }

    private void notifyObservers() {
        for (DocumentObserver observer : observers) {
            observer.update(this);
        }
    }

    // Other operations related to document synchronization, version history, and access controls
}

// CollaborativeEditor class
class CollaborativeEditor {
    private Document document;

    public CollaborativeEditor(Document document) {
        this.document = document;
    }

    public void addCollaborator(User user) {
        document.addCollaborator(user);
    }

    public void removeCollaborator(User user) {
        document.removeCollaborator(user);
    }

    public void editDocument(String newText) {
        document.editDocument(newText);
    }

    // Other operations related to real-time collaboration, presence tracking, and cursor position tracking
}

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

        // Create a document
        Document document = new Document();

        // Create a collaborative editor
        CollaborativeEditor collaborativeEditor = new CollaborativeEditor(document);

        // Add collaborators to the document
        collaborativeEditor.addCollaborator(user1);
        collaborativeEditor.addCollaborator(user2);

        // Edit the document
        collaborativeEditor.editDocument("This is the edited document content.");

        // Update cursor position for user1
        document.updateCursorPosition(user1, 10);

        // Update cursor position for user2
        document.updateCursorPosition(user2, 15);
    }
}

Introduced the DocumentObserver interface and implemented the CursorPositionObserver class, which observes and updates the cursor positions for each user in real-time. The Document class now maintains a list of observers and notifies them about updates to the document.

Additionally, the updateCursorPosition method has been added to the Document class to update the cursor position of each user. This is demonstrated in the main method by updating the cursor positions for user1 and user2 after editing the document.

These changes enhance the collaborative experience by allowing users to see the cursor positions of others as they edit the document in real-time.

Please note that this is still a simplified example, and a complete implementation of a real-time collaborative document editing platform involves more complex components, such as real-time synchronization algorithms, conflict resolution strategies, rich text editing capabilities, and network protocols for real-time communication.