Features Required:

1. User Authentication:

   • Users should be able to sign up, log in, and manage their profiles.

2. Code Editor:

   • Users should be able to create code sessions.

   • Real-time collaborative code editing.

3. Video and Voice Communication:

   • Users should be able to start audio and video sessions.

   • Screen sharing for collaborative coding.

4. Session Management:

   • Managing active code sessions.

   • Inviting other users to join a session.

5. Code Version Control:

   • Recording and managing different versions of code during a session.

   • Ability to roll back to previous code states.

6. Collaborative Whiteboard:

   • A shared whiteboard for drawing diagrams, taking notes, etc.

7. User Roles:

   • Distinction between driver and navigator roles.

8. Real-Time Chat:

   • Users should be able to communicate via chat.

9. Code Review:

   • Review and comment on the shared code.

Design Patterns Involved or Used:

1. Singleton Pattern:

   • Use a singleton pattern for managing user sessions and active code sessions to ensure there is a single point of access.

2. Factory Pattern:

   • Create instances of user sessions and code sessions using factories to encapsulate their creation.

3. Observer Pattern:

   • Implement real-time features like collaborative code editing, chat, and video sessions using the observer pattern to notify participants of changes.

4. State Pattern:

   • Manage different states of a code session such as "editing," "in session," "paused," using state classes.

5. Command Pattern:

   • Implement commands for actions like "share code," "start video," "undo," and "redo" during a code session.

6. Composite Pattern:

   • Organize components of a code session, such as the code editor, whiteboard, and chat, using a composite pattern.

7. Strategy Pattern:

   • Use the strategy pattern for video and voice communication, allowing users to select different communication strategies.

Code: Detailed Implementation of Classes Based on Each Design Pattern Mentioned Above:

// Singleton Pattern
class SessionManager {
    private static SessionManager instance;
    private Map<String, UserSession> activeUserSessions;

    private SessionManager() {
        activeUserSessions = new ConcurrentHashMap<>();
    }

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

    public UserSession createUserSession(User user) {
        UserSession session = new UserSession(user);
        activeUserSessions.put(user.getId(), session);
        return session;
    }

    public UserSession getUserSession(String userId) {
        return activeUserSessions.get(userId);
    }
}

class UserSession {
    private User user;
    private List<CodeSession> activeCodeSessions;
    private List<Collaborator> collaborators;

    public UserSession(User user) {
        this.user = user;
        this.activeCodeSessions = new ArrayList<>();
        this.collaborators = new ArrayList<>();
    }

    public void startCodeSession(CodeSession codeSession, List<User> participants) {
        activeCodeSessions.add(codeSession);
        codeSession.addCollaborators(participants);
    }

    // Other methods for managing code sessions and collaborators
}

class CodeSession {
    private String sessionId;
    private User creator;
    private List<Collaborator> collaborators;
    private CodeEditor codeEditor;

    public CodeSession(User creator, CodeEditor codeEditor) {
        this.sessionId = UUID.randomUUID().toString();
        this.creator = creator;
        this.codeEditor = codeEditor;
        this.collaborators = new ArrayList<>();
    }

    public void addCollaborators(List<User> participants) {
        // Add participants as collaborators
    }

    public void shareCode(String code) {
        codeEditor.addCode(code);
        // Notify collaborators about code changes
    }

    // Other methods for managing code sessions
}

// Observer Pattern
interface Collaborator {
    void update(String message);
}

class User implements Collaborator {
    private String id;
    private String name;

    public User(String id, String name) {
        this.id = id;
        this.name = name;
    }

    @Override
    public void update(String message) {
        // Receive and display messages from the code session
    }
}

// State Pattern
class CodeEditor {
    private CodeSessionState state;

    public void setState(CodeSessionState state) {
        this.state = state;
    }

    public void editCode(String code) {
        state.editCode(code);
    }

    public void undo() {
        state.undo();
    }

    // Other methods for managing code editing
}

interface CodeSessionState {
    void editCode(String code);
    void undo();
}

class EditingState implements CodeSessionState {
    @Override
    public void editCode(String code) {
        // Handle code editing state
    }

    @Override
    public void undo() {
        // Undo code changes
    }
}

// Command Pattern
class CommandExecutor {
    public void execute(Command command) {
        command.execute();
    }
}

interface Command {
    void execute();
}

class ShareCodeCommand implements Command {
    private CodeSession codeSession;
    private String code;

    public ShareCodeCommand(CodeSession codeSession, String code) {
        this.codeSession = codeSession;
        this.code = code;
    }

    @Override
    public void execute() {
        codeSession.shareCode(code);
    }
}

// Composite Pattern
class CodeSessionComponent {
    List<CodeSessionComponent> children;

    public void addComponent(CodeSessionComponent component) {
        children.add(component);
    }

    // Other methods
}

// Strategy Pattern
interface CommunicationStrategy {
    void startCommunication(User user);
}

class VideoCommunication implements CommunicationStrategy {
    @Override
    public void startCommunication(User user) {
        // Start a video session
    }
}

Please note that this code is an outline and doesn't cover the complete implementation of the pair programming platform. You'll need to add more classes and methods to handle features like video and voice communication, chat, and other functionalities. Additionally, you should consider security and error handling aspects as well as the integration of real-time communication libraries for video and voice chats.