Features Required:

User Registration and Authentication: Users should be able to create accounts, log in, and authenticate themselves to access the mentorship platform. This involves securely storing user credentials, handling registration, and providing a seamless login process.
Profile Management: Users (mentors and mentees) should be able to create and update their profiles. This includes personal information (name, contact details, etc.), skills, expertise areas, educational background, work experience, and any other relevant details.
Mentor Dashboard: Mentors should have a dedicated dashboard where they can manage their mentorship sessions. They should be able to set their availability, review mentee requests, accept or reject requests, and schedule sessions.
Mentee Dashboard: Mentees should have a dashboard to interact with the platform. They should be able to search for mentors based on various criteria, send mentorship requests, view mentor profiles, and manage their mentorship sessions.
Mentorship Requests: Mentees should be able to send mentorship requests to specific mentors based on their expertise areas. Mentors should receive these requests and have the option to accept or reject them.
Mentorship Matching: The platform should use intelligent algorithms to match mentees with suitable mentors. This involves analyzing user profiles, skills, preferences, and availability to ensure effective mentor-mentee pairings.
Meeting Sessions: The platform should provide tools for scheduling and conducting mentorship sessions. This includes integrating video conferencing capabilities, allowing in-app messaging, and sending notifications for upcoming sessions.
Payments: The platform should support payment processing for trial sessions and mentor subscriptions. This involves securely handling payment information, processing transactions, generating invoices, and providing payment history.
Subscription Management: Mentees should have the option to purchase mentor subscriptions for a specified duration (e.g., n months). The platform should handle subscription details, renewal, cancellation, and associated billing.
Progress Tracking: Users should be able to track the progress of their mentorship relationships. This involves setting goals, tracking achievements, reviewing session history, and receiving feedback.

Design Patterns Involved or Used:

Model-View-Controller (MVC) Pattern: Implement a clear separation between data models, user interfaces, and application logic. The model represents data and business logic, the view handles presentation, and the controller manages user interactions.
Observer Pattern: Implement a notification system to keep users informed about new mentorship requests, session updates, payment notifications, and subscription changes.
Factory Pattern: Create user objects (mentors, mentees), mentorship request objects, and subscription objects using factory methods. This promotes encapsulation and flexibility.
Singleton Pattern: Ensure that critical classes like user authentication managers or mentorship request managers have a single shared instance to manage resources efficiently.
Proxy Pattern: Use proxies to manage communication between the application and external services like video call platforms and payment gateways, ensuring secure and efficient interactions.
Command Pattern: Encapsulate actions such as sending messages, scheduling sessions, processing payments, and managing subscriptions into command objects. This promotes decoupling and reusability.
Publish-Subscribe Pattern: Implement a publish-subscribe mechanism for notifications, where users subscribe to specific events (mentorship requests, session updates) and receive timely notifications.
Decorator Pattern: Enhance user profiles with additional features such as skills, preferences, and subscription status. Use decorators to dynamically add or modify profile attributes.
Strategy Pattern: Implement different matching algorithms for pairing mentees with mentors. Use the strategy pattern for payment processing, allowing flexible integration with various payment gateways.
State Pattern: Manage the different states of mentorship interactions (request, ongoing, completed), meeting sessions (scheduled, ongoing, completed), and subscription management (active, expired).

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

// User class
class User {
    private String userId;
    private String username;
    private String email;
    private List<String> skills;
    private boolean isMentor;
    private Subscription subscription;
    // Other attributes and methods

    public User(String userId, String username, String email, boolean isMentor) {
        this.userId = userId;
        this.username = username;
        this.email = email;
        this.skills = new ArrayList<>();
        this.isMentor = isMentor;
        this.subscription = new FreeSubscription(); // Default subscription
    }

    // Getters and setters
    // Other user-related methods
}

// MentorshipRequest class
class MentorshipRequest {
    private String requestId;
    private User mentee;
    private User mentor;
    private String areaOfGuidance;
    private LocalDateTime timestamp;
    private RequestStatus status;
    // Other attributes and methods

    public MentorshipRequest(String requestId, User mentee, User mentor, String areaOfGuidance) {
        this.requestId = requestId;
        this.mentee = mentee;
        this.mentor = mentor;
        this.areaOfGuidance = areaOfGuidance;
        this.timestamp = LocalDateTime.now();
        this.status = RequestStatus.PENDING;
    }

    // Getters and setters
    // Other mentorship request-related methods
}

// RequestStatus enum
enum RequestStatus {
    PENDING, ACCEPTED, REJECTED
}

// MentorshipRequestManager class
class MentorshipRequestManager {
    private List<MentorshipRequest> mentorshipRequests;
    // Other attributes and methods

    public MentorshipRequestManager() {
        this.mentorshipRequests = new ArrayList<>();
    }

    public void createMentorshipRequest(MentorshipRequest request) {
        // Add the mentorship request to the list
    }

    public List<MentorshipRequest> getMentorshipRequests(User user) {
        // Retrieve mentorship requests based on user (mentee or mentor)
    }

    public void updateRequestStatus(MentorshipRequest request, RequestStatus status) {
        // Update the status of the mentorship request
    }

    // Other mentorship request management methods
}

// MeetingSession class
class MeetingSession {
    private String sessionId;
    private User mentor;
    private User mentee;
    private LocalDateTime startTime;
    private LocalDateTime endTime;
    private MeetingStatus status;
    // Other attributes and methods

    public MeetingSession(String sessionId, User mentor, User mentee, LocalDateTime startTime) {
        this.sessionId = sessionId;
        this.mentor = mentor;
        this.mentee = mentee;
        this.startTime = startTime;
        this.status = MeetingStatus.SCHEDULED;
    }

    // Getters and setters
    // Other meeting session-related methods
}

// MeetingStatus enum
enum MeetingStatus {
    SCHEDULED, ONGOING, COMPLETED
}

// MeetingSessionManager class
class MeetingSessionManager {
    private List<MeetingSession> meetingSessions;
    // Other attributes and methods

    public MeetingSessionManager() {
        this.meetingSessions = new ArrayList<>();
    }

    public void scheduleMeetingSession(MeetingSession session) {
        // Add the meeting session to the list
    }

    public List<MeetingSession> getMeetingSessions(User user) {
        // Retrieve meeting sessions based on user (mentor or mentee)
    }

    public void updateMeetingStatus(Me

etingSession session, MeetingStatus status) {
        // Update the status of the meeting session
    }

    // Other meeting session management methods
}

// Subscription interface (Strategy)
interface Subscription {
    boolean canBookTrialSession();
    boolean canBuySubscription();
    void handleSessionBooking();
    void handleSubscriptionPurchase();
}

// FreeSubscription class (Strategy)
class FreeSubscription implements Subscription {
    @Override
    public boolean canBookTrialSession() {
        return true;
    }

    @Override
    public boolean canBuySubscription() {
        return true;
    }

    @Override
    public void handleSessionBooking() {
        // Implement session booking logic for free subscription
    }

    @Override
    public void handleSubscriptionPurchase() {
        // Implement subscription purchase logic for free subscription
    }
}

// PaidSubscription class (Strategy)
class PaidSubscription implements Subscription {
    private int months;

    public PaidSubscription(int months) {
        this.months = months;
    }

    @Override
    public boolean canBookTrialSession() {
        return false;
    }

    @Override
    public boolean canBuySubscription() {
        return false;
    }

    @Override
    public void handleSessionBooking() {
        // Implement session booking logic for paid subscription
    }

    @Override
    public void handleSubscriptionPurchase() {
        // Implement subscription purchase logic for paid subscription
    }
}

// SubscriptionManager class
class SubscriptionManager {
    private Map<User, Subscription> subscriptions;
    // Other attributes and methods

    public SubscriptionManager() {
        this.subscriptions = new HashMap<>();
    }

    public void addSubscription(User user, Subscription subscription) {
        subscriptions.put(user, subscription);
    }

    public Subscription getSubscription(User user) {
        return subscriptions.get(user);
    }

    // Other subscription management methods
}

// PaymentProcessor interface (Strategy)
interface PaymentProcessor {
    boolean processPayment(User payer, User payee, double amount);
}

// StripePaymentProcessor class (Strategy)
class StripePaymentProcessor implements PaymentProcessor {
    @Override
    public boolean processPayment(User payer, User payee, double amount) {
        // Implement payment processing using the Stripe payment gateway
    }
}

// PayPalPaymentProcessor class (Strategy)
class PayPalPaymentProcessor implements PaymentProcessor {
    @Override
    public boolean processPayment(User payer, User payee, double amount) {
        // Implement payment processing using the PayPal payment gateway
    }
}

// PaymentManager class
class PaymentManager {
    private PaymentProcessor paymentProcessor;
    // Other attributes and methods

    public void setPaymentProcessor(PaymentProcessor paymentProcessor) {
        this.paymentProcessor = paymentProcessor;
    }

    public boolean processPayment(User payer, User payee, double amount) {
        return paymentProcessor.processPayment(payer, payee, amount);
    }

    // Other payment-related methods
}

This is a comprehensive and detailed implementation of the Mentorship platform, incorporating the specified features and design patterns. The provided code outlines key classes and their methods, but further development and refinement are necessary for a complete and functional system.

Please note that this is a simplified example, and a complete implementation of Mentorship Platform involves more complex components.

Issues in Above Design

1️⃣ Mentorship Request Lifecycle Is Not Concurrency Safe

Current

RequestStatus { PENDING, ACCEPTED, REJECTED }

Race Condition

Mentor accepts request
Mentor clicks accept again
Mentee cancels simultaneously
→ Inconsistent final state

Missing

Versioning
CAS / optimistic locking
Idempotency

📌 Interview Expectation

Requests must transition via atomic state machine, not mutable enum updates.

Solution

will be covered in our premium course.

2️⃣ MentorshipRequestManager Is Not a Singleton (Contradiction)

Design Claim

“Singleton Pattern ensures single shared instance”

Reality

new MentorshipRequestManager()

Problem

Multiple instances → duplicate requests
Inconsistent mentor dashboards
Broken invariants

📌 Expected

Managers that enforce global invariants must be singleton or service-backed.

Solution

will be covered in our premium course.

3️⃣ Mentor Availability Is Missing (Critical)

Problem

No availability slots
No time blocking
No conflict resolution

Failure Scenario

Mentor accepts two mentees
Both book same slot
→ Double booking

📌 Interview Insight

Scheduling systems fail at availability locking, not UI.

Solution

will be covered in our premium course.

4️⃣ MeetingSession Scheduling Is Not Atomic

Current

scheduleMeetingSession(session)

Missing Atomicity

Availability lock
Payment authorization
Calendar confirmation
Notification dispatch

📌 Expected

Session scheduling is a transaction across multiple subsystems.

Solution

will be covered in our premium course.

5️⃣ Payment Processing Is Not Idempotent (Money Bug)

Current

processPayment(payer, payee, amount)

Failure

Payment succeeds
Network timeout
Retry happens
→ User charged twice

📌 Absolute Interview Red Flag

Payments must be idempotent using transaction IDs.

Solution

will be covered in our premium course.

6️⃣ Subscription Strategy Is Misused

Current

FreeSubscription / PaidSubscription

Problem

Subscription holds business rules
Subscription lifecycle not modeled (ACTIVE, EXPIRED, GRACE)

📌 Expected

Subscription is an entity with state machine, not just a strategy.

Solution

will be covered in our premium course.

7️⃣ Subscription Expiry Is Not Time-Aware

Missing

Expiration timestamps
Renewal windows
Grace periods
Webhook reconciliation

📌 Interview Question

“What happens when subscription expires mid-session?”

Solution

will be covered in our premium course.

8️⃣ Matching Algorithm Has No Cost Controls

Current

findPotentialMatches(User user, List<User> users)

Issues

O(N) scan
No ranking
No caching
No explainability

📌 Expected

Matching must be ranked, cached, and bounded.

Solution

will be covered in our premium course.

9️⃣ Profile Decorator Is Conceptually Wrong

Problem

Skills & preferences are data
Decorator duplicates user identity
Breaks persistence

📌 Correct Insight

Decorators are for behavior, not profile metadata.

Solution

will be covered in our premium course.

🔟 Meeting Session State Lacks Enforcement

Current

SCHEDULED → ONGOING → COMPLETED

Missing

Late join handling
Auto-expiry
Cancellation penalties

📌 Interview Signal

State machines must enforce business invariants.

Solution

will be covered in our premium course.

1️⃣1️⃣ Notifications Are Fire-and-Forget

Current

Observer / Pub-Sub without durability

Failure

Server crashes
Notification lost
Mentor misses session

📌 Expected

Notifications must be async + retryable + durable.

Solution

will be covered in our premium course.

1️⃣2️⃣ Progress Tracking Has No Ownership Model

Missing

Who can edit goals?
Mentor vs mentee authority
Audit trail

📌 Expected

Progress data must be immutable logs + derived views.

Solution

will be covered in our premium course.

1️⃣3️⃣ No Authorization Boundaries

Missing

Who can see whose sessions?
Mentor-only notes?
Admin overrides?

📌 Interview Expectation

Mentorship platforms require fine-grained ACLs.

Solution

will be covered in our premium course.

1️⃣4️⃣ No Cancellation & Refund Model

Missing

Late cancellation
No-show penalties
Partial refunds

📌 Money + Time = Mandatory Logic

Solution

will be covered in our premium course.

1️⃣5️⃣ No Versioning on Profiles & Requests

Failure

Mentor updates skills
Mentee matches using stale data

📌 Expected

Profile & request entities must be versioned.

Solution

will be covered in our premium course.

Design (LLD) Mentorship Platform like PrePlaced - Machine Coding