Coupang Machine Coding Round - They were expecting full running code to be completed in two hours

System Design Exercise: Product Bidding Platform (Simplified)
Objective
Design a basic simulation of a product bidding/auction system using object-oriented programming principles. The goal is to evaluate your ability to structure code in a modular, extensible, and logically organized way.
Scenario
You are tasked with designing a simplified bidding platform where:

• Users can register.
• Sellers can post products with a minimum price.
• Buyers can place bids on available products.
• Buyers can withdraw their bids.
• Sellers can view all bids in descending order.
• Sellers can accept the highest bid and complete the sale.
• Product listings are removed after a successful sale.
  This is a simulation, not a real application. You are not expected to write actual APIs or integrate with databases.

Implementation Guidelines
Driver Class (Main Class)

• Acts as the entry point of the application.
• Simulates the frontend using print statements.
• No need to take input from the console.
• Should invoke methods from service classes to simulate flows.

Data Storage

• Use in-memory variables (e.g., arrays, lists, maps) to simulate a database.
• No need to use MySQL or any external storage.
  Class Design Expectations
• Structure your code into POJOs, services, and repositories (if needed).
• Focus on modularity and separation of concerns.
• Avoid unnecessary complexity like interfaces or inheritance unless absolutely needed.
• Ensure that dependencies between classes are logical and minimal.

Functional Requirements

1. User Registration

• Simulate adding users to the system. Done
• Users can act as both buyers and sellers. Done

2. Post Product for Sale

• A seller can create a product listing with:
• Product name/description
• Minimum price
  Product should be marked as "active" and available for bidding.

3. Place Bid. -done

• A buyer can place a bid on an active product.
• The bid amount must be greater than or equal to the minimum price.
• Multiple buyers can bid on the same product.
• A buyer can update their bid (replace their previous bid with a new one).
• Validate that:
• The product exists and is active. —— Done
• The bid meets the minimum price requirement. — Done
• A seller cannot bid on their own product. — Done

4. Withdraw Bid

• A buyer can remove their bid from a product.
• If no bid exists for that buyer-product combination, print an appropriate message.

5. View Bids (Seller Only)

• The seller can view all bids for their product
• Bids should be displayed in descending order (highest to lowest). —————PENDING
• Display bidder information and bid amounts.

6. Accept Bid and Complete Sale

• The seller can accept a bid (typically the highest one).
• Once accepted:
• Mark the product as "sold".
• Remove the product from active listings.
• Record the sale transaction (buyer, seller, final price).
• Optionally, notify other bidders that the product has been sold.

Evaluation Criteria
You will be evaluated on:

• Class Segregation: Are responsibilities clearly divided among classes?
• Modularity: Can the system be extended easily?
• Logical Dependencies: Are classes dependent on the right components?
• Code Readability: Is the code easy to follow?
• System Thinking: Does the solution simulate realistic behavior?
  • Edge Case Handling: How well does the system handle invalid operations?

Bonus Functionalities (Optional)

• Allow sellers to set a reserve price (different from minimum bid).
• Implement automatic sale to highest bidder after a time period.
• Track purchase/sale history per user.
• Allow sellers to reject all bids and relist the product.
• Implement bid notifications (simulated via print statements).
• Support multiple simultaneous active listings per user.
• Add product categories or tags for filtering.

Example Corner Cases to Consider
Case 1: No Bids on a Product

• A seller tries to view bids on their product, but no one has bid yet.
• Expected behavior: Display an appropriate message indicating no bids have been placed.
  Case 2: Bid Below Minimum Price
• A buyer tries to place a bid lower than the minimum price.
• Expected behavior: Reject the bid and notify the user of the minimum price requirement.
  Case 3: Seller Bids on Own Product
• A seller attempts to bid on their own product.
• Expected behavior: Prevent the bid and display an error message.
  Case 4: Bid on Non-existent or Sold Product
• A buyer tries to bid on a product that doesn't exist or has already been sold.
• Expected behavior: Reject the bid with an appropriate error message.
  Case 5: Withdraw Non-existent Bid
• A buyer tries to withdraw a bid they never placed.
• Expected behavior: Display a message indicating no bid exists to withdraw.
  Case 6: Accept Bid on Product with No Bids
• A seller tries to accept a bid when no bids have been placed.
• Expected behavior: Display an error message indicating there are no bids to accept.
  Sample Flow Simulation

=== Product Bidding Platform ===

1. Register Users
   - User1 (Seller)
   - User2 (Buyer)
   - User3 (Buyer)

2. User1 posts Product "Laptop" with minimum price: $500

3. User2 bids $550 on Laptop

4. User3 bids $600 on Laptop

5. User2 updates bid to $650 on Laptop

6. User1 views bids on Laptop:
   - User2: $650-User3:$600

7. User3 withdraws their bid

8. User1 views updated bids:
   - User2: $650

9. User1 accepts User2's bid
   - Product "Laptop" sold to User2 for $650
   - Listing removed from active products

10. User3 tries to bid on Laptop
    - Error: Product no longer available

Suggested Class Structure (High-Level Hint)
Consider organizing your code with classes such as:

• User: Represents a user (can be buyer/seller)
• Product: Represents a product listing with details
• Bid: Represents a bid placed by a buyer on a product
• BiddingService: Handles core bidding logic
• ProductService: Manages product listings
• UserService: Manages user registration
• BiddingPlatformDriver: Main class to simulate the system
  Remember: This is just a suggestion. Feel free to structure your classes as you see fit based on your design principles.