Product, Cart, Order classes in LLD - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the main responsibility of the Product class in a shopping system?

The Product class represents an item available for purchase. It holds details like product ID, name, description, price, and stock quantity.

beginner

How does the Cart class typically manage products added by a user?

The Cart class keeps track of products selected by the user along with their quantities. It allows adding, removing, and updating product quantities before checkout.

beginner

What key information does the Order class store after checkout?

The Order class stores details like order ID, list of products with quantities, total price, user information, order status, and timestamps.

intermediate

Why is it important for the Cart class to validate product availability before adding to cart?

Validating product availability ensures the user cannot add more items than are in stock, preventing overselling and improving user experience.

intermediate

Explain the relationship between Product, Cart, and Order classes in a shopping system.

Products are the items available for purchase. The Cart holds selected products temporarily for the user. When the user checks out, the Cart's contents become an Order, which records the purchase details.

Which class is responsible for storing the user's selected items before purchase?

A Cart

B Product

C Order

D Inventory

The Cart class holds the user's selected products before they complete the purchase.

What information does the Product class NOT typically contain?

A Order status

B Product price

C Product description

D Stock quantity

Order status belongs to the Order class, not the Product class.

After checkout, which class records the purchase details?

A Cart

B Product

C User

D Order

The Order class stores purchase details after checkout.

Why should the Cart class check product stock before adding items?

A To delete the product

B To increase product price

C To prevent overselling

D To update user profile

Checking stock prevents users from adding more items than available.

Which class typically contains a list of products with quantities for a purchase?

A Product

B Order

C Payment

D Cart

The Order class contains the list of products and quantities for the completed purchase.

Describe the roles and key attributes of Product, Cart, and Order classes in a shopping system.

Think about how a user selects items, holds them temporarily, and then completes a purchase.

Explain how the Cart class interacts with Product and Order classes during the shopping process.

Consider the flow from browsing products to placing an order.

Practice

1. Which class is responsible for storing the details like product ID, name, and price?

easy

A. Product class

B. Cart class

C. Order class

D. User class

Solution

1. Step 1: Understand the role of Product class

   The Product class stores item details such as ID, name, and price.

2. Step 2: Compare with other classes

   The Cart class holds selected products and quantities, and Order class records purchased items and status, not product details.

3. Final Answer:

   Product class -> Option A

4. Quick Check:

   Product details = Product class [OK]

Hint: Product details belong to Product class only [OK]

Common Mistakes:

• Confusing Cart with Product class
• Thinking Order stores product details
• Assuming User class stores product info

2. Which of the following is the correct way to add a product to a cart in a typical class design?

easy

A. order.addProduct(product, quantity)

B. product.addToCart(cart, quantity)

C. cart.addProduct(product, quantity)

D. cart.createOrder(product, quantity)

Solution

1. Step 1: Identify the class responsible for holding selected products

   The Cart class holds selected products and their quantities before purchase.

2. Step 2: Check method naming conventions

   Adding a product to a cart is typically done by calling a method on the Cart object, like addProduct(product, quantity).

3. Final Answer:

   cart.addProduct(product, quantity) -> Option C

4. Quick Check:

   Adding product to cart = cart.addProduct() [OK]

Hint: Add products via Cart methods, not Product or Order [OK]

Common Mistakes:

• Calling addToCart on Product class
• Using Order class to add products before purchase
• Confusing method names like createOrder in Cart

3. Given the following code snippet, what will be the total cost stored in the Order after checkout?

product1 = Product(id=1, name='Pen', price=2)
product2 = Product(id=2, name='Notebook', price=5)
cart = Cart()
cart.addProduct(product1, 3)
cart.addProduct(product2, 2)
order = Order(cart)
order.checkout()

medium

A. 16

B. 19

C. 10

D. 7

Solution

1. Step 1: Calculate total cost from cart products and quantities

   Pen price = 2, quantity = 3 -> 2 * 3 = 6
   Notebook price = 5, quantity = 2 -> 5 * 2 = 10
   Total = 6 + 10 = 16

2. Step 2: Check if any additional charges or taxes apply

   No extra charges mentioned, so total cost should be 16.

3. Final Answer:

   16 -> Option A

4. Quick Check:

   2*3 + 5*2 = 16 [OK]

Hint: Multiply price by quantity, then sum all products [OK]

Common Mistakes:

• Adding quantities instead of multiplying by price
• Mixing product prices and quantities incorrectly
• Ignoring one product's cost

4. Identify the error in this Order class method that calculates total cost:

class Order:
    def __init__(self, cart):
        self.cart = cart
        self.total = 0
    def calculate_total(self):
        for product, qty in self.cart.items():
            self.total += product.price * qty
        return self.total

medium

A. Returning total instead of printing it

B. Using self.cart.items() instead of self.cart.products.items()

C. Multiplying price by quantity incorrectly

D. Not resetting self.total before calculation

Solution

1. Step 1: Analyze total calculation logic

   The method adds product price times quantity to self.total in a loop.

2. Step 2: Check for accumulation errors

   Since self.total is not reset before calculation, repeated calls will add to previous total, causing incorrect sums.

3. Final Answer:

   Not resetting self.total before calculation -> Option D

4. Quick Check:

   Reset total before sum to avoid accumulation [OK]

Hint: Reset totals before summing to avoid repeated addition errors [OK]

Common Mistakes:

• Assuming cart.items() is invalid without context
• Thinking multiplication is wrong when it is correct
• Confusing return with print for output

5. You want to design a system where a Cart can hold multiple Products with quantities, and an Order records the purchased items and status. Which design choice best supports scalability and clear responsibility?

hard

A. Make Cart store product IDs only; Order stores full product details and quantities

B. Make Cart hold Product objects with quantities; Order copies Cart items and tracks status separately

C. Make Product class hold quantity and add methods to update Cart and Order directly

D. Make Order class inherit from Cart and add status and total cost fields

Solution

1. Step 1: Understand separation of concerns

   Cart should hold selected products and quantities before purchase. Order should record purchased items and status separately.

2. Step 2: Evaluate design options for scalability and clarity

   Make Cart hold Product objects with quantities; Order copies Cart items and tracks status separately keeps Cart holding Product objects with quantities, and Order copies these items to keep a snapshot and track status, which is clean and scalable.

3. Final Answer:

   Make Cart hold Product objects with quantities; Order copies Cart items and tracks status separately -> Option B

4. Quick Check:

   Separate Cart and Order responsibilities for scalability [OK]

Hint: Keep Cart and Order responsibilities separate and clear [OK]

Common Mistakes:

• Making Order inherit Cart causing tight coupling
• Storing only product IDs in Cart losing details
• Putting quantity in Product class mixing concerns