Design and Implement a Shopping Cart Object Model for an Online Store

Design and implement a shopping cart for an online store, using object-oriented code in a language of your choice. This is a phone-screen object-oriented design exercise, so the goal is a small, runnable model rather than a distributed system. The cart must let a shopper: - add a product to the cart with a quantity; - change the quantity of a product that is already in the cart; - remove a product from the cart; - list what is in the cart; and - get the cart's total price. Products come from a catalog that the cart can look up by product ID. Each product has at least an ID, a display name and a unit price. Write the classes, implement the operations, and show a short usage example or tests that exercise them. ```hint Separate the product from the line A catalog product and a line in the cart change for different reasons. Decide which object owns the price and which owns the quantity before you write any methods. ``` ### Constraints and Clarifications - Assume one shopper using one cart in one process. Persistence, checkout and payment are out of scope unless the interviewer extends the problem. - Money must be computed exactly; state how you represent it. ### Clarifying Questions - If the shopper adds a product that is already in the cart, should the quantities be merged, or should the call replace or reject the existing line? - Should setting a quantity to zero remove the line, or is zero an error? - Should an unknown product ID, a negative quantity or a non-integer quantity raise an error, be ignored, or be clamped? - Is a product's price fixed when it is added to the cart, or read from the catalog when the total is computed? - Does the cart need to check inventory when items are added, or only at checkout? - Are discounts, coupons or taxes part of the total in this version? ### What a Strong Answer Covers - Clear responsibilities for catalog products, cart lines and the cart itself - An explicit, consistent policy for duplicate adds, zero and invalid quantities, and unknown products - Exact money arithmetic with no floating-point rounding errors - Data structures that give constant-time updates and a predictable listing order - A seam for pricing rules such as discounts or tax that does not require rewriting the cart - A runnable demonstration or tests that reach the edge cases, not only the happy path ### Follow-up Questions - Add promotions such as "buy one, get one free" on a product and a percentage-off coupon on the whole cart. Where does that logic live, and how do you decide the order in which they apply? - A product's catalog price changes while it sits in someone's cart. What does the shopper pay at checkout, and how does your model make that explicit? - The same cart is updated from two browser tabs at once. How do you prevent a lost update? - How would you persist the cart so that it survives the shopper signing out and back in on another device?