Stock Brokerage Platform — OOD Interview Walkthrough

Tier: 4 — Hard Domain: Financial Services / Trading Core Patterns: Command (orders), Observer (price alerts), Strategy (order matching/execution), State (order lifecycle), Factory Method (order creation)

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Phase 1: Requirements & Use Cases

Interviewer: "Design the object-oriented model for an online stock brokerage platform."

Candidate: "Before I write a single class, let me frame the scope precisely. We're building the class-level domain model for a brokerage: the core objects, their responsibilities, and how they interact across the lifecycle of placing, executing, and managing orders. We are not designing a distributed trading engine, not covering market microstructure, and not building an API layer. The boundary is the class diagram and the interactions between in-process objects. Does that match your intent?"

Interviewer: "Yes. Class-level model. Proceed."

Candidate: "Great. Let me identify the actors first.

Primary Actors:

• Investor / User — a registered account holder who places and manages orders and views their portfolio
• Market (External Price Feed) — an external data source that pushes price updates into the system; we model its interface, not its internals
• Broker — the platform itself, acting as coordinator and enforcer of trading rules
• Admin — a privileged actor who can suspend accounts or halt trading on a security

Core Use Cases:

1. Place an order — market, limit, or stop variants; buy or sell
2. Cancel a pending or open order
3. View portfolio — current positions, market value, unrealized P&L
4. View order history — orders across all statuses with filtering
5. Track realized and unrealized profit & loss per position
6. Receive price alerts — notify…