Design an Expense Management System Object-Oriented Design Interview

Model the core domain objects, state transitions, approval policies, and audit history for an employee expense management system.

Mock Interview with AI

1. Problem Statement

Design an in-memory expense management system. A candidate can submit an expense, it routes through one or more approvers according to a configurable policy, and eventually reaches a reimbursed or rejected state. Start by clarifying the scope and key objects.

2. Live Coding Format

Work in the provided coding workspace while explaining your decisions to the live interviewer. Validation may use inline tests, examples, comments, or a verbal walkthrough.

3. Key Focus Areas

1
Expense entity modeling and state machine (Draft → Submitted → Approved/Rejected → Reimbursed)
2
ApprovalPolicy abstraction for routing rules
3
Idempotency at the service boundary
4
Immutable audit log derived from transitions
5
Separation of entity, policy, and service responsibilities

4. What Strong Candidates Should Demonstrate

✓Model an Expense entity with explicit state machine invariants and guarded transitions.
✓Separate approval routing policy from the Expense entity itself.
✓Design idempotent service operations at the object/service boundary.
✓Produce an immutable append-only audit history from state transitions.
✓Write handwritten tests that cover state transitions and policy routing.

Want interactive feedback?

Practice this problem in the live coding workspace while the interviewer probes your clarification, implementation, trade-offs, and validation.

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Core Concepts

Object-Oriented DesignState MachinePolicy AbstractionAudit Trail