Felt easy at first glance and that's kind of the danger.
Start by clarifying requirements and constraints, then design a simple Account class with deposit and withdraw methods that enforce the non-negative balance rule. Discuss trade-offs around concurrency, error handling, and scalability, and outline how you would test the implementation.
Pro tip: Mention that in a real banking system, operations must be atomic and thread-safe; even for a basic design, bring up locking or transactional semantics to show you think beyond the happy path.
Ask about expected scale, concurrency needs, error handling, and whether accounts are identified by ID. Confirm that only deposit and withdrawal are needed, and that negative balances are disallowed.
Define an Account class with fields like accountId and balance, and methods deposit(amount) and withdraw(amount). Specify that withdraw returns a boolean or throws an exception if insufficient funds.
Write pseudocode or actual code for deposit (add to balance) and withdraw (check if balance >= amount before subtracting). Include input validation for positive amounts and handle edge cases like zero or negative values.
Discuss how to make operations thread-safe using locks, synchronized methods, or atomic operations. Mention that in a distributed system, you would need transactions or a database with ACID guarantees.
Outline unit tests for deposit, withdrawal, insufficient funds, and concurrent access. Briefly mention possible extensions like transaction history, interest calculation, or multi-account transfers.
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