Vibe codingHardvc-g098

Subject Ai code reviewLevel Senior–Staff~20 minCommon in Databases & SQL interviewsIndustries Software development

Question

An AI assistant wrote this Java method to transfer funds between two accounts. Code review passed, tests pass. Under concurrent load, a few accounts end up with balances that don't reconcile.

java

1@Transactional2public void transfer(long fromId, long toId, BigDecimal amt) {3    Account from = repo.findById(fromId);4    Account to   = repo.findById(toId);5    if (from.getBalance().compareTo(amt) < 0)6        throw new InsufficientFundsException();7    from.setBalance(from.getBalance().subtract(amt));8    to.setBalance(to.getBalance().add(amt));9    repo.save(from);10    repo.save(to);11}

What concurrency hazard exists despite `@Transactional`, and how do you fix it?

What a strong answer looks like

Treat the AI’s output as a draft to verify, not an answer to trust. Name the specific flaw and the input that triggers it, say how you’d catch it — tests, edge cases, reading critically — and how you’d re-prompt or decompose to get it right.

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