Interview guides / android

Double-charge checkout race: Android Interview Question

Double-charge checkout race interview question with a clear example answer for Android engineers. Practice with AceMobileInterview.

This is a common Android interview topic: Double-charge checkout race.

Below is a concise answer you can adapt in a real interview, then go deeper in the full AceMobileInterview track.

1. 1.Double-tapping Pay charges twice; cart state races. Fix concurrency (≈45–60 min).

   ## Scenario Checkout isn’t single-flight; shared cart mutates unsafely. ## How to investigate - Double-tap Pay with delay in fake API - Watch duplicate requests in Logcat - Trace `Cart.items` mutations ## Planted bug 1: No in-flight guard **Symptom:** Two checkout calls **Root cause:** Button stays enabled; ViewModel method re-entrant **Fix:** `if (isCheckingOut) return`; disable button; use `Mutex` or atomic flag ## Planted bug 2: Unsynchronized mutable list **Symptom:** Wrong totals / rare CME **Root cause:** `MutableList` edited from IO + Main **Fix:** Confine to Main; or `Mutex`; expose immutable snapshots via StateFlow ## Planted bug 3: UI update off main **Symptom:** Occasional threading issues **Root cause:** Updating LiveData/state from wrong context inconsistently **Fix:** Always emit UI state on Main ## Optimization tips - Mention server idempotency keys as the real backstop - `callbackFlow` / structured concurrency over ad-hoc threads ## What to say in the interview - UI disable vs true idempotency - How you’d write a stress test