Sealed Interfaces + Exhaustive Switch: The Pattern the Senior Java Interview Expects (opens in new tab)

Six progressively harder questions — from exhaustiveness guarantees and non-sealed subtypes to generic bounds and cross-package visibility. Everything a senior engineer is expected to know cold in 2026. Sealed interfaces landed as a stable feature in Java 17 (JEP 409) and became production baseline in Java 21 once pattern matching for switch was finalized in JEP 441. By …