Java Interview Questions

HashMap is unsynchronised and can corrupt or infinite-loop under concurrent writes. ConcurrentHashMap uses per-bin locking and CAS so reads are lock-free and writes contend only on the affected bin.

Each thread has its own stack holding frames and primitives; objects live on the shared heap. Escaping references, not allocation size, are what force objects to survive across frames.

Visibility and ordering, not atomicity. A volatile read sees the latest write from any thread, but compound operations such as i++ still need synchronisation or an atomic class.

Checked exceptions model recoverable, caller-actionable conditions; unchecked exceptions model programming errors. Modern codebases lean unchecked and wrap at boundaries rather than propagating checked types everywhere.

Generational collectors split the heap into young and old regions, collecting the young generation frequently and cheaply. G1 and ZGC add region-based, mostly-concurrent collection to keep pause times bounded.