JAVA · MEDIUM CODEX

Java Interview Questions — Medium

Q: What is the difference between fail-fast and fail-safe iterators?

Fail-Fast: Mechanism: Operates directly on the collection's structure. If the collection is structurally modified (added/removed) after the iterator is created, it immediately throws ConcurrentModific

Q: Explain how ConcurrentHashMap achieves high concurrency in Java 8.

Locking Mechanism: Instead of locking the entire map (like Hashtable or Collections.synchronizedMap), it uses Node-level synchronization using CAS (Compare-And-Swap) for empty buckets and locks only t

Q: Explain the difference between ExecutorService.submit() and execute().

execute(): Defined in the Executor interface. Accepts a Runnable task, returns void (fire-and-forget), and propagates unhandled exceptions directly to the thread's uncaught exception handler. submit()

Q: Detail the parameters of ThreadPoolExecutor.

corePoolSize: The minimum number of threads kept alive in the pool, even if they are idle. maximumPoolSize: The maximum number of active threads allowed to run concurrently in the pool. keepAliveTime:

Q: What are the different types of thread pools created by the Executors utility?

Fixed Thread Pool (newFixedThreadPool(n)): Uses a fixed number of threads and an unbounded queue. Cached Thread Pool (newCachedThreadPool()): Spawns new threads dynamically as needed, reclaiming idle

Real-world mechanisms, state management, edge cases, performance trade-offs, and practical coding.

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Q1What is the difference between fail-fast and fail-safe iterators?

Fail-Fast:
- Mechanism: Operates directly on the collection's structure. If the collection is structurally modified (added/removed) after the iterator is created, it immediately throws ConcurrentModificationException using a modCount check.
- Examples: ArrayList, HashMap, HashSet iterators.
Fail-Safe (Weakly Consistent):
- Mechanism: Operates on a clone or snapshot of the collection, or utilizes lock-free concurrent structures. It does not throw exceptions if modifications occur during iteration.
- Examples: CopyOnWriteArrayList, ConcurrentHashMap iterators.

Q2Explain how ConcurrentHashMap achieves high concurrency in Java 8.

Locking Mechanism: Instead of locking the entire map (like Hashtable or Collections.synchronizedMap), it uses Node-level synchronization using CAS (Compare-And-Swap) for empty buckets and locks only the head node of a specific bucket segment during write operations.
Concurrency Level: Allows concurrent read operations without locking and locks individual buckets during writes, enabling high throughput under heavy multithreaded loads.

Q3Explain the difference between ExecutorService.submit() and execute().

execute(): Defined in the Executor interface. Accepts a Runnable task, returns void (fire-and-forget), and propagates unhandled exceptions directly to the thread's uncaught exception handler.
submit(): Defined in ExecutorService. Accepts Runnable or Callable tasks, returns a Future object wrapping the execution state, and swallows exceptions, returning them inside the Future.get() call.

Q4Detail the parameters of ThreadPoolExecutor.

corePoolSize: The minimum number of threads kept alive in the pool, even if they are idle.
maximumPoolSize: The maximum number of active threads allowed to run concurrently in the pool.
keepAliveTime: The maximum duration that excess idle threads (beyond core size) wait for new tasks before terminating.
workQueue: The queue (BlockingQueue) holding tasks before they are picked up by active threads (e.g., LinkedBlockingQueue, SynchronousQueue).

Q5What are the different types of thread pools created by the Executors utility?

Fixed Thread Pool (newFixedThreadPool(n)): Uses a fixed number of threads and an unbounded queue.
Cached Thread Pool (newCachedThreadPool()): Spawns new threads dynamically as needed, reclaiming idle threads after 60 seconds; uses a SynchronousQueue (no storage capacity).
Single Thread Executor (newSingleThreadExecutor()): Spawns exactly one worker thread executing tasks sequentially in FIFO order.
Scheduled Thread Pool (newScheduledThreadPool(n)): Scheduled to execute commands after a delay or periodically.

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