Java Concurrency Interview Questions and Answers for 10 years experience

1. What is the difference between process and thread?

   • Answer: A process is an independent, self-contained execution environment, while a thread is a lightweight unit of execution within a process. Processes have separate memory spaces, while threads share the same memory space within a process. Processes are heavier to create and manage than threads. Inter-process communication is more complex than inter-thread communication.

2. Explain different ways to achieve thread synchronization in Java.

   • Answer: Java offers several mechanisms for thread synchronization:
     • synchronized keyword: Used with methods or blocks to ensure only one thread can access the critical section at a time.
     • ReentrantLock: Provides more flexibility than `synchronized`, allowing for features like tryLock and fairness settings.
     • Semaphore: Controls access to a limited number of resources.
     • CountDownLatch: Allows one or more threads to wait until a set of operations are completed by other threads.
     • CyclicBarrier: Synchronizes a set of threads until they all reach a common barrier point.
     • ReadWriteLock: Allows multiple reader threads to access a shared resource concurrently, but only one writer thread at a time.

3. What is a deadlock? How can you prevent it?

   • Answer: A deadlock occurs when two or more threads are blocked indefinitely, waiting for each other to release the resources that they need. Prevention strategies include:
     • Avoid circular dependencies: Ensure that threads don't acquire locks in a circular order.
     • Acquire locks in a consistent order: All threads should acquire locks in the same order.
     • Use timeouts: When acquiring locks, use tryLock with timeouts to avoid indefinite blocking.
     • Resource ordering: If multiple resources are needed, acquire them all at once or in a predetermined order.

4. What is the difference between `wait()`, `notify()`, and `notifyAll()`?

   • Answer: These methods are used for inter-thread communication and are called on objects that are locked using `synchronized`.
     • wait(): Causes the current thread to release the lock and wait until another thread calls `notify()` or `notifyAll()` on the same object.
     • notify(): Wakes up a single thread that is waiting on the object's monitor.
     • notifyAll(): Wakes up all threads that are waiting on the object's monitor.

5. Explain the concept of thread starvation.

   • Answer: Thread starvation occurs when a thread is unable to acquire the resources it needs to execute, even though the resources are not perpetually unavailable. This often happens due to unfair scheduling or high contention for resources.

6. What are ConcurrentHashMap and its advantages over HashMap?

   • Answer: `ConcurrentHashMap` is a thread-safe alternative to `HashMap`. It offers better concurrency performance because it uses internal segmentation (multiple locks) to allow concurrent access to different parts of the map. This reduces contention compared to `HashMap` which is synchronized using a single lock, leading to significant performance improvements in multi-threaded environments.

7. Explain the use of Executor framework in Java.

   • Answer: The Executor framework provides a high-level API for managing the lifecycle and execution of threads. It simplifies thread creation, management, and resource reuse. Key components include `ExecutorService`, `ThreadPoolExecutor`, `ScheduledExecutorService`, and `Future`. It allows for the creation of thread pools, handling of task scheduling, and efficient management of thread resources.

8. What are Atomic variables and when to use them?

   • Answer: Atomic variables provide atomic operations on variables, guaranteeing that operations are indivisible and thread-safe. Examples include `AtomicInteger`, `AtomicLong`, `AtomicBoolean`, etc. Use them when you need simple, thread-safe updates to variables without the overhead of locks.

9. What is a thread pool and why is it beneficial?

   • Answer: A thread pool is a collection of reusable threads that are managed by the `ExecutorService`. It avoids the overhead of creating and destroying threads for each task, improving performance and resource management. It efficiently handles a large number of tasks using a limited number of threads.

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