Java Concurrency Interview Questions and Answers
Freshers / Beginner level questions & answers
Ques 1. What is concurrency in Java?
Concurrency in Java is the ability of multiple threads to execute independently and simultaneously.
Ques 2. Explain the difference between processes and threads.
A process has its own memory space, while threads within a process share the same memory space.
Ques 3. What is the purpose of the synchronized keyword in Java?
The synchronized keyword is used to control access to critical sections of code by allowing only one thread at a time to execute it.
Ques 4. Explain the concept of Thread Safety.
Thread Safety ensures that a piece of code can be safely executed by multiple threads without causing data corruption or unexpected behavior.
Intermediate / 1 to 5 years experienced level questions & answers
Ques 5. What is the Executor framework in Java?
The Executor framework provides a higher-level replacement for managing threads and thread pools.
Ques 6. How does the wait-notify mechanism work in Java?
wait() and notify() are methods used for inter-thread communication. wait() makes a thread wait until another thread invokes notify() or notifyAll().
Ques 7. Explain the concept of the volatile keyword in Java.
The volatile keyword ensures that a variable's value is always read and written from and to the main memory, preventing thread-local caching.
Ques 8. What is the purpose of the java.util.concurrent package?
The java.util.concurrent package provides a framework for concurrent programming, including high-level abstractions such as Executors, Concurrent Collections, and synchronization utilities.
Ques 9. Explain the concept of the ReentrantLock class.
ReentrantLock is an implementation of the Lock interface that allows a thread to acquire the lock multiple times without deadlocking.
Ques 10. What is the purpose of the AtomicInteger class?
AtomicInteger provides atomic operations for integer variables, ensuring that operations are performed in an atomic and thread-safe manner.
Experienced / Expert level questions & answers
Ques 11. Explain the concept of deadlock.
Deadlock occurs when two or more threads wait indefinitely for each other to release the resources, resulting in a situation where no thread can proceed.
Ques 12. What is the ConcurrentHashMap class?
ConcurrentHashMap is a thread-safe implementation of the Map interface, providing high concurrency without the need for explicit synchronization.
Ques 13. Explain the Phaser class in Java.
Phaser is a synchronization barrier that allows a set of threads to wait for each other to reach a common phase before proceeding.
Ques 14. What is the ForkJoinPool in Java?
ForkJoinPool is a specialized implementation of ExecutorService designed for parallel processing and efficient handling of recursive algorithms.
Ques 15. Explain the concept of the CountDownLatch class.
CountDownLatch is a synchronization aid that allows one or more threads to wait until a set of operations in other threads completes.
Ques 16. What is the purpose of the CyclicBarrier class?
CyclicBarrier is a synchronization aid that allows a set of threads to wait for each other to reach a common barrier point before proceeding.
Ques 17. Explain the concept of the Future and FutureTask in Java.
Future represents the result of an asynchronous computation, and FutureTask is a concrete implementation of the Future interface.
Ques 18. What is the purpose of the Semaphore class?
Semaphore is a synchronization aid that allows a fixed number of threads to access a certain resource concurrently.
Ques 19. Explain the concept of the Exchanger class.
Exchanger is a synchronization point at which threads can pair and swap elements within pairs.
Ques 20. What is the purpose of the LockSupport class?
LockSupport provides low-level thread park and unpark mechanisms, useful for building higher-level synchronization abstractions.
Ques 21. Explain the concept of the CopyOnWriteArrayList class.
CopyOnWriteArrayList is a thread-safe variant of ArrayList in which all mutative operations (add, set, etc.) are implemented by making a fresh copy of the underlying array.
Ques 22. What is the purpose of the Phaser class in Java?
Phaser is a synchronization barrier that allows a set of threads to wait for each other to reach a common phase before proceeding.
Ques 23. Explain the concept of the ReadWriteLock interface.
ReadWriteLock provides a pair of associated locks for read and write access, allowing multiple threads to read the data concurrently or a single thread to write it exclusively.
Ques 24. What is the purpose of the ScheduledExecutorService interface?
ScheduledExecutorService is an extension of ExecutorService that supports delayed and periodic task execution.
Ques 25. Explain the concept of the BlockingQueue interface.
BlockingQueue is a queue that supports operations that wait for the queue to become non-empty when retrieving an element and wait for space to become available when storing an element.
Ques 26. What is the purpose of the ForkJoinPool in Java?
ForkJoinPool is a specialized implementation of ExecutorService designed for parallel processing and efficient handling of recursive algorithms.
Ques 27. Explain the concept of the CompletableFuture class.
CompletableFuture is a higher-level replacement for the Future interface, providing a more flexible and composable way to handle asynchronous computations.
Ques 28. What is the purpose of the Lock interface in Java?
Lock interface provides a more flexible and sophisticated way to control access to shared resources compared to the traditional synchronized keyword.
Ques 29. Explain the concept of the StampedLock class.
StampedLock is a read-write lock that supports optimistic reading, allowing for better concurrency in certain scenarios.
Ques 30. What is the purpose of the ThreadLocal class in Java?
ThreadLocal provides thread-local variables, allowing each thread to have its own instance of a variable.
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