Java Multithreading Interview Questions and Answers
Experienced / Expert level questions & answers
Ques 1. What is deadlock in multithreading?
Deadlock is a situation where two or more threads are blocked forever, waiting for each other to release the locks.
Example:
Thread 1 locks resource A and waits for resource B. Thread 2 locks resource B and waits for resource A.
Ques 2. Explain the concept of the wait-notify mechanism.
The wait-notify mechanism is used for inter-thread communication, where one thread can signal the other that a certain condition has been met.
Example:
Using wait(), notify(), and notifyAll() methods in synchronized blocks.
Ques 3. How does the join() method work in Java?
The join() method is used to wait for a thread to die. It causes the current thread to pause execution until the specified thread completes its execution.
Example:
Thread anotherThread = new Thread(() -> {
/* Thread logic */ });
anotherThread.start();
anotherThread.join();
Ques 4. Explain the concept of a race condition.
A race condition occurs when two or more threads access shared data concurrently, and the final outcome depends on the order of execution.
Example:
When two threads increment a shared counter without proper synchronization.
Ques 5. What is the ReentrantLock class in Java?
The ReentrantLock class is a part of the java.util.concurrent package and provides a flexible locking mechanism with the ability to interrupt a thread that is waiting for the lock.
Example:
ReentrantLock lock = new ReentrantLock();
lock.lock();
try { /* Critical section */ }
finally { lock.unlock(); }
Ques 6. Explain the concept of the producer-consumer problem in multithreading.
The producer-consumer problem involves two types of threads: producers that produce data and consumers that consume the data. The challenge is to synchronize their activities.
Example:
Using a shared buffer and synchronization mechanisms to ensure proper communication between producers and consumers.
Ques 7. Explain the concept of thread dumping.
Thread dumping involves generating a snapshot of all the threads' current state and stack traces. It is useful for diagnosing performance or deadlock issues.
Example:
In Java, you can use tools like jstack or VisualVM to generate thread dumps.
Ques 8. Explain the concept of the happens-before relationship in Java.
The happens-before relationship is a partial ordering of memory operations in Java. It ensures that the result of one operation is visible to another, providing a consistent view of shared data among threads.
Example:
Using synchronized blocks establishes a happens-before relationship.
Ques 9. What is the purpose of the Phaser class in Java?
The Phaser class in Java provides a more flexible alternative to the CountDownLatch and CyclicBarrier for synchronizing threads. It supports dynamic registration of threads and phase advancement.
Example:
Phaser phaser = new Phaser();
phaser.register(); // Register the current thread
Ques 10. What is the purpose of the Exchanger class in Java?
The Exchanger class provides a synchronization point at which threads can pair and swap elements within pairs. Each thread presents some object on entry to the exchange method, matches with a partner thread, and receives its partner's object on return.
Example:
Exchangerexchanger = new Exchanger<>();
Ques 11. Explain the concept of the ForkJoinPool in Java.
The ForkJoinPool is a special-purpose ExecutorService designed for parallelizing divide-and-conquer algorithms. It is particularly useful for recursive tasks with multiple subtasks.
Example:
RecursiveTask and RecursiveAction are classes commonly used with ForkJoinPool.
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