Java interviews and campus tests lean heavily on object oriented behaviour rather than API recall. This set covers inheritance and polymorphism, interfaces and abstract classes, the collections framework, exception handling, multithreading, string handling, and JVM basics such as garbage collection. Output based questions are included because they expose the gaps that theory questions hide.
What is the behavior of ReentrantReadWriteLock when multiple threads perform read operations?
Answer: B
ReentrantReadWriteLock allows multiple threads to acquire the read lock simultaneously, but only one thread can hold the write lock. This improves concurrency for read-heavy workloads.
Q.22Hard
Which scenario can lead to livelock in multithreading?
Answer: A
Livelock occurs when threads are not blocked but continuously change state in response to each other (like two people trying to pass each other), preventing progress. This differs from deadlock where threads are blocked.
Q.23Hard
In a ForkJoinPool, what is the primary advantage over ExecutorService for recursive tasks?
Answer: B
ForkJoinPool uses a work-stealing algorithm where idle threads can 'steal' tasks from busy threads' queues, providing better load balancing for divide-and-conquer problems.
Q.24Hard
What is the output of the following code? Thread t = new Thread(() -> { throw new RuntimeException("Error"); }); t.setUncaughtExceptionHandler((thread, ex) -> System.out.println("Caught")); t.start();
Answer: A
The UncaughtExceptionHandler is invoked when an exception is thrown in a thread and not caught. It will print 'Caught' before the thread terminates.
Q.25Hard
Which statement about StampedLock is TRUE?
Answer: B
StampedLock provides optimistic reads that don't require acquiring a lock. If the data changes during an optimistic read, validation fails and a pessimistic lock can be acquired.
Q.26Hard
In the context of Java 21 Virtual Threads, what is a major limitation of traditional threading that Virtual Threads solve?
Answer: B
Virtual Threads are lightweight and can be created in large numbers (millions) with minimal memory footprint, unlike platform threads which are heavy OS-level constructs. This solves scalability issues in high-concurrency applications.
Q.27Hard
Consider a ThreadLocal variable initialized in a thread pool executor with 10 threads. If the same thread is reused from the pool for a different task, what is the state of its ThreadLocal variable?
Answer: B
ThreadLocal values persist across task executions in the same thread. ThreadPools reuse threads, so previous ThreadLocal values remain unless explicitly removed. This can cause data leakage. Developers must call remove() to clean up.
Q.28Hard
A high-frequency trading system uses AtomicInteger for concurrent counter updates. However, performance degrades as more threads access the same counter. What is the primary cause and best solution?
Answer: A
AtomicInteger suffers from false sharing when multiple threads frequently update adjacent memory locations on the same CPU cache line. LongAdder uses striped updates across multiple cells, reducing contention. This is a 2024-25 performance optimization best practice for high-concurrency scenarios.
