Prev Next

Java / Java Concurrency and Multi Threading in Java 17 and Java 21 Interview questions

What is the difference between ConcurrentHashMap and Hashtable?
ConcurrentHashMapHashtable
Locks per-bin; high concurrency for reads and writes.Synchronizes every method on a single lock for the whole map.
Iterators are weakly consistent and never throw ConcurrentModificationException.The single shared lock itself becomes the concurrency bottleneck.
Disallows null keys and null values.Also disallows null keys and null values.
Modern class, actively used and improved since Java 5/8.Legacy class from Java 1.0, effectively superseded.

The practical difference is performance under concurrent access: because Hashtable serializes every get and put through one lock, it becomes a bottleneck as thread count grows, while ConcurrentHashMap's fine-grained locking lets unrelated operations proceed in parallel. For new code, ConcurrentHashMap is essentially always the right choice.

Why does Hashtable scale poorly under high thread contention?
What do both ConcurrentHashMap and Hashtable have in common regarding nulls?

More Related questions...

What is a thread in Java? What is multithreading in Java? What are the different states of a thread in Java? How do you create a thread in Java? What is the Runnable interface used for? What is a daemon thread in Java? What is thread priority in Java? What is the purpose of the synchronized keyword? What is the purpose of the volatile keyword? What are the types of locks available in java.util.concurrent.locks? Define a race condition in multithreading? What is the Executor framework in Java? What is a virtual thread in Java 21? What is a thread pool? Describe the purpose of the java.util.concurrent package? How does the synchronized keyword achieve mutual exclusion internally? Why should you prefer ReentrantLock over synchronized in some cases? What is the difference between wait() and sleep()? What is the difference between notify() and notifyAll()? How does ConcurrentHashMap achieve thread safety? What is the difference between ConcurrentHashMap and Hashtable? Why should you use CompletableFuture instead of Future? How does the ForkJoinPool execute tasks? What is the difference between Runnable and Callable? How do you handle thread interruption in Java? What happens when a deadlock occurs in a multithreaded application? When should you use CountDownLatch instead of CyclicBarrier? How does compare-and-swap (CAS) work in atomic classes? What is the difference between ExecutorService shutdown() and shutdownNow()? How is the Java Memory Model relevant to concurrent programming? How does ThreadLocal work internally? What is the difference between CopyOnWriteArrayList and a synchronized ArrayList? When would you choose a fixed thread pool over a cached thread pool? How does a BlockingQueue support the producer-consumer pattern? What happens when you call start() twice on the same thread? Explain the lifecycle of a virtual thread in Java 21? Explain the internal working of structured concurrency in Java 21? Why does a synchronized block pin a virtual thread to its carrier thread? What is the difference between ScopedValue and ThreadLocal? Explain the internal working of the ForkJoinPool work-stealing algorithm? How can you optimize code to avoid false sharing? Why does LongAdder outperform AtomicLong under high contention? Why does the ABA problem occur in lock-free CAS-based algorithms? Explain the execution flow of a parallel stream operation? How do you troubleshoot a deadlock in a production Java application? What is the difference between StampedLock and ReadWriteLock? Which is better for high-throughput I/O - platform or virtual threads, and why? How can you optimize a thread pool for mixed CPU-bound and I/O-bound workloads? Explain the internal working of AbstractQueuedSynchronizer (AQS)? Why doesn't increasing the thread pool size always improve throughput?
Show more question and Answers...

Spring

Comments & Discussions

Interviews Questions

About Javapedia.net

Javapedia.net is for Java and J2EE developers, technologist and college students who prepare of interview. Also this site includes many practical examples.

This site is developed using J2EE technologies by Steve Antony, a senior Developer/lead at one of the logistics based company.

Follow Javapedia updates

contact: javatutorials2016[at]gmail[dot]com

Kindly consider donating for maintaining this website. Thanks.

Copyright © 2026, javapedia.net, all rights reserved. · .

DMCA.com Protection Status