Java & the JVM · topic 4 of 6

Multithreading

Threads, locks, executors, the memory model, deadlocks, async.

Questions below are grouped from foundations to advanced. Work top to bottom if the topic is new to you, or jump to a level if you are brushing up.

Questions: 30
Difficulty mix: 5 easy · 12 medium · 13 hard

Topics covered

Threads & Pools9
Locks6
volatile & Memory Model3
Deadlocks3
CompletableFuture3
Atomics2
Virtual Threads1
Cancellation1
Fundamentals1
Coordination1

Foundations

Easy · 5 questions

Definitions and building blocks — start here if the topic is new to you.

Runnable vs Callable vs Future — how do you run a task and get its result back?
Threads & PoolsEasy
What's the difference between calling thread.start() and thread.run()?
Threads & PoolsEasy
Walk through the thread lifecycle states — and which one signals a problem in a thread dump?
Threads & PoolsEasy
Thread.sleep() vs Object.wait() — what's the real difference?
LocksEasy
What is a daemon thread, and why does the JVM exit even while daemon threads are still running?
FundamentalsEasy

Core concepts

Medium · 12 questions

The questions most interviews spend their time on: how it works and when to use it.

What does the volatile keyword guarantee, and what does it NOT?
volatile & Memory ModelMedium
Why use an ExecutorService instead of new Thread(), and which pool type do you pick?
Threads & PoolsMedium
How do corePoolSize, maxPoolSize, the queue and rejection policy interact in ThreadPoolExecutor?
Threads & PoolsMedium
Why must wait() always be called in a while-loop, not an if?
LocksMedium
ReentrantLock vs synchronized — when do you reach for the explicit lock?
LocksMedium
What causes a deadlock, and how do you prevent it in a real codebase?
DeadlocksMedium
How does CompletableFuture let you compose async calls without blocking?
CompletableFutureMedium
How do Atomic classes achieve thread-safety without locks (CAS)?
AtomicsMedium
CountDownLatch vs CyclicBarrier vs Semaphore — which coordination tool when?
LocksMedium
What is a check-then-act race condition, and how do you fix it correctly?
volatile & Memory ModelMedium
thenApply vs thenApplyAsync — which thread actually runs your callback, and why does that matter?
CompletableFutureMedium
How does Thread.interrupt() actually work, and why must you never swallow InterruptedException?
CancellationMedium

Advanced

Hard · 13 questions

Internals, trade-offs and edge cases — useful for senior rounds and follow-ups.

What is the Java Memory Model and the happens-before relationship?
volatile & Memory ModelHard
Why did the double-checked locking singleton need volatile to be correct?
LocksHard
What is false sharing, and how does it silently kill multi-threaded throughput?
AtomicsHard
Deadlock vs livelock vs starvation — how do you tell them apart and fix each?
DeadlocksHard
How does ForkJoinPool's work-stealing work, and what's the parallel-stream pitfall?
Threads & PoolsHard
How do you diagnose high latency or a hang from a production thread dump?
DeadlocksHard
When would you reach for StampedLock's optimistic locking over ReentrantReadWriteLock?
LocksHard
How does thread-pool exhaustion in one service cascade into a system-wide outage, and how does the bulkhead pattern prevent it?
Threads & PoolsHard
Does the 'one thread per request' mental model still hold with virtual threads — and why can ThreadLocal-based caching patterns silently stop working?
Virtual ThreadsHard
How does StructuredTaskScope let you treat a family of subtasks as a single unit of work?
Threads & PoolsHard
How does synchronized interact with virtual-thread carrier pinning, and how did that change in Java 24?
Threads & PoolsHard
What actually happens to the underlying work when CompletableFuture.orTimeout() fires?
CompletableFutureHard
How do you implement a producer-consumer queue with wait() and notifyAll() (not BlockingQueue)?
CoordinationHard

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Topics covered

Foundations

Runnable vs Callable vs Future — how do you run a task and get its result back?

What's the difference between calling thread.start() and thread.run()?

Walk through the thread lifecycle states — and which one signals a problem in a thread dump?

Thread.sleep() vs Object.wait() — what's the real difference?

What is a daemon thread, and why does the JVM exit even while daemon threads are still running?

Core concepts

What does the volatile keyword guarantee, and what does it NOT?

Why use an ExecutorService instead of new Thread(), and which pool type do you pick?

How do corePoolSize, maxPoolSize, the queue and rejection policy interact in ThreadPoolExecutor?

Why must wait() always be called in a while-loop, not an if?

ReentrantLock vs synchronized — when do you reach for the explicit lock?

What causes a deadlock, and how do you prevent it in a real codebase?

How does CompletableFuture let you compose async calls without blocking?

How do Atomic classes achieve thread-safety without locks (CAS)?

CountDownLatch vs CyclicBarrier vs Semaphore — which coordination tool when?

What is a check-then-act race condition, and how do you fix it correctly?

thenApply vs thenApplyAsync — which thread actually runs your callback, and why does that matter?

How does Thread.interrupt() actually work, and why must you never swallow InterruptedException?

Advanced

What is the Java Memory Model and the happens-before relationship?

Why did the double-checked locking singleton need volatile to be correct?

What is false sharing, and how does it silently kill multi-threaded throughput?

Deadlock vs livelock vs starvation — how do you tell them apart and fix each?

How does ForkJoinPool's work-stealing work, and what's the parallel-stream pitfall?

How do you diagnose high latency or a hang from a production thread dump?

When would you reach for StampedLock's optimistic locking over ReentrantReadWriteLock?

How does thread-pool exhaustion in one service cascade into a system-wide outage, and how does the bulkhead pattern prevent it?

Does the 'one thread per request' mental model still hold with virtual threads — and why can ThreadLocal-based caching patterns silently stop working?

How does StructuredTaskScope let you treat a family of subtasks as a single unit of work?

How does synchronized interact with virtual-thread carrier pinning, and how did that change in Java 24?

What actually happens to the underlying work when CompletableFuture.orTimeout() fires?

How do you implement a producer-consumer queue with wait() and notifyAll() (not BlockingQueue)?