☕ Java Concurrency & Threads — Practice Problem Sets

A structured collection of 50 Java concurrency problems organized by difficulty, both interview preparation and hands-on learning.

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📚 Structure

java-concurrency-practice/
├── beginner/       # Problems 01–15  (Foundations)
├── intermediate/   # Problems 16–25  (Core Concurrency APIs)
└── advanced/       # Problems 26–35  (Core advanced patterns)
└── expert/         # Problems 36–50  (Deep systems design)

Each problem folder contains:

• README.md — Problem description, concepts covered, hints
• src/main/java/ — Skeleton class(es) for you to implement
• src/test/java/ — JUnit 5 tests to validate your solution

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🗺️ Problem Index

🟢 Beginner (01–15)

#	Problem	Key Concept
01	Thread Basics	Thread class, start(), run()
02	Runnable vs Thread	Runnable, lambda threads
03	Synchronized Counter	synchronized, race conditions
04	Producer-Consumer Basic	wait(), notify()
05	Thread Join	join(), ordering guarantees
06	Volatile Keyword	Memory visibility, JMM, stop-flags
07	Daemon Threads	Daemon vs user threads, background services
08	Thread Interruption	Cooperative cancellation, InterruptedException
09	AtomicInteger & CAS	Lock-free programming, Compare-And-Swap
10	ThreadLocal	Per-thread storage, memory-leak pitfall
11	ReentrantLock	Explicit locking, Condition, deadlock-free transfer
12	ReadWriteLock	Concurrent reads, exclusive writes, lock downgrading
13	CountDownLatch	Startup gate, starting-gun pattern, timed await
14	Thread-Safe Singleton	Eager / Synchronized / DCL+volatile / Holder
15	FizzBuzz Threads	4-thread coordination with wait/notifyAll

🟡 Intermediate (16–25)

#	Problem	Classes	Core Concept
16	Semaphore	ConnectionPool, RateLimiter	Permits, bounded concurrency, token-bucket
17	CyclicBarrier	ParallelMatrixMultiplier, PhaseSimulator	Reusable barrier, barrier action, phased execution
18	ExecutorService	TaskDispatcher, PrioritizedExecutor	ThreadPoolExecutor, CallerRunsPolicy, PriorityBlockingQueue
19	Future & Callable	AsyncTaskRunner, CompletablePipeline	Future.get(), timeout, first-completed, CompletableFuture chaining
20	ForkJoinPool	ParallelSum, ParallelMergeSort, ParallelSearch	RecursiveTask, RecursiveAction, work-stealing, divide-and-conquer
21	BlockingQueue Deep Dive	WorkStealingPipeline, DelayedTaskScheduler	Poison-pill shutdown, DelayQueue, Delayed interface
22	Exchanger	DoubleBufferedLogger, GeneticCrossover	Bidirectional handoff, double-buffering pattern
23	Phaser	DynamicWorkerPool, PipelinedDataProcessor	Dynamic registration, onAdvance, arriveAndDeregister
24	StampedLock	OptimisticPoint, StampedCache	Optimistic reads, validate(), tryConvertToWriteLock
25	ConcurrentHashMap	WordFrequencyCounter, ConcurrentInventory	merge, compute, replace CAS loop, parallel bulk ops

🔴 Advanced (26–35)

#	Problem	Skeleton Classes	Core Concept
26	Lock-Free Data Structures	TreiberStack, LockFreeQueue, ABADemonstrator	CAS retry loops, sentinel node, ABA problem
27	Java Memory Model	SafePublicationShowcase, HappensBeforeChain, MemoryVisibilityProbe	happens-before rules, safe publication, volatile
28	Deadlock Detection	ResourceAllocationGraph, DeadlockPreventer, DeadlockDemo	RAG cycle detection, lock ordering, tryLock, ThreadMXBean
29	ScheduledExecutorService	TaskScheduler, CircuitBreaker	scheduleAtFixedRate vs fixedDelay, circuit breaker state machine
30	Custom ThreadPoolExecutor	InstrumentedThreadPool, WorkerThreadFactory, BoundedCallerRunsPool	hook methods, ThreadLocal latency, CallerRunsPolicy
31	Async Pipeline	AsyncOrderProcessor, PipelineMerger, Order	thenCompose, thenCombine, allOf, anyOf, exceptionally, fan-out/fan-in
32	Work-Stealing Deque	WorkStealingDeque<T>, WorkStealingScheduler	Owner LIFO pop, thief FIFO steal, circular array, grow(), ForkJoinPool internals
33	STM Simulation	TVar<T>, Transaction, STM	Optimistic concurrency, versioned reads, commit/retry protocol, composable atomicity
34	Actor Model	Actor<S,M>, BankAccountActor, ActorSystem, PingPongActors	Message-passing, mailbox, tell/ask, fire-and-forget vs request-reply, no shared state
35	Concurrent Skip List	ConcurrentSkipListSet (lock-based), LockFreeSkipListSet (lock-free), SkipListBenchmark	O(log n) probabilistic structure, hand-over-hand locking, AtomicMarkableReference, logical deletion, linearisability

🔴 Expert (36–50)

#	Problem	Key Concept
36	Reactive Streams	Flow.Publisher/Subscriber, back-pressure, TransformProcessor
37	Distributed Counter	StripedCounter, LongAdder, MetricsCollector
38	Priority Task Executor	PriorityBlockingQueue, PriorityTask, FIFO tie-breaking
39	Concurrent LRU Cache	LinkedHashMap accessOrder, ReadWriteLock
40	Thread-Safe Object Pool	LinkedBlockingQueue, AutoCloseable borrow/return
41	Custom Barrier	ReentrantLock+Condition, generation counter, BrokenBarrierException
42	Async Event Bus	ConcurrentHashMap, CopyOnWriteArrayList, class hierarchy dispatch
43	Concurrent Graph Traversal	Parallel BFS (ExecutorService), Parallel DFS (ForkJoinPool)
44	Rate-Limited Executor	Token bucket (lazy refill), tryAcquire with timeout
45	Lock-Free Ring Buffer	SPSC volatile head/tail, MPSC AtomicLong+ready flags
46	Concurrent Trie	computeIfAbsent, volatile isEnd, DFS collect
47	2PC Transaction Manager	Parallel prepare/commit/rollback, ExecutorService
48	Thread Confinement Pool	ThreadLocal, DateFormatterPool, ConfinedWorkerPool
49	Parallel Merge Sort Advanced	ForkJoinPool, RecursiveAction, adaptiveThreshold, binarySearch
50	Mini Job Scheduler (Capstone)	Priority queue, token bucket, metrics, graceful shutdown

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🚀 Getting Started

Prerequisites

• Java 11+
• Maven 3.6+
• ⚠️ Java 17 required — Problems 34 and 35 use sealed interface and record types. Update your root pom.xml compiler target to 17:

<maven.compiler.source>17</maven.compiler.source>
<maven.compiler.target>17</maven.compiler.target>

Run Tests for a Single Problem

cd beginner/01-thread-basics
mvn test

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💡 How to Use This Repo

1. Read the README.md inside each problem folder
2. Open the skeleton class in src/main/java/
3. Implement the solution — do not modify the test files
4. Run mvn test to verify

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