Java Concurrency Mastery

A hands-on, scenario-based repository to master Java concurrency from basics to advanced concepts.

Philosophy

NO THEORY. ONLY CODE.

Each problem is a real-world scenario requiring you to implement thread-safe solutions. Learn by building, not by reading.

Structure

Level 1: Basics (Thread Creation & Lifecycle)

• Thread creation (Thread class vs Runnable)
• Thread lifecycle states
• sleep, join, interrupt
• Daemon threads

Level 2: Synchronization

• Race conditions
• synchronized methods vs blocks
• volatile keyword
• Deadlock scenarios

Level 3: Inter-Thread Communication ⭐

• wait/notify mechanism
• Producer-Consumer pattern
• Thread data exchange
• Covers your interview gap!

Level 4: Concurrent Collections

• BlockingQueue
• ConcurrentHashMap
• CopyOnWriteArrayList
• Thread-safe data structures

Level 5: Executors Framework

• ThreadPoolExecutor
• Callable and Future
• CompletableFuture
• Custom thread pools

Level 6: Advanced Synchronizers

• CountDownLatch
• CyclicBarrier
• Semaphore
• ReentrantLock, ReadWriteLock
• Phaser

Level 7: Real-World Patterns

• Bounded buffer
• Rate limiter
• Connection pool
• Task scheduler

How to Use

1. Start from Level 1 - Even if basics seem trivial, they build muscle memory
2. Read problem statement - Understand what needs to be built
3. Implement solution - Fill in TODOs in skeleton code
4. Run tests - Validate your implementation
5. Review solution notes - After solving, check key learnings

Running Problems

Each problem is a standalone Java file with:

• Problem description in comments
• Skeleton code with TODOs
• main() method to test manually
• Expected output

# Compile and run a problem
cd src/main/java
javac com/buildmuse/concurrency/level1_basics/Q01_CreateThread.java
java com.buildmuse.concurrency.level1_basics.Q01_CreateThread

Progress Tracking

• Level 1: Basics (8 problems)
• Level 2: Synchronization (10 problems)
• Level 3: Communication (12 problems)
• Level 4: Collections (8 problems)
• Level 5: Executors (10 problems)
• Level 6: Advanced (12 problems)
• Level 7: Patterns (10 problems)

Total: 70 problems

Interview Preparation

After completing Level 3, you'll be able to:

• Implement thread communication mechanisms
• Build producer-consumer patterns
• Handle data exchange between threads
• Debug concurrency issues

Level 6+ prepares you for senior-level questions.

Tips

• Don't skip levels - Each builds on previous concepts
• Run code, don't just read - Concurrency is about behavior
• Break things intentionally - Remove synchronization to see race conditions
• Time yourself - Interview problems should take 15-20 minutes

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Let's master concurrency. Start with Level 1, Problem 1.