Thread # Multithreading + Concurrency + Java 8

Multithreading + Concurrency + Java 8 questions and answers.

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✅ TOP MULTITHREADING INTERVIEW QUESTIONS & ANSWERS (With Java 8)

(Short, powerful, and expected in interviews)

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🔹 1. What is a Thread in Java?

A thread is the smallest unit of execution inside a process.
Java supports multithreading using Thread class, Runnable, and Executors.

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🔹 2. Difference between Process vs Thread

Process	Thread
Independent memory	Shares memory of process
Heavyweight	Lightweight
Slow context-switching	Fast switching

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🔹 3. Ways to create a thread in Java

1. Extend Thread

2. Implement Runnable

3. Implement Callable

4. Using ExecutorService

5. Using Java 8 CompletableFuture

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🔹 4. Runnable vs Callable

Runnable	Callable
No return value	Returns value
No checked exception	Can throw exception
Used with Thread	Used with ExecutorService

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🔹 5. What is Executor Framework?

A framework to manage thread pools.
Prevents creating many threads manually.

Example:

ExecutorService executor = Executors.newFixedThreadPool(10);
executor.submit(() -> System.out.println("Task executed"));

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🔹 6. What is Thread Pooling & Why?

Instead of creating new threads every time, reuse existing threads.

Benefits:
✔ Avoids overhead
✔ Improves performance
✔ Stable under heavy load

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🔹 7. Types of Thread Pools

• FixedThreadPool

• CachedThreadPool

• ScheduledThreadPool

• SingleThreadExecutor

• WorkStealingPool (Java 8)

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🔹 8. What is Synchronization?

A mechanism to control multiple threads accessing shared resources.

Ways:

• synchronized keyword

• ReentrantLock

• Atomic classes

• synchronized blocks

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🔹 9. synchronized method vs synchronized block

synchronized method	synchronized block
Locks entire method	Locks only specific code
Less efficient	More efficient
Easy to use	More control

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🔹 10. What is ReentrantLock? Why better?

ReentrantLock is an advanced lock mechanism.

Benefits:

• TryLock()

• Lock interruptibility

• Fairness policy

• Better performance under contention

Example:

lock.lock();
try {
    // critical section
} finally {
    lock.unlock();
}

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🔹 11. What is volatile?

volatile ensures:

✔ Visibility of changes across threads
✔ Prevents caching variables inside thread-local memory

But volatile does NOT ensure atomicity.

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🔹 12. What is Atomic package?

java.util.concurrent.atomic
Provides lock-free thread-safe operations.

Example:

AtomicInteger counter = new AtomicInteger();
counter.incrementAndGet();

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🔹 13. What is Deadlock?

When two threads wait forever for each other’s lock.

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🔹 14. What is Livelock?

Threads keep running but make no progress (continuously responding to each other).

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🔹 15. What is Starvation?

Low-priority thread never gets CPU because high-priority threads always run.

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🔹 16. What is ThreadLocal?

Provides thread-specific variable.

Example:

ThreadLocal<String> tl = ThreadLocal.withInitial(() -> "default");

Used in:

• Per-request context

• Tenant context

• Security context

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🔹 17. Difference between wait(), sleep(), yield()?

Method	Belongs	Releases lock?	Purpose
wait()	Object	YES	Thread communication
sleep()	Thread	NO	Pause thread
yield()	Thread	NO	Hint to scheduler

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🔹 18. What is Thread.join()?

Makes one thread wait for another to complete.

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🔹 19. What is Future & FutureTask?

Represents result of async computation.

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🔹 20. CompletableFuture (Java 8)

Provides powerful async programming.

Example:

CompletableFuture.supplyAsync(() -> "Hello")
                 .thenApply(String::toUpperCase)
                 .thenAccept(System.out::println);

Features:

• async pipelines

• chaining

• exception handling

• combining tasks

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🔹 21. Parallel Streams (Java 8 Multithreading)

list.parallelStream()
    .map(e -> e * 2)
    .forEach(System.out::println);

Uses ForkJoinPool (common pool) internally.

Warning:

Not good for:

• I/O tasks

• Stateful operations

• Shared mutable data

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🔹 22. What is ForkJoinPool?

A pool using work-stealing algorithm.
Used for parallel streams, RecursiveTask, RecursiveAction.

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🔹 23. What is CountDownLatch?

Allows threads to wait until other threads complete.

Example:

CountDownLatch latch = new CountDownLatch(3);
latch.await();

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🔹 24. What is CyclicBarrier?

Lets threads wait for each other repeatedly.

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🔹 25. Difference between CountDownLatch vs CyclicBarrier

CountDownLatch	CyclicBarrier
One-time use	Reusable
One thread waits	All threads wait

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🔹 26. Semaphore

Controls number of threads accessing a resource.

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🔹 27. ReadWriteLock

Allows:

• multiple readers

• one writer

Better performance for read-heavy systems.

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🔹 28. What is concurrency vs parallelism?

Concurrency	Parallelism
Multiple tasks progressing	Multiple tasks running at same time
Single CPU possible	Requires multiple CPUs

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🔹 29. What is Immutable Object?

Thread-safe because state cannot change.

Examples:
String, Integer, LocalDate

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🔹 30. Best practices for multithreading

✔ Use ExecutorService → not “new Thread()”
✔ Avoid shared mutable state
✔ Prefer immutability
✔ Use locks only when required
✔ Avoid parallelStream if your use case is I/O
✔ Always shutdown executor
✔ Handle exceptions in CompletableFuture

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