NEW CD SET 4

What is the difference between notify() and notifyAll()?

notify() wakes one random thread. notifyAll() wakes every waiting thread, and they compete for the lock.

What does the volatile keyword guarantee, and what doesn't it guarantee?

volatile makes sure every thread sees the latest value, but it doesn't make things like x++ atomic.

What's the difference when using a binary semaphore instead of a lock?

A binary semaphore has no ownership check, so any thread can release it — a ReentrantLock only lets the owner unlock.

How many methods does the Externalizable interface have?

Externalizable has two methods: writeExternal() and readExternal().

Why do static and instance variables together complicate concurrent access?

A synchronized instance method locks the object; a synchronized static method locks the class — so they don't protect each other.

What is the difference between CountDownLatch and CyclicBarrier?

A CountDownLatch counts down once and can't reset. A CyclicBarrier makes threads rendezvous repeatedly and resets automatically.

How is it determined which thread wakes up when notify() is called?

The JVM decides — it's not guaranteed to be FIFO or based on priority.

Why is concurrent programming hard to debug?

Bugs depend on thread timing, so a program can run fine for ages then fail unpredictably.

How does a fair lock affect waiting threads? Give an example.

If five threads call lock() one after another, a fair lock lets them through in that same order.

What is a cyclic barrier? Give an example of when you'd use one.

Three threads could each fill a third of an array, call await(), then all proceed once everyone's done.