NEW CD SET 1

What's the difference between run() and start() on a Thread?

run() just calls the method on the current thread. start() creates a new thread and has the JVM call run() on it.

What is a race condition and how can it be prevented?

A race condition happens when multiple threads access shared state at once and the outcome depends on timing. Prevent it with locks, semaphores, or atomic variables.

What is the difference between a semaphore and a lock or monitor?

A semaphore is just a counter with acquire() and release(), and any thread can release it, so it has no ownership. A lock does have ownership — only the thread that acquired it can release it.

Describe Java's 6-state thread model (Thread.State).

A thread goes from NEW to RUNNABLE, then maybe BLOCKED, WAITING, or TIMED_WAITING, then finally TERMINATED.

What is meant by "signal and continue" semantics for Condition variables?

Calling signal() doesn't hand over the lock right away. The signalled thread waits to reacquire the lock, then rechecks its condition.

What is a critical section?

A critical section is code that touches shared state and must run on only one thread at a time.

What are the three general approaches to resolving deadlock?

Prevention removes a deadlock condition outright, avoidance checks before granting resources, and detection and recovery lets deadlock happen then fixes it.

What is the difference between Callable and Runnable?

Runnable's run() returns nothing and can't throw checked exceptions. Callable's call() returns a value and can throw one, retrieved via a Future.

What happens to an object's references during serialization?

When you serialize an object, its references get serialized too, but only if they're also Serializable — otherwise you get a NotSerializableException.

What is a non-blocking, lock-free data structure?

A lock-free structure stays thread safe without locks, usually using compare-and-swap. Some thread always makes progress, so there's no deadlock, but threads may retry under contention.