wait/notify, sleep/yield & Producer-Consumer

wait()/notify() uses a single wait-set per object (may wake wrong thread), while Condition.await()/signal() supports multiple wait-sets per lock for targeted wake-ups.

wait() releases the lock and waits for a signal (communication), sleep() holds the lock and pauses for a time (delay), and yield() is an unreliable scheduler hint.

wait() must be in a while loop because spurious wake-ups can return without notify(), and the condition may change between notify() and lock re-acquisition.

wait() must release the lock so the signaling thread can acquire it and call notify(); sleep() keeps the lock because it is a simple delay, not a communication mechanism.

Thread.yield() is a scheduler hint that may be ignored, keeps the thread RUNNABLE, releases no locks, and is unreliable in production.

The producer-consumer pattern uses a shared bounded buffer with BlockingQueue (simplest), Lock+Conditions (targeted), or wait/notifyAll (classic) to decouple production from consumption.