Java / Java Concurrency and Multi Threading in Java 17 and Java 21 Interview questions

What is the difference between wait() and sleep()?

wait()	sleep()
Defined on `Object`.	Defined as a static method on `Thread`.
Must be called while holding the object's monitor (inside synchronized).	Can be called from anywhere, no lock required.
Releases the held lock while waiting.	Does not release any lock it holds.
Woken by `notify()`/`notifyAll()`, or a timeout if given.	Wakes automatically after the specified duration.

wait() is a coordination primitive: a thread gives up the CPU and the lock, telling other threads "wake me up when a condition changes." sleep() is a simple pause and has nothing to do with inter-thread signaling or locks.

Both throw InterruptedException if the thread is interrupted while waiting, which needs to be handled or have the interrupt status restored.

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Which method releases the object's lock while it waits? sleep()

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wait()

✓ Correct! Well done.

Neither releases the lock

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What wakes a thread blocked in wait() besides a timeout? A call to notify() or notifyAll() on the same object

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The garbage collector running

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Another thread calling sleep()

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