What will or won't cause a thread to block (a question from a test)

Question

I've had a test, and there was a question I lost some points on, because I wasn't able to answer it :

Which of the following is NOT a condition which can cause a thread to block :

• Calling an objects's wait() method
• Waiting for an I/O operation
• Calling sleep()
• Calling yield()
• Calling join()

As far as I know, all of these are blocking calls :

• wait() returns when an something calls notify(), blocks until then
• If the thread is WAITING for an I/O operation then it's obviously blocked
• sleep(), obviously, blocks until the time runs out, or something wakes up the thread
• yield() "cancels the rest of the thread's timeslice" (lacking a better term), and returns only when the thread is active again
• join() blocks until the thread it's waiting for terminates.

Am I missing something here?

Answer 1

yield(). it does not block the thread, what I mean it does not not put the thread in the BLOCK state, but it put the thread in the state READY - so it is available to the scheduler choose it again to execute. try to thing about the states of a thread. all operations but yield put the thread in the BLOCK state.

Answer 2

Also, calling join() on a thread that has already finished will NOT block. And sleep(0) will behave like yield() if there are no other threads that are eligible for execution.