How does wait() method work in java ?

Question

How do threads communicate using wait() and notify()/notifyAll()? and Why are these methods present in Object class rather than Thread class?

Answer 1

Hello,

About classes, you need to know that any object is build with a lock, also called monitor.

These monitors are used when you use a synchronized method. but in some case, you may need to wait an event before doing some action (for example, the reader-consumer problem). To do that, you use the method wait.

and when the condition is ok, you can call notify, to notify a thread waiting with the wait method to run again, or notifyAll, to notify all suspended thread.

Evidently, you should use wait inside a loop which check if the condition is satisfied.

Answer 2

the goal of join is to wait a thread to complete. the goal of wait is to do nothing before a specific condition is satisfied

Answer 3

what if we use join() instead of wait? a thread will still wait until the completion of the other thread, then why is wait() required?

Answer 4

you can find the "producer consurmer" problem here: https://www.geeksforgeeks.org/producer-consumer-solution-using-threads-java/ it contains an example