In a Producer Consumer scenario, one thread is a Producer and

another thread is a Consumer.

For this scenario to start working, a Consumer has to know when

the Producer has produced. In Object class, there is a wait()

method. A Consumer calls wait method to wait on Producer. The

Producer used notify() method of Object class to inform Consumer

that it has produced.

In this way the processor time between produce and consume

operations is freed due to the use of wait() and notify() methods.

https://www.baeldung.com/java-producer-consumer-problem