Object.wait() and Object. notify() are methods used to co-ordinate Threads. Note these are methods of all objects, not just Threads. These methods are not something you will use directly in ordinary code. For most tasks, you would use one of the classes in the java.util.concurrent package.

Let’s pretend though you were writing some sort of producer-consumer logic from scratch where thread A produced objects for Thread B to consume.

In the ideal situation, with a multiple CPU machine, both Threads would whip along at full bore, but if B got behind consuming, (full queue), (Lucy in the chocolate factory), then A should wait() for B to catch up, and B should notify() A when A can start working again. Similarly if A can’t keep up producing fast enough, (empty queue), B should wait() and A should notify() B when B can start working again.

You need an arbitrary rendezvous object for one Thread to wait() on and the other to notify(). It can get hairier with multiple waiters and multiple notifiers and timeouts on the waits.

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