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When should I use notify() and when notifyAll() methods on threads?

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Question: When should I use notify() and when notifyAll() methods on threads?

Answer: Generally it is more reliable to use notifyAll() method. Notify() method wakes up a single thread that is waiting on this object's monitor. If any threads are waiting on this object, one of them is chosen to be awakened. The choice is arbitrary and occurs at the discretion of the implementation. A thread waits on an object's monitor by calling one of the wait methods. Since it is internally implemented we do not know exactly which thread will be chosen.

This method should only be called by a thread that is the owner of this object's monitor. A thread becomes the owner of the object's monitor in one of three ways:
By executing a synchronized instance method of that object.

By executing the body of a synchronized statement that synchronizes on the object.
For objects of type Class, by executing a synchronized static method of that class.
Only one thread at a time can own an object's monitor.

It is much safer to use from the beginning notifyAll() method and later trying to optimize your program you can try to switch to notify(). You have no idea which thread will be awakened. You must be sure that every waiting thread will be able to continue if you use notify() method. The behavior of your program will be more predictable.
From another hand using notify() method will decrease CPU and memory consumption since just one of multiple thread is awaken and rest are in waiting mode.
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