WHY notify(), notifyall() and wait() are there in object class and not in Thread class?

Question

what is the use of declaring notify(), notifyall() and wait() in object class and not in Thread class?

`Object` is the ultimate super class of `Java`. And these method's context is a lock associated with every object in `Java` — Ruchira Gayan Ranaweera, Aug 15 '14 at 06:31

score 1 · Answer 1 · answered Aug 15 '14 at 06:33

Threads synchronize with each other using a shared object. It's on this shared object, which is not a thread, that these methods are called. BTW, the documentation explicitely recommends to never use these methods on Thread instances, as that confuses things.

For example, let's say that you have a thread adding dished to a buffet, and several threads trying to take dishes out of this buffet. The buffet will be the synchronization point: the filling thread will call notifyAll() on the shared buffet once it has added a dish to wake up threads waiting on the buffet to be filled.

score 0 · Answer 2 · answered Aug 15 '14 at 06:33

Those methods are in Object to provide means of synchronisation for threads (for example through locks):

synchronize(lock) {
    lock.wait(); // Will block until lock.notify() is called
}

// Later on the same day in another thread...
synchronize(lock) {
    lock.notify(); // Will wake up lock.wait()
}

This behaviour is not Thread specific therefore not in the Thread class

WHY notify(), notifyall() and wait() are there in object class and not in Thread class?

2 Answers2