This is another tricky questions from core Java interviews, since multiple threads can wait on single monitor lock, Java API designer provides method to inform only one of them or all of them, once waiting condition changes, but they provide half implementation. There notify() method doesn't provide any way to choose a particular thread, that's why its only useful when you know that there is only one thread is waiting. On the other hand, notifyAll() sends notification to all threads and allows them to compete for locks, which ensures that at-least one thread will proceed further.
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