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Threads K&B 724

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i am unable to understand the code can any one explain me whats happening

1. class Reader extends Thread {
2. Calculator c;
4. public Reader(Calculator calc) {
5. c = calc;
When the wait() method is invoked on an object, the thread executing
that code gives up its lock on the object immediately. However, when notify() is called,
that doesn�t mean the thread gives up its lock at that moment. If the thread is still
completing synchronized code, the lock is not released until the thread moves out of
synchronized code. So just because notify() is called doesn�t mean the lock becomes
available at that moment.
6. }
8. public void run() {
9. synchronized(c) {
10. try {
11. System.out.println("Waiting for calculation...");
12. c.wait();
13. } catch (InterruptedException e) {}
14. System.out.println("Total is: " + c.total);
15. }
16. }
18. public static void main(String [] args) {
19. Calculator calculator = new Calculator();
20. new Reader(calculator).start();
21. new Reader(calculator).start();
22. new Reader(calculator).start();
23. calculator.start();
24. }
25. }
27. class Calculator extends Thread {
28. int total;
30. public void run() {
31. synchronized(this) {
32. for(int i=0;i<100;i++) {
33. total += i;
34. }
35. notifyAll();
36. }
37. }
38. }

Thanks in advance
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All three reader threads are synchronized & waiting on the same Calculator object.

The calculator thread starts and generates some number. It then notifies all the threads that were waiting for the calculator. All Readers can now return to runnable state where they may compete to use the Calculator object.

It is important that the threads that are waiting for an object (Readers) should be started before the notifying thread(Calculator) -

otherwise the calculator could have started first and sent the notification before any of the reader threads were in waiting. As a result the readers could have missed the notification.
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