Mark Jackson

Davin,
Nice score! Which mocks did you take?

Kathy,
Thanks very much for the learning opportunity
Yes, I made it through the last chapter of the SCJP material, but I'm actually working *backwards* through your book (chapter by chapter). I had started with the Heller / Roberts book.
After reading on javaranch that the thread questions are some of the more difficult exam questions, I jumped right to chapter 9 in your book.
I plan to schedule my exam once I see how I'm doing on the mock exams (current results are confidential ).
Thanks for your well-written study guide!
Regards,
Mark.

Hello --
When running the Calculator Thread example from the Kathy Sierra 1.4 book (see below, p. 533), it hangs with the following output when the calculator thread is started first [1].
Output:
Calculator running... waiting for lock Calc has the lock. Calc: notifyAll Waiting for calculation... Waiting for calculation... Waiting for calculation...
I believe the reason is that the calculator thread starts first and grabs its object lock first. This allows the notifyAll() call to occur *before* any of the other threads are waiting. Therefore, notifyAll() does nothing.
If I modify the code to start the calculator thread *after* the Reader threads, the code appears to work correctly and generates the following output:
Waiting for calculation... Waiting for calculation... Waiting for calculation... Calculator running... waiting for lock Calc has the lock. Calc: notifyAll Total is: 4950 Total is: 4950 Total is: 4950
Q. Is the example really incorrect?

public class Reader extends Thread { Calculator c; public Reader(Calculator calc) { c = calc; } public void run() { synchronized(c) { try { System.out.println("Waiting for calculation..."); c.wait(); } catch (InterruptedException e) { } System.out.println("Total is: " + c.total); } } public static void main(String[] args) { Calculator calculator = new Calculator(); calculator.start(); // won't work [1] new Reader(calculator).start(); new Reader(calculator).start(); new Reader(calculator).start(); // calculator.start();// does work [2] } } class Calculator extends Thread { int total; public void run() { System.out.println("Calculator running... waiting for lock"); synchronized(this) { System.out.println("Calc has the lock."); for (int i = 0; i<100; i++) { total += i; } System.out.println("Calc: notifyAll"); notifyAll(); } } }