Why does it get stuck?

[Yongping Wang]

I just couldn't figure out why it gets stuck when running the following code:

class Reader extends Thread
{
Calculator c;
public Reader (Calculator c)
{
this.c=c;
}
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 c= new Calculator();
c.start();
new Reader(c).start();
new Reader(c).start();
new Reader(c).start();
}
}

class Calculator extends Thread
{
int total;
public void run()
{
synchronized(this)
{
for(int i=0;i<100;i++)
{
total += i;
}
notifyAll();
}
}
}

Thanks for any comments.
Y.Wang

[Barry Gaunt]

What happens if the Calculator thread finishes before the readers get started? The notifyAll() notifies nothing, it is not queued. The readers will then wait for ever.
I think the authors of the book where this comes from have made a correction in the book's errata.

Page 533,     Bottom code block                                                                   2/07/03
Change:
    20.   calculator.start();
    21.   new Reader(calculator).start();
    22.   new Reader(calculator).start();
    23.   new Reader(calculator).start();
To:
    20.   new Reader(calculator).start();
    21.   new Reader(calculator).start();
    22.   new Reader(calculator).start();
    23.   calculator.start();
   

[ April 18, 2003: Message edited by: Barry Gaunt ]

[Dirk Schreckmann]

So, which book are we talking about here?