K&B Chapter 9 Question # 16

[Paras Jain]

As suggested in the book
I have modified the method by synchrozing the method call. I end up in getting infinite loop. What is wrong?
When I write wait(1000) instead of wait() then also nothing gets printed

    
public class Test {
    public static void main(String [] args) {
        final Foo f = new Foo();
        Thread t = new Thread(new Runnable() {
            public void run() {
                f.doStuff();
            }
        });
        Thread g = new Thread() {
            public void run() {
                f.doStuff();
            }
        };
            t.start();
            g.start();
        
}
    
}
    
    
class Foo {
    int x = 5;
    public void doStuff() {
        synchronized(this){
        if (x < 10) {
// nothing to do
            try {
                wait();
            } catch(InterruptedException ex) { }
        } else {
            System.out.println("x is " + x++);
            if (x >= 10) {
                notify();
            }
        }
    }
    }
}
 

[Burkhard Hassel]

Hi, Paras,

not a problem of synchronization alone.

In your method, nothing is printed if(x<10)
And x is always five. You never get to the else{} block.

As your whole method block is synchronized to this, I made the code a bit easier to read by defining the whole method as sync.

And I put some output lines at the end of doStuff()

class Test {
public static void main(String[] args) {
final Foo f = new Foo();
Thread t = new Thread(new Runnable() {
public void run() {
f.doStuff();
}
});
Thread g = new Thread() {
public void run() {
f.doStuff();
}
};
t.start();
g.start();
}
}
 
class Foo {
int x = 5;
 
public synchronized void doStuff() {
if (x < 10) {
try {
wait([B]1234[/B]);
} catch (InterruptedException ex) {
}
} else {
System.out.println("x is " + x++);
if (x >= 10)
notify();
}
System.out.print("method doStuff,");
System.out.print("Current thread:");
System.out.println(Thread.currentThread().getName());
}
 
}
 

Without the 1234 it will wait for ever. Or up to a spontanous wakeup.
Also here, x is never printed.

Yours,
Bu.

[wise owen]

I have modified the method by synchrozing the method call. I end up in getting infinite loop. What is wrong?
When I write wait(1000) instead of wait() then also nothing gets printed

By code design.
Case 1: x always less than 10 and threads call wait() and no other threads to wake up them.

Case 2: threads call wait(1000) and will wake up after 1000ms. and returns from run(). The thread ends.