a Q from mugal engine

[aymen esawey]

hi
if anyone here saw this Q on the mugal exam and answered it right,please help me and explain it it's totaly ambiguous:

 
class Counter {
    int v = 0;
    synchronized void inc() { v++; }
    synchronized void dec() { v--; }
}
public class Mugal {
    Counter i;
    Counter j;
    Counter k;
    public synchronized void a() {
        i.inc();
        System.out.println("a");
        i.dec();
    }
    public synchronized void b() {
        i.inc(); j.inc(); k.inc();
        System.out.println("b");
        i.dec(); j.dec(); k.dec();
    }
    public void c() {
        k.inc();
        System.out.println("c");
        k.dec();
    }
    
    
}
 

the question asks if any thread can call methods a,b,and c at any time what is the guaranteed values for i.v,j.v and k.v?
i tried it even the main thread can't go in because they are all initialized to null
only those who saw this Q reply
thanks

[Rajinder Yadav]

As per your request, I never saw this Q...but I hope my answer will suffice?
Any thread can call any method on an instance of class Counter, each instance of class Counter has it's own "thread monitor key", what this means is that access to any synchronized method must happen one thread at a time. If more than one thread tries to access one of the synchronized methods, only one will be given access while the others will block until that thread is done processing the synchronized method.
Since each method has the same number of decrement operations as it does increments, this guarantees that at any one time the values for i.v, j.v, and k.v will be zero!
[ February 03, 2002: Message edited by: Rajinder Yadav ]

[Valentin Crettaz]

Rajinder,
the problem that aymen wanted to stress is that the Counter variables i,j and k are not explicitely initialized (that is they will be null by default), thus, as soon as you invoke either method a, b or c on an instance of class Test, there is gonna be a NullPointerException.
Add the following main method to the code provided by aymen:

public static void main(String args[]){
final Test t = new Test();
Thread t1 = new Thread(new Runnable(){
public void run(){
t.a();//whatever
t.b();//whatever
t.c();//whatever
}
});
Thread t2 = new Thread(new Runnable(){
public void run(){
t.a();//whatever
t.b();//whatever
t.c();//whatever
}
});
t1.start();
t2.start();
}

The first method to be invoked is a() and its first statement is i.inc()
Since i is null, the invocation i.inc() will throw a NullPointerException at runtime.

[Rick Reumann]

Originally posted by Rajinder Yadav:

Since each method has the same number of decrement operations as it does increments, this guarantees that at any one time the values for i.v, j.v, and k.v will be zero!
[ February 03, 2002: Message edited by: Rajinder Yadav ]

Excluding the deal with thowing the null pointers since Counters wouldn't be intialized, but assuming they were, how you guarantee k will always be zero? Since c() isn't declared as a synchronized, the inc() and dec() could be called from different threads meaning it's possible one could get called and another get called more than once, which would make the answer not being equal for k's values.
Am I wrong here? I'm more concerned with this thread issue. I'm assuming that the Counter references are all initialized at some point first.

[aymen esawey]

hi
thanks for your interest in the Q but according to
maha ana mock list she has done every Q so can't anyone get her to answer i know she is not a moderator here.
or could any scjp of you take the mugal exam(won't be that hard for you now).

here is the whole Q and thanks:

Given the following code, which statements concerning the objects referenced through the member variables i, j and k are true, given that any thread may call the methods a, b and c at any time?

class Counter {
    int v = 0;
    synchronized void inc() { v++; }
    synchronized void dec() { v--; }
}
public class Q7ed5 {
    Counter i;
    Counter j;
    Counter k;
    public synchronized void a() {
        i.inc();
        System.out.println("a");
        i.dec();
    }
    public synchronized void b() {
        i.inc(); j.inc(); k.inc();
        System.out.println("b");
        i.dec(); j.dec(); k.dec();
    }
    public void c() {
        k.inc();
        System.out.println("c");
        k.dec();
    }
}
 

common it wouldn't hurt me if i found that am stupid
[ February 04, 2002: Message edited by: aymen esawey ]

[aymen esawey]

by the way all the methods end up doing nothing to the values in question(they inc then dec whats inc'ed)

[Brian Lugo]

Should'nt the inc/dec methods in the Counter class be declared as static? And should'nt the Counter class member variable v should be static too.
Am I missing something here?
[ February 05, 2002: Message edited by: Brian Lugo ]