Variable increment problem (threads)

[Naveen Koneti]

Hi All,

I have a problem in my project.Some thing like I will be spawning n number of threads.These threads have a increment two variables x and y.And these threads have to run infinitly each time incrementing and using those variables.After some time I am getting these variables being increments out of sync though i am incrementing them in a syncronized method.
ex: if I give initial variables as x = 100 and y =200.after running the program say after 5 min those variables are becoming something like 144 & 251 but I am expecting them to be 191 & 291.
my method is like this

static long x = 0;
static long y = 0;
public void run()
{
while(true)
{ increment();//increments the variables }
if(someCondition)
{ thread.yield(); return;//terminating execution }
}

static synchronized increment()
{
x++;
y++;
}

[Ernest Friedman-Hill]

They should appear to be in sync to any thread that reads them in a matching synchronized method or block.

Please don't post the same question in multiple forums. I'm going to move this one to the Threads forum and delete the other copy.

[Arun Bommannavar]

Originally posted by Naveen Koneti:
Hi All,

I have a problem in my project.Some thing like I will be spawning n number of threads.These threads have a increment two variables x and y.And these threads have to run infinitly each time incrementing and using those variables.After some time I am getting these variables being increments out of sync though i am incrementing them in a syncronized method.
ex: if I give initial variables as x = 100 and y =200.after running the program say after 5 min those variables are becoming something like 144 & 251 but I am expecting them to be 191 & 291.
my method is like this

static long x = 0;
static long y = 0;
public void run()
{
while(true)
{ increment();//increments the variables }
if(someCondition)
{ thread.yield(); return;//terminating execution }
}

static synchronized increment()
{
x++;
y++;
}

Have you tried making x and y volatile?

[Jim Yingst]

Volatile will not help here - you may read the values when x has been incremented but y has not. EFH's answer is correct:

static synchronized void readValues() {
    System.out.println("x=" + x);
    System.out.println("y=" + y);
}

Because this method is synchronized the same way the increment() method is, it should always see the same values for x and y. (Assuming there are no other unsynchronized methods that change x and y.)