Threads again

[sai kumar]

Hi all,

Here is a question:
What is the result of the code:

public class Z implements Runnable{
       private int x;
       private int y;
       public static void main(String[] args){
            Z that = new Z();
            (new Thread(that)).start();
            (new Thread(that)).start();
      }
      public void run(){
              for(; ;) {
                     x++;
                     y++;
                     System.out.println("x="+x+"y="+y);
              }
      }
}

Answer given:
The progam prints pairs of values for x and y that are always the same on the same line)for example "x=1,y=1". In addition, each value appears only for once(for example "x=1,y=1" followed by "x=2,y=2").
There is another option among the answers:
The program prints pairs of values for x and y that might not always be the same on the same line(for example "x=2, y=1").
Why this option could not be a solution.
I mean, since there is no synchronization betweent he two threads, how can it be garunteed that the pairs of values for x and y will always be same.
Someone please clarify this.
Thx,
bye
sk
[ edited to turn off smilies -ds ]
[ April 05, 2004: Message edited by: Dirk Schreckmann ]

[Corey McGlone]

You're right, Sai. There is no guarantee that the two values will have the same value on every line. In fact, I modified the code slightly to make the run() method look like this:

public void run()
{
    for(; ;) 
    {
        x++;
        for ( int i = 0; i < 1000; i++ ) {System.out.print("");}
      y++;
      System.out.println("x="+x+"y="+y);
    }
}
 
// Excerpt from output
 
x=1y=1
x=3y=2
x=4y=3
x=5y=4
x=6y=5

I put in the extra for loop to increase the chance of a thread losing its time on the processor between the two increment statements and you can see the effect. If one thread increments x and then leaves the running state, the other thread can increment both x and y and you're left with two different values on the same line.
I hope that helps,
Corey

[sai kumar]

Thx a lot corey,
You are doing a great job. I appreciate your help a lot.
Thx again.
bye
sk