while loop without body is falling into a infinite loop

Question

I have searched a lot but didn't find solution of this problem so I asked the question here.

I made a small chunk of code to replicate the problem. so following are those Java classes.

The class with the main function:

package test;

public class Main {
    public static void main(String[] args) {
        Background ob = new Background();

        while(ob.val > 0);

        System.out.println("Program Completed");
    }
}

The runnable class:

package test;

public class Background implements Runnable {
    int val;
    Thread t;

    public Background() {
        val = 500;
        t=new Thread(this); 
        t.start();
    }

    @Override
    public void run() {
        while(val > -1000) {
            try {
                Thread.sleep(1);
            } catch (InterruptedException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            }
            val--;
        }
    }

}

Now this code is not printing "Program Completed", so I'm thinking that the while loop is in infinite loop. But if I replace while(ob.val > 0); with

while(ob.val > 0){
    System.out.println("val: "+ob.val);
};

or any System.out.println() statement then I can see "Program Completed". But yes only any System.out.println() statement. If I replace while(ob.val > 0); with

int g;
while(ob.val > 0){
    g = 0;
};

Above code also not showing "Program Completed".

The main code is too large to post here so I have replicated the problem. Tested th replicated code in ubuntu 14.04, JDK -> jdk-8u31-linux-x64

I didnt tested this replicated code in Windows. But I tested the main code in windows & its working fine.

I'm really confused with this type of behavior. Will any one help me?? Thanks in advance.

So will I just use a semaphore variable or I have to do anything else? — Jayadratha Mondal, Apr 23 '15 at 23:22
why not just use a global flag that you can set in your runnable that will indicate that it is done, then put while(!done); instead of while(ob.val > 0); since you have not access to ob.val — faljbour, Apr 23 '15 at 23:25
@faljbour - he has access to ob.val, if it class variable is not specified by private/protected/public, it is accessible in its package. — libik, Apr 23 '15 at 23:27
@faljbour The same issue should happen with `boolean` variables. He would still need some form of synchronization. — Jeffrey, Apr 23 '15 at 23:28
yes mr. @faljbour I already done it in my main code what you said. But I was very curious to know why its happening. — Jayadratha Mondal, Apr 23 '15 at 23:29
@JayadrathaMondal You might want to take a look at [this SO answer](http://stackoverflow.com/a/12193970/758280) for an explanation of what `volatile` does. — Jeffrey, Apr 23 '15 at 23:33
Mr. @Jeffrey I read it and working now. But I want to know if I don't make that val to be volatile then how by adding a simple system.out.println() under the while solving the issue? — Jayadratha Mondal, Apr 23 '15 at 23:40
@I.K. I dont think that the method System.out.println() is sysnchronised it uses synchronised block inside public void println(String x) { synchronized (this) { print(x); newLine(); } } — Harshit Gupta, Jun 29 '15 at 05:25

while loop without body is falling into a infinite loop

0 Answers0