Why is program not terminating?

Question

I am new to using the Timer class, and so am trying to experiment with it before incorporating it into my project. I am wondering why this program does not terminate when the count reaches 5. The program keeps running even though the condition for the while loop is not satisfied.

package Timer;

import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import javax.swing.Timer;

public class demo {

    private static int count;


    public static void main(String[] args) {
        ActionListener executeThis = new ActionListener(){


            @Override
            public void actionPerformed(ActionEvent arg0) {
                // TODO Auto-generated method stub
                System.out.println("Hello");
                count++;

            }

        };

        Timer timer = new Timer(500, executeThis);
        timer.setInitialDelay(1000);
        timer.start();

        while(count < 5){

        }
    }

}

why should it stop? you are executing it as a timer job. Which basically schedules it and run it after the given time interval — Danyal Sandeelo, Mar 22 '16 at 09:02
Possible duplicate of [How to stop immediately the task scheduled in Java.util.Timer class](http://stackoverflow.com/questions/4050442/how-to-stop-immediately-the-task-scheduled-in-java-util-timer-class) — itzmukeshy7, Mar 22 '16 at 09:05
Why is it, then, that when I don't have a while loop, the timer never sends out an actionEvent? If the while loop is not needed why does this occur? — lb91, Mar 22 '16 at 09:09

BobTheBuilder · Answer 1 · 2016-03-22T11:56:10.917

2

You should add:

timer.stop();

In order to stop the timer scheduling.

Another issue is the usage of non atomic variable in multi-threaded ennvironment. You can use AtomicInteger instead.

You can do it inside the actionPerformed method if count == 5.

edited Mar 22 '16 at 11:56

answered Mar 22 '16 at 09:03

BobTheBuilder

18,858
6
40
61

This did not work, by the way. The program still continues. – lb91 Mar 22 '16 at 09:15
Timer stops but program continues with your update. Some other actions are required. – RubioRic Mar 22 '16 at 09:40
1

The problem is there is the `while-loop` is never seeing the changes been made to `count` as they are on different threads, it's basically a dirty read problem – MadProgrammer Mar 22 '16 at 10:10

Romeo Sheshi · Answer 2 · 2016-03-22T10:02:25.833

2

for stoping the timer

import javax.swing.*;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.util.concurrent.atomic.AtomicInteger;


public class Test1 {
    private static AtomicInteger count = new AtomicInteger(0);

    public static void main(String[] args) {
        ActionListener executeThis = new ActionListener(){


            @Override
            public void actionPerformed(ActionEvent arg0) {
                // TODO Auto-generated method stub
                System.out.println("Hello");
                count.getAndIncrement();

            }

        };

        Timer timer = new Timer(500, executeThis);
        timer.setInitialDelay(1000);
        timer.start();

        while(count.get() < 5){

        }
        timer.stop();

    }
}

but i think this will be the correct way to do this

import javax.swing.*;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.util.concurrent.atomic.AtomicInteger;

/**
 * User: Romeo Sheshi 
 * Date: 21/03/16
 * Time: 12:12
 */
public class Test {
    private static AtomicInteger count = new AtomicInteger(0);
    private static Timer timer;
    public static void main(String[] args) {


        ActionListener executeThis = new ActionListener(){


            @Override
            public void actionPerformed(ActionEvent arg0) {
                // TODO Auto-generated method stub
                System.out.println("Hello");

                if( count.incrementAndGet()==5){
                   stopTimer();
                }

            }

        };
        timer = new Timer(500, executeThis);
        timer.setInitialDelay(1000);
        startTimer();

        while ( count.get()<5){}

    }

    public static void startTimer(){
        timer.start();
    }

    public static void stopTimer(){
        timer.stop();
    }
}

edited Mar 22 '16 at 10:02

answered Mar 22 '16 at 09:23

Romeo Sheshi

901
5
7

yes i test it works, but i think is not the correct way to use the timer the anwser of BobTheBuilder is the correct way – Romeo Sheshi Mar 22 '16 at 09:42
I have copied your first excerpt and the program is still running while I'm writing this. – RubioRic Mar 22 '16 at 09:48
Sorry for that may be my ide stopping the program I'm going to make a test by terminal. Your observations could be right – Romeo Sheshi Mar 22 '16 at 09:51
Yes you right the first extracts is not working the problem is the counter not incrementing like tak3shi said in his answer. i change the first extract to make it work – Romeo Sheshi Mar 22 '16 at 10:01
@RubioRic I have basically the same code as the first example and it works fine in both the IDE and terminal. What OS are you using? – MadProgrammer Mar 22 '16 at 10:09
That's not the reason either. count is incremented each time actionPerformed is invoked. – RubioRic Mar 22 '16 at 10:09
@MadProgrammer I'm using Eclipse Mars 2.0 on Windows 7 – RubioRic Mar 22 '16 at 10:09
Im using eclipse Mars on macosx . In the ide is working but not in terminal – Romeo Sheshi Mar 22 '16 at 10:12
@RubioRic What version of Java? – MadProgrammer Mar 22 '16 at 10:12
java version "1.7.0_75" – Romeo Sheshi Mar 22 '16 at 10:12
@RomeoSheshi Both Netbeans and the MacOS terminal worked just fine for me; Java 8 MacO 10.11.3 – MadProgrammer Mar 22 '16 at 10:13
@MadProgrammer jdk1.8.0_74 – RubioRic Mar 22 '16 at 10:13
whith private static AtomicInteger count = new AtomicInteger(0); yes me too but with private static int count; not – Romeo Sheshi Mar 22 '16 at 10:14
As Romeo indicates, I'm using the original int count. – RubioRic Mar 22 '16 at 10:15
@RubioRic I have now tested and verified it works in Windows 7 using Java 7 (1.7.0_75) just fine. I've posted my test code as well, but it's basically the same as Romeo – MadProgrammer Mar 22 '16 at 10:48
@MadProgrammer Yes, It works fine with AtomicInteger. Romeo has been changing the answer during the morning. I was mentioning the original solution that defines count as an int. – RubioRic Mar 22 '16 at 11:19

score 2 · Answer 3 · edited May 23 '17 at 10:28

There are two issues:

The timer keeps running when not stopped.
The static count variable does not get synchronised between threads.

Check this explanation from Gray about synchronization between threads: Static variables and multithreading in java He explains that each thread has its own copy of a static variable. To avoid this declare the variable volatile.

For simple testing like this example, a volatile variable is all you need. For critical situations where you need a realtime synchronization between threads use AtomicInteger.

private static volatile int count;

Make sure to stop the timer with timer.stop(). It is not wrong to add a Thread.sleep in the loop, this safes CPU resources and allows thread synchronization for non-volatile variables.

while (count < 5) {
  Thread.sleep(10);
}    
timer.stop();

You can read more about the difference between a volatile variable and AtomicInteger here: What is the difference between atomic / volatile / synchronized?

You'd have to read deep into the memory model and concurrency in Java — MadProgrammer, Mar 22 '16 at 10:56
While volatile is a step in the right direction, AtomicInteger is a far better solution (at least from my experience :P) — MadProgrammer, Mar 22 '16 at 20:10

score 1 · Answer 4 · edited Jun 20 '20 at 09:12

This is a supporting answer to Romeo Sheshi

This code has been test on:

Windows; Java 1.7.0_75 using Netbeans 8 AND the command line
MacOS 10.11.3; Java 1.8.0_20 using Netbeans 8 and the terminal

In both cases, the program terminated after the count reached 5

import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.util.concurrent.atomic.AtomicInteger;
import javax.swing.Timer;

public class Test {

    private static AtomicInteger count = new AtomicInteger(0);

    public static void main(String[] args) {
        ActionListener executeThis = new ActionListener() {

            @Override
            public void actionPerformed(ActionEvent arg0) {
                // TODO Auto-generated method stub
                System.out.println("Hello");
                count.getAndIncrement();
                System.out.println(count);
            }

        };

        Timer timer = new Timer(500, executeThis);
        timer.setInitialDelay(1000);
        timer.start();

        while (count.get() < 5) {
        }
        System.out.println("On the outside...");
        timer.stop();
    }

}

score -1 · Answer 5 · edited Jun 20 '20 at 09:12

-1

Best And Easiest Way to do this is --

import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import javax.swing.Timer;

public class demo {

    private static int count;


    public static void main(String[] args) {
        ActionListener executeThis = new ActionListener(){


            @Override
            public void actionPerformed(ActionEvent arg0) {
                // TODO Auto-generated method stub
                System.out.println("Hello");
                count++;
                if(count==5){
                    System.exit(0);                 
                }
            }

        };

        Timer timer = new Timer(500, executeThis);
        timer.setInitialDelay(1000);
        timer.start();

        while(count < 5){

        }
    }

}

edited Jun 20 '20 at 09:12

Community

1
1

answered Mar 22 '16 at 10:02

Pritpal Singh

163
13

1

There's no need to use such a blunt method, nor does it explain why the `while-loop` doesn't exit (or at least doesn't exit when the `count` reaches `5`, think I got it to `20`) – MadProgrammer Mar 22 '16 at 10:50
2

the code is not running without while loop i dont know why...if u have solution then explain little bit – Pritpal Singh Mar 23 '16 at 05:00
I did, have a look if you're interested ;) – MadProgrammer Mar 23 '16 at 06:01

Why is program not terminating?

5 Answers5

This is a supporting answer to Romeo Sheshi

Best And Easiest Way to do this is --