Help to stop a thread

[raghu_lzybns]

Hi every1,
I'm new to java and was trying to write a code which moves the character ">" from left to right in a frame when 'Start' button is pressed and stops when 'Stop' button is pressed..

I was able to move the character when started but ..unable to stop the thread..

Plz help..

import java.awt.*;
import java.awt.event.*;
import javax.swing.*;

class AnimCar extends JFrame implements ActionListener{
    JButton bstart;
    JButton bstop;
    JButton bexit;
    int x=0;
    Font f=null;
    String t;

    public AnimCar(){
        bstart=new JButton("START");
        bstop=new JButton("STOP");
        bexit=new JButton("EXIT");
        Panel p1=new Panel();
        p1.add(bstart);
        p1.add(bstop);
        p1.add(bexit);
        add("South",p1);
        bstart.addActionListener(this);
        bstop.addActionListener( this);
        bexit.addActionListener(this);
        f=new Font("Arial",Font.BOLD,48);
    }

    @Override
    public void paint(Graphics g){
        g.setFont(f);
        g.clearRect(0, 0, 1000, 100);
        x+=10;
        if(x<=1000)
            g.drawString(">", x, 50);
        else
            x=0;
    }

    public static void main(String [] args){
        AnimCar ac=new AnimCar();
        ac.setVisible(true);
        ac.setSize(1000,100);
        ac.setTitle("Animation");
    }

   class AnimThread implements Runnable{
       volatile Thread thread;
       @Override
       public void run(){
           try{
	           while(thread==Thread.currentThread()){
			repaint();
                        Thread.sleep(100);
                   }
           }catch(Exception ex){
                   ex.printStackTrace();
           }
       }

       public void start(){
           thread=new Thread(this);
           thread.start();
       }

       public void stop(){
           thread=null;
       }
   }

   public void actionPerformed(ActionEvent ae){
	   AnimThread at=new AnimThread();
	   try{
		   if(bstart==ae.getSource()){
			   at.start();
                   }else if(ae.getSource()==bstop){
                           at.stop();
                   }else if(ae.getSource()==bexit){
                           System.exit(0);
                   }
            }catch(Exception ex){
                   ex.printStackTrace();
            }
   }
}

[stevemilw]

concurrr mmy little ones

[RamyaSivakanth]

Hi,
U can't stop like this..It will not work..
Do logically.

1.I kept a boolean "flag";

2.Whenever start button is pressed I have set the flag true

3.Whenever stop button is pressed I have set the flag false

4.In run iam running the while condition till the flag is true.

import java.awt.*;
import java.awt.event.*;
import javax.swing.*;

class AnimCar extends JFrame implements ActionListener{
    JButton bstart;
    JButton bstop;
    JButton bexit;
    int x=0;
    Font f=null;
    String t;
	boolean flag = true;

    public AnimCar(){
        bstart=new JButton("START");
        bstop=new JButton("STOP");
        bexit=new JButton("EXIT");
        Panel p1=new Panel();
        p1.add(bstart);
        p1.add(bstop);
        p1.add(bexit);
        add("South",p1);
        bstart.addActionListener(this);
        bstop.addActionListener( this);
        bexit.addActionListener(this);
        f=new Font("Arial",Font.BOLD,48);
    }


    public void paint(Graphics g){
        g.setFont(f);
        g.clearRect(0, 0, 1000, 100);
        x+=10;
        if(x<=1000)
            g.drawString(">", x, 50);
        else
            x=0;
    }

    public static void main(String [] args){
        AnimCar ac=new AnimCar();
        ac.setVisible(true);
        ac.setSize(1000,100);
        ac.setTitle("Animation");
    }

   class AnimThread implements Runnable{
       volatile Thread thread;

       public void run(){
           try{
	           while(flag == true){
			repaint();
                        Thread.sleep(100);
                   }
           }catch(Exception ex){
                   ex.printStackTrace();
           }
       }

       public void start(){
           thread=new Thread(this);
           thread.start();
       }

       public void stop(){
           thread.interrupt();
       }
   }

   public void actionPerformed(ActionEvent ae){
	   AnimThread at=new AnimThread();
	   try{
		   if(bstart==ae.getSource()){
			   at.start();
                   }else if(ae.getSource()==bstop){
					       flag = false;
                         
                   }else if(ae.getSource()==bexit){
                           System.exit(0);
                   }
            }catch(Exception ex){
                   ex.printStackTrace();
            }
   }
}

[raghu_lzybns]

ThanQ Ramya..
boolean flag worked well...but can u explain the flaw in my logic

[Singing Boyo]

@Ramya: your way will terminate the thread, not wait like he wants.


This one has pseudocode interspersed in it, but should work. I did extend thread, but the difference should not matter that much.

public class AnimThread extends Thread{
     private boolean terminate=false, waitFlag=false, inSleepOrWait=false;
     private Object wait=new Object;
     public AnimThread(){
          super("Animation Thread);//Always name threads!!! 
//Thread names appear in runtime exceptions, and can be useful in tracking down the bug
//This is especially important if you are going to use a debugger
     }

     public void run(){
          //I'm leaving out your actual animation
          while(!terminate){
               //do animation
               try{
                    inSleepOrWait=true;
                    sleep(100);
               } catch (InterruptedException ie){
                    return;
               }
               if(waitFlag){
                    try{
                         wait.wait();
                    } catch(InterruptedException ie){
                         return;
                    }
               }
               inSleepOrWait=false;
          }
     }
     public void terminate(){
          if(inSleepOrWait)
               interrupt();
          else terminate=true;
     }
     public void halt(){
          waitFlag=true;
     }
     public void continue(){
          waitFlag=false;
          wait.notifyAll();
     }
}

Also, instead of just calling system.exit(0) when the exit button is pressed, try this... It ensures that your thread gets terminated "normally" rather than abruptly closing, or worse, getting stuck.

final AnimThread at = new AnimThread();
.....
if(e.getSource()==bexit)
     at.terminate();
     System.exit(0);
}

EDIT: Missed OPs last post, though I still think this could be a better way of doing it.

The flaw in the logic was that a thread can ignore the interrupt() method. To ensure that the thread is terminated, you need to use some form of flag.