Code causing exceptions at its own sweet time and place

Question

import java.lang.*;
class newthread implements Runnable
     Thread t;
     int click=0;
     public volatile boolean running=true;
     newthread(int p)
          t=new Thread(this);
          t.setPriority(p);
     void start()
          t.start();
     public void run()
          while(running)
               click++;
     void start1()
          running=true;
     void stop()
          running=false;     
class threadpriority2
     public static void main(String argv[])
          newthread ob1=new newthread(Thread.NORM_PRIORITY+2);
          newthread ob2=new newthread(Thread.NORM_PRIORITY-2);
          ob1.start();
          ob2.start();
          try
               Thread.sleep(5000);
          catch(Exception e)
               System.out.println("error detected:"+e);
          System.out.println("first one:"+ob1.click);
          System.out.println("first one:"+ob2.click);
          System.out.println("the priority of ob1 is:"+ob1.t.getPriority());
          System.out.println("the priority of ob2 is:"+ob2.t.getPriority());
          ob1.stop();
          ob2.stop();
          ob2.t.setPriority(Thread.NORM_PRIORITY+3);      
          ob1.t.setPriority(Thread.NORM_PRIORITY-3);
          ob1.start1();
          ob2.start1();
          ob1.start();
          ob2.start();
          System.out.println("the priority of ob1 is:"+ob1.t.getPriority());
          System.out.println("the priority of ob2 is:"+ob2.t.getPriority());
          try
               Thread.sleep(5000);
          catch(Exception e)
               System.out.println("error detected:"+e);
          ob1.stop();
          ob2.stop();
          System.out.println("first one:"+ob1.click);
          System.out.println("first one:"+ob2.click);
errors during runtime:
Exception in thread "main" java.lang.IllegalThreadStateException
at java.lang.Thread.start(Unknown Source)
at newthread.start(threadpriority2.java:17)
at threadpriority2.main(threadpriority2.java:77)
when i run this code it causes exceptions at its own sweet locations when i run it separately.if once it displays some valid input it would then be followed by the exception
next time if i run it it will throw exception as soon as i run it at the very start without displaying even an iota of valid data like the previous case
is ti because the os is placing restrictions on the threads growth or something else.incase there is an error in my code it should atleast pop up the exceptions at one particular loaction in the output and not give different outputs

Answer 1

1) When you post code, please use and tags as described in Formatting tips on the message entry page. It makes it much easier to read.
wasnt aware of it.will keep it in mind i post nexttime.sorry for the inconvenience[\b]
3) You have multiple threads, right? The scheduling of which thread gets how much CPU time when is not subject to your control or prediction, so of course you'll see different timings in subsequent runnings of the same code.[b]actually the problem is not with multiple threads showing different times.whats happening is that exceptions are being thrown randomly at runtime.once when i ran it the exception was displayed at the very beginning of the output and the program snapped.next when i ran it first some output which was slated to be displayed got displayed and then the exceptions appeared in the output.now if the exceptions are thrown at the very beginning its obvious theres some problem at the background coz in the second case that never happened[\b]
i guess the formatting tips have taken effect[:D]