Threads not running concurrent?

[Jörg Hagemann]

Hi,

I'm trying to understand how threads work in Java. For this I use this small test program:

public class Threads01 extends Thread {

  public static void main(String[] args) {
    new Threads01().start();
    new Threads01().start();
  }

  public void run() {
    for (int i = 0; i < 10; i++) {
        System.out.print(i + " ");
    }
  }
} 

As output, I expect a mixed sequence of numbers, e.g.: 0 0 1 1 2 2 3 3 4 5 4 6 5 7 6 8 9 7 8 9
But what I get is: 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9
It looks as if the threads were running consecutively. I use netbeans to run the program.

Somebody has an explanation for this?

Thank you and best regards
polarbear

[Rahul P Kumar]

When you say concurrent, that does not mean that threads are running side by side, they will take their turn to execute, and it is not that you will get the output in that sequence, it may change depending upon which thread was running and which one went in wait state. Multi threading is for situations say there are two threads, one is opening the file and another is calculating something, now in absence of multi threading, works will be sequential, so unless that file is open, no business proess will happen. But because of multi threading, in the meanwhile this thread is in waiting state for file opening, another thread will start running and do some other business.

[Miklos Szeles]

Put a Thread.sleep(100) into the for loop, and check again. Check the concurrency tutorial to understand how threading works.

[Ulf Dittmer]

The run method finishes much too quickly. The thread scheduler doesn't switch contexts *that* often.

[Campbell Ritchie]

Try adding something to print the names of the Threads as well as those numbers. Look at this method; you can write

System.out.println(Thread.currentThread().getName());

in your run() method; then you can see whether the threads (probablay called thread1, thread2 or some such poetic names) are running in a particular order.

[Campbell Ritchie]

Not quite the same as what you showed earlier . . .

public class ConcurrentDemo
{
    public static void main(String[] arg)
    {
         for (String s : args)
         {
              new Thread(
                  new Runnable()
                  {
                      public void run()
                      {
                          String name = thread.currentThread.getName();
                          for (int i = 0; i < 10; i++)
                          {
                               System.out.printf("%s: %d%n", name, i);
                          }//end for
                      }//end run
                  },       //end Runnable
                  s
              ).start();
         }//end for-each
    }//end main
} // end of class

java ThreadTest Thread1 Thread2 Thread3 Thread4

. . . and see what happens if you run that class several times.