//File ProvaThread.java
public class ProvaThread extends Thread
{

  private int cont = 0;
  private String id = "";

  public ProvaThread(int max, String id, ThreadGroup tg)
  {
    super(tg, id);
    cont = max;
    this.id = id;
  }

  public void run()
  {
    while (cont >0)
    {
      System.out.println(id+" "+cont); cont--;
    }
  }

}

---------------------------

//File Main.java
public class Main
{
  ProvaThread pt1;
  ProvaThread pt2;
  ProvaThread pt3;

  public Main()
  {
    ThreadGroup tg = new ThreadGroup("test");
    pt1 = new ProvaThread(20,"t1", tg);
    pt2 = new ProvaThread(20,"t2", tg);
    pt3 = new ProvaThread(20,"t3", tg);
    pt1.start();
    pt2.start();
    pt3.start();
    while (tg.activeCount()>0){}
    System.out.println("\n\nDONE");
  }

  public static void main(String[] args)
  {
    Main m = new Main();
  }
}