Execution of Multiple Threads in Java

Can anyone tell me how multiple threads get executed in java??I mean to say that after having called the start method,the run is also invoked, right??Now in my main method if I want to instantiate more than two instances of the same class which extends Thread,how and when will each of the child thread enter the run() and get executed??

class NewThread extends Thread {

NewThread() {

  // Create a new, second thread                                      
  super("Demo Thread");
  System.out.println("Child thread: " + this);
  start(); // Start the thread

}

// This is the entry point for the second thread.

public void run() {
try { for(int i = 5; i > 0; i--) {

        System.out.println("Child Thread: " + i);                                    
        // Let the thread sleep for a while.
        Thread.sleep(500);
     }
  } catch (InterruptedException e) {
     System.out.println("Child interrupted.");
  }
  System.out.println("Exiting child thread.");

} }

class ExtendThread {

public static void main(String args[]) {

  new NewThread(); // create a new thread
  try {
     for(int i = 5; i > 0; i--) {
        System.out.println("Main Thread: " + i);
        Thread.sleep(1000);
     }
  } catch (InterruptedException e) {
     System.out.println("Main thread interrupted.");
  }
  System.out.println("Main thread exiting.");

} }

If inside the ExtendThread class if I add another statement "new NewThread()" the first child thread after being created enters the run() along with the second child thread which also enters the run and both of them executes at the same time.But isn't it this way, the first child thread after being created is supposed to execute completely and only then the second child comes into the scene and executes.