What if we call run() method directly instead start() method?

Each thread starts in a separate call stack. Invoking the run() method from main thread, the run() method goes onto the current call stack rather than at the beginning of a new call stack.

1. class TestCallRun1 extends Thread{  
2.  public void run(){  
3.    System.out.println("running...");  
4.  }  
5.  public static void main(String args[]){  
6.   TestCallRun1 t1=new TestCallRun1();  
7.   t1.run();//fine, but does not start a separate call stack  
8.  }  
9. }  

Output:running...