multithreading with start()

public class cl extends Thread{ 
 public void run(){ 
  for(int i=1;i<5;i++){ 
    try{
        Thread.sleep(300);
       
    }
    catch(InterruptedException e)
    {System.out.println(e);} 
    System.out.println(i); 
  } 
 } 
 public static void main(String args[]){ 
  cl t1=new cl(); 
  cl t2=new cl(); 
 
  t1.start(); 
  t2.start(); 
 } 
} 

$javac cl.java
$java -Xmx128M -Xms16M cl
1
1
2
2
3
3
4
4