Can we start a thread twice?

No. After staring a thread, it can never be started again. If you does so, an IllegalThreadStateException is thrown. For Example:

public class TestThreadTwice1 extends Thread{  

 public void run(){  

   System.out.println("running...");  

 }  

 public static void main(String args[]){  

  TestThreadTwice1 t1=new TestThreadTwice1();  

  t1.start();  

  t1.start();  

 }  

}  

Output:running

       Exception in thread "main" java.lang.IllegalThreadStateException