What is a Thread identifer and how can I obtain it?

public long getId() 

class ThreadID{ 
 public static void main(String[] args){
  Thread t1 = new Thread("Thread one");
  Thread t2 = new Thread("Thread two");
  Thread t3 = new Thread("Thread three");
  System.out.println(Thread.currentThread().getName()+ "'s" + " identifier is " + Thread.currentThread().getId());
  System.out.println(t1.getName()+ "'s" + " identifier is " + t1.getId());
  System.out.println(t2.getName()+ "'s" + " identifier is " + t2.getId());
  System.out.println(t3.getName()+ "'s" + " identifier is " + t3.getId());
 }
}