public class Rolex { 
 private final long start;
 
 public Rolex() {
 start = System.currentTimeMillis();
 } 
 
 // return time (in seconds) since this object was created
 public double elapsedTime() {
 long now = System.currentTimeMillis();
 return (now - start) / 1000.0;
 } 
 
 public static void main(String[] args) {
 Rolex watchOne = new Rolex();
 // waste time so it is large enough to be measurable
 try {Thread.sleep(500);} catch (InterruptedException e) {}
 
 Rolex watchTwo = new Rolex(); 
 // waste time so it is large enough to be measurable
 try {Thread.sleep(500);} catch (InterruptedException e) {}

 // right now, watchOne is older
 System.out.println("watchOne " + watchOne.elapsedTime());
 System.out.println("watchTwo " + watchTwo.elapsedTime());
 
 Rolex watchTmp = watchOne;
 watchOne = watchTwo;
 watchTwo = watchTmp; 
 
 // swapped! now watchTwo is the older one
 // e.g. watchTwo.elapsedTime() returns a value
 // (slightly) larger than watchOne.elapsedTime()
 System.out.println("watchOne " + watchOne.elapsedTime());
 System.out.println("watchTwo " + watchTwo.elapsedTime());
 }
}