public double stop(){
  if (_start == -1)   throw new RuntimeException("Stop time measurement without prior start is invalid.");
  long end=System.nanoTime();
  double duration=((double)(end - _start)) / 1000000;
  _start=end;
  return duration;
}

public double stop ( ) { if ( _start == - 1 ) throw new RuntimeException ( "Stop time measurement without prior start is invalid." ) ; long end = System . nanoTime ( ) ; double duration = ( ( double ) ( end - _start ) ) / 1000000 ; _start = end ; return duration ; }
measures and returns the time since the last start ( ) or stop ( ) invocation and restarts the measurement .
