public long restart(){
  long start=_startTimeNanos;
  _startTimeNanos=System.nanoTime();
  return _startTimeNanos - start;
}

public long restart ( ) { long start = _startTimeNanos ; _startTimeNanos = System . nanoTime ( ) ; return _startTimeNanos - start ; }
sets the reference time and returns the amount of time between now and the previous time the reference time was set . this is largely an optimization to avoid reading the monotonic clock twice , as well as to avoid losing track of nanoseconds between the read and update .
