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