code: public void stop() {
          if (this.isRunning()) {
              long duration = System.nanoTime() - this.lastStarted;
              this.lastStarted = 0l;
              this.stats.add(new Duration(duration));
          }
      }
docs: Stop the stopwatch and record the statistics for the latest run. This method does nothing if the stopwatch is not currently
  {@link #isRunning() running}
  
  @see #isRunning()