code: private void computeFrameRate(Timing updateFpsTimer, long lastTime, long currentTime)
      {
          if (updateFpsTimer.elapsed(Constant.ONE_SECOND_IN_MILLI))
          {
              currentFrameRate = (int) Math.round(Constant.ONE_SECOND_IN_NANO / (double) (currentTime - lastTime));
              updateFpsTimer.restart();
          }
      }
docs: Compute the frame rate depending of the game loop speed.
  
  @param updateFpsTimer The update timing
  @param lastTime The last time value before game loop in nano.
  @param currentTime The current time after game loop in nano (must be superior or equal to lastTime).