code: private void validateCoefficients() {
          if (coefsValid)
              return;
  
          if (n >= 2) {
              double xBar = (double) sumX / n;
              double yBar = (double) sumY / n;
  
              a1 = (double) ((n * sumXY - sumX * sumY) / (n * sumXX - sumX * sumX));
              a0 = (double) (yBar - a1 * xBar);
          } else {
              a0 = a1 = Double.NaN;
          }
  
          coefsValid = true;
      }
docs: Validate the coefficients.