code: public double matthewsCorrelation(int classIdx) {
          return EvaluationUtils.matthewsCorrelation((long) truePositives.getCount(classIdx),
                          (long) falsePositives.getCount(classIdx), (long) falseNegatives.getCount(classIdx),
                          (long) trueNegatives.getCount(classIdx));
      }
docs: Calculate the binary Mathews correlation coefficient, for the specified class.<br>
  MCC = (TP*TN - FP*FN) / sqrt((TP+FP)(TP+FN)(TN+FP)(TN+FN))<br>
  
  @param classIdx Class index to calculate Matthews correlation coefficient for