code: public static double averageCommonFeatureRank(Vector a, Vector b) {
          return averageCommonFeatureRank(Vectors.asDouble(a).toArray(),
                                          Vectors.asDouble(b).toArray());
      }
docs: Computes the Average Common Feature Rank between the two feature arrays.
  Uses the top 20 features for comparison. Converts types and calls
  averageCommonFeatureRank(double[],double[])
  
  @throws IllegalArgumentException when the length of the two vectors are
  not the same.