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