code: public Counter<F> getFeatureCounter()
    {
      Counter<F> featureCounts = new ClassicCounter<F>();
      for (int i=0; i < this.size(); i++)
      {
        BasicDatum<L, F> datum = (BasicDatum<L, F>) getDatum(i);
        Set<F> featureSet   = new HashSet<F>(datum.asFeatures());
        for (F key : featureSet) {
          featureCounts.incrementCount(key, 1.0);
        }
      }
      return featureCounts;
    }
docs: Get Number of datums a given feature appears in.