code: public List<Set<K>> kmeans(int nclusters)
      {
          Cluster<K> cluster = new Cluster<K>();
          for (Document<K> document : documents_.values())
          {
              cluster.add_document(document);
          }
          cluster.section(nclusters);
          refine_clusters(cluster.sectioned_clusters());
          List<Cluster<K>> clusters_ = new ArrayList<Cluster<K>>(nclusters);
          for (Cluster<K> s : cluster.sectioned_clusters())
          {
              s.refresh();
              clusters_.add(s);
          }
          return toResult(clusters_);
      }
docs: k-means聚类
  
  @param nclusters 簇的数量
  @return 指定数量的簇（Set）构成的集合