code: public final List<CellName> clusteringKeys(ColumnFamily columnFamily) {
          List<CellName> clusteringKeys = new ArrayList<>();
          CellName lastClusteringKey = null;
          for (Cell cell : columnFamily) {
              CellName cellName = cell.name();
              if (!isStatic(cellName)) {
                  CellName clusteringKey = extractClusteringKey(cellName);
                  if (lastClusteringKey == null || !lastClusteringKey.isSameCQL3RowAs(cellNameType, clusteringKey)) {
                      lastClusteringKey = clusteringKey;
                      clusteringKeys.add(clusteringKey);
                  }
              }
          }
          return sort(clusteringKeys);
      }
docs: Returns the common clustering keys of the specified column family.
  
  @param columnFamily A storage engine {@link ColumnFamily}.
  @return The common clustering keys of the specified column family.