code: public Collection<FullColumnDescription> getPartitionKeyColumnDescriptions()
      {
          if (partitionKeys == null)
          {
              partitionKeys = new LinkedList<>();
  
              for (FullColumnDescription col : fullColumnDescriptions.values())
              {
                  if (col.isPartitionKeyMember())
                  {
                      this.partitionKeys.add(col);
                  }
              }
  
              Collections.sort(this.partitionKeys, PartitionKeyComparator.INSTANCE);
          }
  
          return this.partitionKeys;
      }
docs: Get the collection of Partition Key FullColumnDescriptions, in order.
  @return an iterable collection.