code: public static Map<String,Set<Writable>> getUnique(List<String> columnNames, Schema schema, RecordReader data){
          Map<String,Set<Writable>> m = new HashMap<>();
          for(String s : columnNames){
              m.put(s, new HashSet<>());
          }
          while(data.hasNext()){
              List<Writable> next = data.next();
              for(String s : columnNames){
                  int idx = schema.getIndexOfColumn(s);
                  m.get(s).add(next.get(idx));
              }
          }
          return m;
      }
docs: Get a list of unique values from the specified columns.
  For sequence data, use {@link #getUniqueSequence(String, Schema, SequenceRecordReader)}
  
  @param columnNames   Names of the column to get unique values from
  @param schema        Data schema
  @param data          Data to get unique values from
  @return              List of unique values, for each of the specified columns