code: public static List<Writable> getUnique(String columnName, Schema schema, JavaRDD<List<Writable>> data) {
          int colIdx = schema.getIndexOfColumn(columnName);
          JavaRDD<Writable> ithColumn = data.map(new SelectColumnFunction(colIdx));
          return ithColumn.distinct().collect();
      }
docs: Get a list of unique values from the specified columns.
  For sequence data, use {@link #getUniqueSequence(List, Schema, JavaRDD)}
  
  @param columnName    Name 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