code: public static List<Writable> sampleFromColumnSequence(int count, String columnName, Schema schema,
                      JavaRDD<List<List<Writable>>> sequenceData) {
          JavaRDD<List<Writable>> flattenedSequence = sequenceData.flatMap(new SequenceFlatMapFunction());
          return sampleFromColumn(count, columnName, schema, flattenedSequence);
      }
docs: Randomly sample values from a single column, in all sequences.
  Values may be taken from any sequence (i.e., sequence order is not preserved)
  
  @param count         Number of values to sample
  @param columnName    Name of the column to sample from
  @param schema        Schema
  @param sequenceData  Data to sample from
  @return              A list of random samples