code: public static List<Writable> sampleInvalidFromColumn(int numToSample, String columnName, Schema schema,
                      JavaRDD<List<Writable>> data, boolean ignoreMissing) {
          //First: filter out all valid entries, to leave only invalid entries
          int colIdx = schema.getIndexOfColumn(columnName);
          JavaRDD<Writable> ithColumn = data.map(new SelectColumnFunction(colIdx));
  
          ColumnMetaData meta = schema.getMetaData(columnName);
  
          JavaRDD<Writable> invalid = ithColumn.filter(new FilterWritablesBySchemaFunction(meta, false, ignoreMissing));
  
          return invalid.takeSample(false, numToSample);
      }
docs: Randomly sample a set of invalid values from a specified column.
  Values are considered invalid according to the Schema / ColumnMetaData
  
  @param numToSample    Maximum number of invalid values to sample
  @param columnName     Same of the column from which to sample invalid values
  @param schema         Data schema
  @param data           Data
  @param ignoreMissing  If true: ignore missing values (NullWritable or empty/null string) when sampling. If false: include missing values in sampling
  @return               List of invalid examples