public static DoubleWritable nextDouble(ColumnVector vector,
                                   int row,
                                   Object previous) {
    if (vector.isRepeating) {
      row = 0;
    }
    if (vector.noNulls || !vector.isNull[row]) {
      DoubleWritable result;
      if (previous == null || previous.getClass() != DoubleWritable.class) {
        result = new DoubleWritable();
      } else {
        result = (DoubleWritable) previous;
      }
      result.set(((DoubleColumnVector) vector).vector[row]);
      return result;
    } else {
      return null;
    }
  }
If the current batch is empty, get a new one.
@return true if we have rows available.
ql/src/java/org/apache/hadoop/hive/ql/io/BatchToRowReader.java