public DoubleWritable evaluate(ByteWritable i) {
    if (i == null) {
      return null;
    } else {
      doubleWritable.set(i.get());
      return doubleWritable;
    }
  }
Convert from boolean to a double. This is called for CAST(... AS DOUBLE)

@param i
         The byte value to convert
@return DoubleWritable
ql/src/java/org/apache/hadoop/hive/ql/udf/UDFToDouble.java