public BooleanWritable evaluate(DateWritableV2 d) {
    // date value to boolean doesn't make any sense.
    return null;
  }
Convert from a string to boolean. This is called for CAST(... AS BOOLEAN)

@param i
         The string value to convert
@return BooleanWritable
ql/src/java/org/apache/hadoop/hive/ql/udf/UDFToBoolean.java