public String getUdfName() {
    return getClass().getName();
  }
Gets the negative function of the current one. E.g., GenericUDFOPNotEqual for
GenericUDFOPEqual, or GenericUDFOPNull for GenericUDFOPNotNull.
@return Negative function
ql/src/java/org/apache/hadoop/hive/ql/udf/generic/GenericUDF.java