public Class<? extends UDAF> getUDAFClass() {
    return udaf.getClass();
  }
This class is a bridge between GenericUDAF and UDAF. Old UDAF can be used
with the GenericUDAF infrastructure through this bridge.
ql/src/java/org/apache/hadoop/hive/ql/udf/generic/GenericUDAFBridge.java