@Override
  public GenericUDAFEvaluator getEvaluator(TypeInfo[] parameters) throws SemanticException {

    Class<? extends UDAFEvaluator> udafEvaluatorClass = udaf.getResolver()
        .getEvaluatorClass(Arrays.asList(parameters));

    return new GenericUDAFBridgeEvaluator(udafEvaluatorClass);
  }
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