@Override
  public GenericUDAFEvaluator getEvaluator(TypeInfo[] parameters)
      throws SemanticException {
    if (parameters.length != 1) {
      throw new UDFArgumentTypeException(parameters.length - 1,
          "Exactly one argument is expected.");
    }

    if (parameters[0].getCategory() != ObjectInspector.Category.PRIMITIVE) {
      throw new UDFArgumentTypeException(0,
          "Only primitive type arguments are accepted but "
          + parameters[0].getTypeName() + " is passed.");
    }
    switch (((PrimitiveTypeInfo) parameters[0]).getPrimitiveCategory()) {
    case BYTE:
    case SHORT:
    case INT:
    case LONG:
    case FLOAT:
    case DOUBLE:
    case STRING:
    case VARCHAR:
    case CHAR:
    case TIMESTAMP:
    case DECIMAL:
      return new GenericUDAFStdEvaluator();
    case BOOLEAN:
    case DATE:
    default:
      throw new UDFArgumentTypeException(0,
          "Only numeric or string type arguments are accepted but "
          + parameters[0].getTypeName() + " is passed.");
    }
  }
Compute the standard deviation by extending GenericUDAFVariance and
overriding the terminate() method of the evaluator.
ql/src/java/org/apache/hadoop/hive/ql/udf/generic/GenericUDAFStd.java