[GitHub] twalthr commented on a change in pull request #7059: [FLINK-10689][table] Port UDFs in Table API extension points to flink-table-common

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twalthr commented on a change in pull request #7059: [FLINK-10689][table] Port UDFs in Table API extension points to flink-table-common
URL: https://github.com/apache/flink/pull/7059#discussion_r235888819
 
 

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 File path: flink-libraries/flink-table-common/src/main/java/org/apache/flink/table/functions/AggregateFunction.java
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+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.apache.flink.table.functions;
+
+import org.apache.flink.api.common.typeinfo.TypeInformation;
+
+/**
+ * Base class for User-Defined Aggregates.
+ *
+ * <p>The behavior of an {@link AggregateFunction} can be defined by implementing a series of custom
+ * methods. An {@link AggregateFunction} needs at least three methods:
+ *  - createAccumulator,
+ *  - accumulate, and
+ *  - getValue.
+ *
+ * <p>There are a few other methods that can be optional to have:
+ *  - retract,
+ *  - merge, and
+ *  - resetAccumulator
+ *
+ * <p>All these methods must be declared publicly, not static and named exactly as the names
+ * mentioned above. The methods createAccumulator and getValue are defined in the
+ * {@link AggregateFunction} functions, while other methods are explained below.
+ *
+ * <p>Processes the input values and update the provided accumulator instance. The method
+ * accumulate can be overloaded with different custom types and arguments. An AggregateFunction
+ * requires at least one accumulate() method.
+ *
+ * {@code
+ * param accumulator           the accumulator which contains the current aggregated results
+ * param userDefinedInputs the input value (usually obtained from a new arrived data).
+ *
+ * def accumulate(accumulator: ACC, userDefinedInputs): Unit
+ * }
+ *
+ * <p>Retracts the input values from the accumulator instance. The current design assumes the
+ * inputs are the values that have been previously accumulated. The method retract can be
+ * overloaded with different custom types and arguments. This function must be implemented for
+ * datastream bounded over aggregate.
+ *
+ * {@code
+ * param accumulator           the accumulator which contains the current aggregated results
+ * param userDefinedInputs the input value (usually obtained from a new arrived data).
+ *
+ * def retract(accumulator: ACC, userDefinedInputs): Unit
+ * }
+ *
+ * <p>Merges a group of accumulator instances into one accumulator instance. This function must be
+ * implemented for datastream session window grouping aggregate and dataset grouping aggregate.
+ *
+ * {@code
+ * param accumulator  the accumulator which will keep the merged aggregate results. It should
+ *                     be noted that the accumulator may contain the previous aggregated
+ *                     results. Therefore user should not replace or clean this instance in the
+ *                     custom merge method.
+ * param its          an {@link java.lang.Iterable} pointed to a group of accumulators that will be
+ *                     merged.
+ *
+ * def merge(accumulator: ACC, its: java.lang.Iterable[ACC]): Unit
+ * }
+ *
+ * <p>Resets the accumulator for this {@link AggregateFunction}. This function must be implemented for
+ * dataset grouping aggregate.
+ *
+ * {@code
+ * param accumulator  the accumulator which needs to be reset
+ *
+ * def resetAccumulator(accumulator: ACC): Unit
+ * }
+ *
+ *
+ * @param T   the type of the aggregation result
+ * @param ACC the type of the aggregation accumulator. The accumulator is used to keep the
+ *             aggregated values which are needed to compute an aggregation result.
+ *             AggregateFunction represents its state using accumulator, thereby the state of the
+ *             AggregateFunction must be put into the accumulator.
+ */
+public abstract class AggregateFunction<T, ACC> extends UserDefinedFunction {
 
 Review comment:
   Add proper annotations to all classes.

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