Github user haohui commented on a diff in the pull request: https://github.com/apache/flink/pull/4556#discussion_r140823719 --- Diff: flink-libraries/flink-table/src/main/scala/org/apache/flink/table/functions/aggfunctions/FirstValueAggFunction.scala --- @@ -0,0 +1,107 @@ +/* + * 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.aggfunctions + +import java.math.BigDecimal + +import org.apache.flink.api.common.typeinfo.{BasicTypeInfo, TypeInformation} +import org.apache.flink.api.java.tuple.{Tuple1 => JTuple1} +import org.apache.flink.api.java.typeutils.TupleTypeInfo +import org.apache.flink.table.functions.AggregateFunction + +/** + * The initial accumulator for first value aggregate function + * + * @tparam T the type for the aggregation result + */ +class FirstValueAccumulator[T] extends JTuple1[T] + +abstract class FirstValueAggFunction[T] extends AggregateFunction[T, FirstValueAccumulator[T]] { + + override def createAccumulator(): FirstValueAccumulator[T] = { + val acc = new FirstValueAccumulator[T] + acc.f0 = getInitValue + acc + } + + def accumulate(acc: FirstValueAccumulator[T], value: Any): Unit = { + if (null == acc.f0) { + acc.f0 = value.asInstanceOf[T] + } + } + + override def getValue(acc: FirstValueAccumulator[T]): T = { + acc.f0 + } + + def resetAccumulator(acc: FirstValueAccumulator[T]): Unit = { + acc.f0 = getInitValue + } + + def getInitValue: T = { + null.asInstanceOf[T] + } + + def getValueTypeInfo: TypeInformation[T] --- End diff -- Looks like the parameter can be a generic parameter
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