Github user wuchong commented on a diff in the pull request:
https://github.com/apache/flink/pull/4355#discussion_r134151559
--- Diff:
flink-libraries/flink-table/src/main/scala/org/apache/flink/table/api/dataview/ListView.scala
---
@@ -0,0 +1,122 @@
+/*
+ * 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.api.dataview
+
+import java.lang.{Iterable => JIterable}
+import java.util
+
+import org.apache.flink.api.common.typeinfo.{TypeInfo, TypeInformation}
+import org.apache.flink.table.dataview.ListViewTypeInfoFactory
+
+/**
+ * ListView provides List functionality for accumulators used by
user-defined aggregate functions
+ * {{AggregateFunction}}.
+ *
+ * A ListView can be backed by a Java ArrayList or a state backend,
depending on the context in
+ * which the function is used.
+ *
+ * At runtime `ListView` will be replaced by a
[[org.apache.flink.table.dataview.StateListView]]
+ * when use state backend..
+ *
+ * Example:
+ * {{{
+ *
+ * public class MyAccum {
+ * public ListView<String> list;
+ * public long count;
+ * }
+ *
+ * public class MyAgg extends AggregateFunction<Long, MyAccum> {
+ *
+ * @Override
+ * public MyAccum createAccumulator() {
+ * MyAccum accum = new MyAccum();
+ * accum.list = new ListView<>(Types.STRING);
+ * accum.count = 0L;
+ * return accum;
+ * }
+ *
+ * public void accumulate(MyAccum accumulator, String id) {
+ * accumulator.list.add(id);
+ * ... ...
+ * accumulator.get()
+ * ... ...
+ * }
+ *
+ * @Override
+ * public Long getValue(MyAccum accumulator) {
+ * accumulator.list.add(id);
+ * ... ...
+ * accumulator.get()
+ * ... ...
+ * return accumulator.count;
+ * }
+ * }
+ *
+ * }}}
+ *
+ * @param elementTypeInfo element type information
+ * @tparam T element type
+ */
+@TypeInfo(classOf[ListViewTypeInfoFactory[_]])
+class ListView[T](val elementTypeInfo: TypeInformation[T]) extends
DataView {
+
+ def this() = this(null)
+
+ val list = new util.ArrayList[T]()
+
+ /**
+ * Returns an iterable of the list.
+ *
+ * @return The iterable of the list or { @code null} if the list is
empty.
+ */
+ def get: JIterable[T] = {
+ if (!list.isEmpty) {
+ list
+ } else {
+ null
+ }
+ }
+
+ /**
+ * Adding the given value to the list.
+ *
+ * @param value element to be appended to this list
+ */
+ def add(value: T): Unit = list.add(value)
+
+ /**
+ * Removes all of the elements from this list.
+ *
+ * The list will be empty after this call returns.
+ */
+ override def clear(): Unit = list.clear()
+
+ /**
+ * Copy from a list instance.
+ *
+ * @param t List instance.
+ * @return A copy of this list instance
+ */
+ def copyFrom(t: util.List[T]): ListView[T] = {
--- End diff --
the `copyFrom` method can be accessed by users. I should avoid this.
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