Github user fhueske commented on a diff in the pull request:
https://github.com/apache/flink/pull/4355#discussion_r135550350
--- Diff:
flink-libraries/flink-table/src/main/scala/org/apache/flink/table/api/dataview/ListView.scala
---
@@ -0,0 +1,133 @@
+/*
+ * 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]]
+ * if it is backed by a 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](
+ @transient private[flink] val elementTypeInfo: TypeInformation[T])
+ extends DataView {
+
+ def this() = this(null)
+
+ private[flink] var list: util.List[T] = new util.ArrayList[T]()
--- End diff --
right now an empty `ArrayList` is always created when a `ListView` is
instantiated.
This is unnecessary overhead when the `ListView` is copied or deserialized
using `ListViewSerializer` because the empty instance is immediately replaced.
We should add an option to create a `ListView` without an `ArrayList`
instance. This means we have to move the creation of the `ArrayList` out of the
primary constructor.
The same applies to the `MapView`.
---
If your project is set up for it, you can reply to this email and have your
reply appear on GitHub as well. If your project does not have this feature
enabled and wishes so, or if the feature is enabled but not working, please
contact infrastructure at infrastruct...@apache.org or file a JIRA ticket
with INFRA.
---
