Github user wuchong commented on a diff in the pull request:
https://github.com/apache/flink/pull/4355#discussion_r134118759
--- Diff:
flink-libraries/flink-table/src/main/scala/org/apache/flink/table/api/dataview/MapView.scala
---
@@ -0,0 +1,192 @@
+/*
+ * 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.MapViewTypeInfoFactory
+
+/**
+ * MapView provides Map functionality for accumulators used by
user-defined aggregate functions
+ * [[org.apache.flink.table.functions.AggregateFunction]].
+ *
+ * A MapView can be backed by a Java HashMap or a state backend,
depending on the context in
+ * which the function is used.
+ *
+ * At runtime `MapView` will be replaced by a
[[org.apache.flink.table.dataview.StateMapView]]
+ * when use state backend.
+ *
+ * Example:
+ * {{{
+ *
+ * public class MyAccum {
+ * public MapView<String, Integer> map;
+ * public long count;
+ * }
+ *
+ * public class MyAgg extends AggregateFunction<Long, MyAccum> {
+ *
+ * @Override
+ * public MyAccum createAccumulator() {
+ * MyAccum accum = new MyAccum();
+ * accum.map = new MapView<>(Types.STRING, Types.INT);
+ * accum.count = 0L;
+ * return accum;
+ * }
+ *
+ * public void accumulate(MyAccum accumulator, String id) {
+ * try {
+ * if (!accumulator.map.contains(id)) {
+ * accumulator.map.put(id, 1);
+ * accumulator.count++;
+ * }
+ * } catch (Exception e) {
+ * e.printStackTrace();
+ * }
+ * }
+ *
+ * @Override
+ * public Long getValue(MyAccum accumulator) {
+ * return accumulator.count;
+ * }
+ * }
+ *
+ * }}}
+ *
+ * @param keyTypeInfo key type information
+ * @param valueTypeInfo value type information
+ * @tparam K key type
+ * @tparam V value type
+ */
+@TypeInfo(classOf[MapViewTypeInfoFactory[_, _]])
+class MapView[K, V](
+ private[flink] val keyTypeInfo: TypeInformation[K],
+ private[flink] val valueTypeInfo: TypeInformation[V])
+ extends DataView {
+
+ def this() = this(null, null)
+
+ val map = new util.HashMap[K, V]()
+
+ /**
+ * Returns the value to which the specified key is mapped, or { @code
null } if this map
+ * contains no mapping for the key.
+ *
+ * @param key The key of the mapping.
+ * @return The value of the mapping with the given key.
+ * @throws Exception Thrown if the system cannot get data.
+ */
+ @throws[Exception]
+ def get(key: K): V = map.get(key)
+
+ /**
+ * Put a value with the given key into the map.
+ *
+ * @param key The key of the mapping.
+ * @param value The new value of the mapping.
+ * @throws Exception Thrown if the system cannot put data.
+ */
+ @throws[Exception]
+ def put(key: K, value: V): Unit = map.put(key, value)
+
+ /**
+ * Copies all of the mappings from the specified map to this map view.
+ *
+ * @param map The mappings to be stored in this map.
+ * @throws Exception Thrown if the system cannot access the map.
+ */
+ @throws[Exception]
+ def putAll(map: util.Map[K, V]): Unit = map.putAll(map)
+
+ /**
+ * Deletes the mapping of the given key.
+ *
+ * @param key The key of the mapping.
+ * @throws Exception Thrown if the system cannot access the map.
+ */
+ @throws[Exception]
+ def remove(key: K): Unit = map.remove(key)
+
+ /**
+ * Returns whether there exists the given mapping.
+ *
+ * @param key The key of the mapping.
+ * @return True if there exists a mapping whose key equals to the given
key.
+ * @throws Exception Thrown if the system cannot access the map.
+ */
+ @throws[Exception]
+ def contains(key: K): Boolean = map.containsKey(key)
+
+ /**
+ * Returns all the mappings in the map.
+ *
+ * @return An iterable view of all the key-value pairs in the map.
+ * @throws Exception Thrown if the system cannot access the map.
+ */
+ @throws[Exception]
+ def entries: JIterable[util.Map.Entry[K, V]] = map.entrySet()
+
+ /**
+ * Returns all the keys in the map.
+ *
+ * @return An iterable view of all the keys in the map.
+ * @throws Exception Thrown if the system cannot access the map.
+ */
+ @throws[Exception]
+ def keys: JIterable[K] = map.keySet()
+
+ /**
+ * Returns all the values in the map.
+ *
+ * @return An iterable view of all the values in the map.
+ * @throws Exception Thrown if the system cannot access the map.
+ */
+ @throws[Exception]
+ def values: JIterable[V] = map.values()
+
+ /**
+ * Iterates over all the mappings in the map.
+ *
+ * @return An iterator over all the mappings in the map.
+ * @throws Exception Thrown if the system cannot access the map.
+ */
+ @throws[Exception]
+ def iterator: util.Iterator[util.Map.Entry[K, V]] =
map.entrySet().iterator()
+
+ /**
+ * Removes all of the mappings from this map (optional operation).
+ *
+ * The map will be empty after this call returns.
+ */
+ override def clear(): Unit = map.clear()
+
+ /**
+ * Copy from a map instance.
+ *
+ * @param m Map instance.
+ * @return A copy of this map instance
+ */
+ def copyFrom(m: util.Map[K, V]): MapView[K, V] = {
--- End diff --
the `copyFrom` method can be accessed by users. I would like to avoid this.
We can implement the serializer's `createInstance` method as following instead
of using `copyFrom`:
```
override def createInstance(): MapView[K, V] = {
val mapview = new MapView[K, V]
mapview.putAll(mapSerializer.createInstance())
mapview
}
```
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