Github user suez1224 commented on a diff in the pull request:
https://github.com/apache/flink/pull/4585#discussion_r141507177
--- Diff:
flink-core/src/main/java/org/apache/flink/api/java/typeutils/MultisetTypeInfo.java
---
@@ -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.api.java.typeutils;
+
+import org.apache.flink.annotation.PublicEvolving;
+import org.apache.flink.api.common.typeinfo.BasicTypeInfo;
+import org.apache.flink.api.common.typeinfo.TypeInformation;
+
+import java.util.Map;
+
+import static org.apache.flink.util.Preconditions.checkNotNull;
+
+/**
+ * A {@link TypeInformation} for the Multiset types of the Java API.
+ *
+ * @param <T> The type of the elements in the Multiset.
+ */
+@PublicEvolving
+public final class MultisetTypeInfo<T> extends MapTypeInfo<T, Integer> {
--- End diff --
I took a look at Calcite tests for Collect function, null will be ignored.
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