Github user dongjoon-hyun commented on a diff in the pull request:
https://github.com/apache/spark/pull/23124#discussion_r235851554
--- Diff:
sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/util/ArrayBasedMapBuilder.scala
---
@@ -0,0 +1,118 @@
+/*
+ * 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.spark.sql.catalyst.util
+
+import scala.collection.mutable
+
+import org.apache.spark.sql.catalyst.InternalRow
+import org.apache.spark.sql.types.{AtomicType, CalendarIntervalType,
DataType, MapType}
+
+/**
+ * A builder of [[ArrayBasedMapData]], which fails if a null map key is
detected, and removes
+ * duplicated map keys w.r.t. the last wins policy.
+ */
+class ArrayBasedMapBuilder(keyType: DataType, valueType: DataType) extends
Serializable {
+ assert(!keyType.existsRecursively(_.isInstanceOf[MapType]), "key of map
cannot be/contain map")
--- End diff --
Shall we add assert to prevent `NullType` here, too?
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