GH-399: Check for null writers in DenseUnionWriter#setPosition

.gitignore

@@ -9,6 +9,8 @@
 .classpath
 .factorypath
 .idea/
+.vscode/
+.cursor/
 .project
 .settings/
 /*-build/

vector/src/main/codegen/templates/DenseUnionWriter.java

@@ -55,7 +55,9 @@ public DenseUnionWriter(DenseUnionVector vector, NullableStructWriterFactory nul
   public void setPosition(int index) {
     super.setPosition(index);
     for (BaseWriter writer : writers) {
-      writer.setPosition(index);
+      if (writer != null) {
+        writer.setPosition(index);
+      }
     }
   }
 

vector/src/test/java/org/apache/arrow/vector/complex/impl/TestDenseUnionWriterNPE.java

@@ -0,0 +1,86 @@
+/*
+ * 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.arrow.vector.complex.impl;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+import java.util.List;
+import org.apache.arrow.memory.BufferAllocator;
+import org.apache.arrow.memory.RootAllocator;
+import org.apache.arrow.vector.complex.ListVector;
+import org.apache.arrow.vector.types.Types;
+import org.apache.arrow.vector.types.pojo.FieldType;
+import org.junit.jupiter.api.AfterEach;
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Test;
+
+public class TestDenseUnionWriterNPE {
+
+  private BufferAllocator allocator;
+
+  @BeforeEach
+  public void init() {
+    allocator = new RootAllocator(Long.MAX_VALUE);
+  }
+
+  @AfterEach
+  public void terminate() {
+    allocator.close();
+  }
+
+  @Test
+  public void testListOfDenseUnionWriterNPE() {
+    try (ListVector listVector = ListVector.empty("list", allocator)) {
+      listVector.addOrGetVector(FieldType.nullable(Types.MinorType.DENSEUNION.getType()));
+      UnionListWriter listWriter = listVector.getWriter();
+
+      listWriter.startList();
+      listWriter.endList();
+    }
+  }
+
+  @Test
+  public void testListOfDenseUnionWriterWithData() {
+    try (ListVector listVector = ListVector.empty("list", allocator)) {
+      listVector.addOrGetVector(FieldType.nullable(Types.MinorType.DENSEUNION.getType()));
+
+      UnionListWriter listWriter = listVector.getWriter();
+      listWriter.startList();
+      listWriter.writeInt(100);
+      listWriter.writeBigInt(200L);
+      listWriter.endList();
+
+      listWriter.startList();
+      listWriter.writeFloat4(3.14f);
+      listWriter.endList();
+
+      listVector.setValueCount(2);
+
+      assertEquals(2, listVector.getValueCount());
+
+      List<?> value0 = (List<?>) listVector.getObject(0);
+      List<?> value1 = (List<?>) listVector.getObject(1);
+
+      assertEquals(2, value0.size());
+      assertEquals(100, value0.get(0));
+      assertEquals(200L, value0.get(1));
+
+      assertEquals(1, value1.size());
+      assertEquals(3.14f, value1.get(0));
+    }
+  }
+}