From 835ae1359267be688ed1eec97da81a6569ded785 Mon Sep 17 00:00:00 2001
From: David Li <li.davidm96@gmail.com>
Date: Tue, 30 Jun 2020 13:25:44 -0400
Subject: [PATCH] ARROW-9284: [Java] fix getMinorTypeForArrowType with dense
 unions

---
 .../org/apache/arrow/vector/types/Types.java  |  9 ++++-
 .../arrow/vector/TestDenseUnionVector.java    | 33 +++++++++++++++++++
 2 files changed, 41 insertions(+), 1 deletion(-)

diff --git a/java/vector/src/main/java/org/apache/arrow/vector/types/Types.java b/java/vector/src/main/java/org/apache/arrow/vector/types/Types.java
index 40d20842d5d..99ee937a6dc 100644
--- a/java/vector/src/main/java/org/apache/arrow/vector/types/Types.java
+++ b/java/vector/src/main/java/org/apache/arrow/vector/types/Types.java
@@ -809,7 +809,14 @@ public MinorType visit(FixedSizeList type) {
 
       @Override
       public MinorType visit(Union type) {
-        return MinorType.UNION;
+        switch (type.getMode()) {
+          case Sparse:
+            return MinorType.UNION;
+          case Dense:
+            return MinorType.DENSEUNION;
+          default:
+            throw new IllegalArgumentException("Unknown union mode: " + type.getMode());
+        }
       }
 
       @Override
diff --git a/java/vector/src/test/java/org/apache/arrow/vector/TestDenseUnionVector.java b/java/vector/src/test/java/org/apache/arrow/vector/TestDenseUnionVector.java
index dcd60507fa8..eb003aa4334 100644
--- a/java/vector/src/test/java/org/apache/arrow/vector/TestDenseUnionVector.java
+++ b/java/vector/src/test/java/org/apache/arrow/vector/TestDenseUnionVector.java
@@ -23,6 +23,7 @@
 import static org.junit.Assert.assertTrue;
 
 import java.util.ArrayList;
+import java.util.Collections;
 import java.util.HashMap;
 import java.util.List;
 import java.util.Map;
@@ -31,6 +32,7 @@
 import org.apache.arrow.memory.BufferAllocator;
 import org.apache.arrow.vector.complex.DenseUnionVector;
 import org.apache.arrow.vector.complex.StructVector;
+import org.apache.arrow.vector.complex.UnionVector;
 import org.apache.arrow.vector.holders.NullableBigIntHolder;
 import org.apache.arrow.vector.holders.NullableBitHolder;
 import org.apache.arrow.vector.holders.NullableFloat4Holder;
@@ -43,6 +45,7 @@
 import org.apache.arrow.vector.types.pojo.ArrowType;
 import org.apache.arrow.vector.types.pojo.Field;
 import org.apache.arrow.vector.types.pojo.FieldType;
+import org.apache.arrow.vector.types.pojo.Schema;
 import org.apache.arrow.vector.util.JsonStringHashMap;
 import org.apache.arrow.vector.util.Text;
 import org.apache.arrow.vector.util.TransferPair;
@@ -613,6 +616,36 @@ public void testChildVectorValueCounts() {
     }
   }
 
+  @Test
+  public void testDenseUnionFromMinorType() {
+    final Field denseField = new Field(
+        "union",
+        new FieldType(
+            /* nullable */ false,
+            new ArrowType.Union(UnionMode.Dense, new int[]{0}),
+            /* dictionary */ null),
+        Collections.emptyList());
+    final Field sparseField = new Field(
+        "union",
+        new FieldType(
+            /* nullable */ false,
+            new ArrowType.Union(UnionMode.Sparse, new int[]{0}),
+            /* dictionary */ null),
+        Collections.emptyList());
+    final Schema denseSchema = new Schema(Collections.singletonList(denseField));
+    final Schema sparseSchema = new Schema(Collections.singletonList(sparseField));
+    try (final VectorSchemaRoot root = VectorSchemaRoot.create(sparseSchema, allocator)) {
+      assertTrue(root.getVector("union") instanceof UnionVector);
+    }
+    try (final VectorSchemaRoot root = VectorSchemaRoot.create(denseSchema, allocator)) {
+      assertTrue(root.getVector("union") instanceof DenseUnionVector);
+    }
+    assertEquals(MinorType.DENSEUNION,
+        Types.getMinorTypeForArrowType(new ArrowType.Union(UnionMode.Dense, new int[]{0})));
+    assertEquals(MinorType.UNION,
+        Types.getMinorTypeForArrowType(new ArrowType.Union(UnionMode.Sparse, new int[]{0})));
+  }
+
   private static NullableIntHolder newIntHolder(int value) {
     final NullableIntHolder holder = new NullableIntHolder();
     holder.isSet = 1;