apacheGH-40773: [Java] add DENSEUNION case to StructWriters, resolves

apache#40773 (apache#40809)

### What changes are included in this PR?

Adding a `DENSEUNION` case to the `StructWriters` template so that one can create StructVectors with a DenseUnionVector child.

### Are these changes tested?

Yes

### Are there any user-facing changes?

No
* GitHub Issue: apache#40773

Authored-by: James Henderson <james@jarohen.dev>
Signed-off-by: David Li <li.davidm96@gmail.com>

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

@@ -73,6 +73,12 @@ public class ${mode}StructWriter extends AbstractFieldWriter {
         map(child.getName(), arrowType.getKeysSorted());
         break;
       }
+      case DENSEUNION: {
+        FieldType fieldType = new FieldType(addVectorAsNullable, MinorType.DENSEUNION.getType(), null, null);
+        DenseUnionWriter writer = new DenseUnionWriter(container.addOrGet(child.getName(), fieldType, DenseUnionVector.class), getNullableStructWriterFactory());
+        fields.put(handleCase(child.getName()), writer);
+        break;
+      }
       case UNION:
         FieldType fieldType = new FieldType(addVectorAsNullable, MinorType.UNION.getType(), null, null);
         UnionWriter writer = new UnionWriter(container.addOrGet(child.getName(), fieldType, UnionVector.class), getNullableStructWriterFactory());

java/vector/src/test/java/org/apache/arrow/vector/TestValueVector.java

@@ -60,6 +60,7 @@
 import org.apache.arrow.vector.testing.ValueVectorDataPopulator;
 import org.apache.arrow.vector.types.Types;
 import org.apache.arrow.vector.types.Types.MinorType;
+import org.apache.arrow.vector.types.UnionMode;
 import org.apache.arrow.vector.types.pojo.ArrowType;
 import org.apache.arrow.vector.types.pojo.Field;
 import org.apache.arrow.vector.types.pojo.FieldType;
@@ -2974,6 +2975,20 @@ public void testStructVectorEqualsWithDiffChild() {
     }
   }
 
+  @Test
+  public void testStructVectorAcceptsDenseUnionChild() {
+    Field childField = new Field("child",
+             FieldType.notNullable(new ArrowType.Union(UnionMode.Dense, new int[] {})),
+             Collections.emptyList());
+    Field structField = new Field("struct",
+             FieldType.notNullable(ArrowType.Struct.INSTANCE),
+             Collections.singletonList(childField));
+
+    try (FieldVector structVec = structField.createVector(allocator)) {
+      assertEquals(structField, structVec.getField());
+    }
+  }
+
   @Test
   public void testUnionVectorEquals() {
     try (final UnionVector vector1 = new UnionVector("union", allocator, /* field type */ null, /* call-back */ null);