From 5dd83f117ad81d31a6ec96b760141ce81d20dd8e Mon Sep 17 00:00:00 2001
From: William Suppiger <wsuppiger@palantir.com>
Date: Thu, 20 Feb 2025 22:09:10 -0500
Subject: [PATCH 1/2] GH-625: Map MinorType getNewFieldWriter returns
 UnionMapWriter

---
 vector/src/main/java/org/apache/arrow/vector/types/Types.java | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/vector/src/main/java/org/apache/arrow/vector/types/Types.java b/vector/src/main/java/org/apache/arrow/vector/types/Types.java
index e9b963b62c..17503f98c8 100644
--- a/vector/src/main/java/org/apache/arrow/vector/types/Types.java
+++ b/vector/src/main/java/org/apache/arrow/vector/types/Types.java
@@ -116,6 +116,7 @@
 import org.apache.arrow.vector.complex.impl.UnionLargeListViewWriter;
 import org.apache.arrow.vector.complex.impl.UnionLargeListWriter;
 import org.apache.arrow.vector.complex.impl.UnionListWriter;
+import org.apache.arrow.vector.complex.impl.UnionMapWriter;
 import org.apache.arrow.vector.complex.impl.UnionWriter;
 import org.apache.arrow.vector.complex.impl.VarBinaryWriterImpl;
 import org.apache.arrow.vector.complex.impl.VarCharWriterImpl;
@@ -721,7 +722,7 @@ public FieldVector getNewVector(
 
       @Override
       public FieldWriter getNewFieldWriter(ValueVector vector) {
-        return new UnionListWriter((MapVector) vector);
+        return new UnionMapWriter((MapVector) vector);
       }
     },
     TIMESTAMPSECTZ(null) {

From 07afd46ce446b38f0116fbcd7cec6c9bd4c63908 Mon Sep 17 00:00:00 2001
From: William Suppiger <wsuppiger@palantir.com>
Date: Mon, 24 Feb 2025 10:35:04 -0500
Subject: [PATCH 2/2] add test

---
 .../apache/arrow/vector/TestMapVector.java    | 22 +++++++++++++++++++
 1 file changed, 22 insertions(+)

diff --git a/vector/src/test/java/org/apache/arrow/vector/TestMapVector.java b/vector/src/test/java/org/apache/arrow/vector/TestMapVector.java
index a4197c50b5..313d83ec91 100644
--- a/vector/src/test/java/org/apache/arrow/vector/TestMapVector.java
+++ b/vector/src/test/java/org/apache/arrow/vector/TestMapVector.java
@@ -35,6 +35,7 @@
 import org.apache.arrow.vector.complex.reader.FieldReader;
 import org.apache.arrow.vector.complex.writer.BaseWriter.ListWriter;
 import org.apache.arrow.vector.complex.writer.BaseWriter.MapWriter;
+import org.apache.arrow.vector.complex.writer.FieldWriter;
 import org.apache.arrow.vector.types.Types.MinorType;
 import org.apache.arrow.vector.types.pojo.ArrowType;
 import org.apache.arrow.vector.types.pojo.Field;
@@ -1241,4 +1242,25 @@ public void testMakeTransferPairPreserveNullability() {
     assertEquals(intField, vec.getField().getChildren().get(0));
     assertEquals(intField, res.getField().getChildren().get(0));
   }
+
+  @Test
+  public void testMapTypeReturnsSupportedMapWriter() {
+    try (final MapVector vector = MapVector.empty("map", allocator, false)) {
+      vector.allocateNew();
+      FieldWriter mapWriter = MinorType.MAP.getNewFieldWriter(vector);
+
+      mapWriter.startMap();
+      mapWriter.startEntry();
+      mapWriter.key().bigInt().writeBigInt(1);
+      mapWriter.value().integer().writeInt(11);
+      mapWriter.endEntry();
+      mapWriter.endMap();
+
+      Object result = vector.getObject(0);
+      ArrayList<?> resultSet = (ArrayList<?>) result;
+      Map<?, ?> resultStruct = (Map<?, ?>) resultSet.get(0);
+      assertEquals(1L, getResultKey(resultStruct));
+      assertEquals(11, getResultValue(resultStruct));
+    }
+  }
 }