GH-625: Map MinorType getNewFieldWriter returns UnionMapWriter (#627)

## What's Changed

Closes #625

---------

Co-authored-by: William Suppiger <[email protected]>

Author: wsuppiger

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) {

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));
+    }
+  }
 }