GH-586: Add unit test for first time key and value initialization

Author: axreldable

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

@@ -21,6 +21,7 @@
 import static org.junit.jupiter.api.Assertions.assertFalse;
 import static org.junit.jupiter.api.Assertions.assertNull;
 import static org.junit.jupiter.api.Assertions.assertSame;
+import static org.junit.jupiter.api.Assertions.assertThrows;
 import static org.junit.jupiter.api.Assertions.assertTrue;
 
 import java.nio.ByteBuffer;
@@ -1536,4 +1537,47 @@ public void testFixedSizeBinaryWriter() {
       assertArrayEquals(new byte[] {32, 21}, (byte[]) resultStruct.get(MapVector.VALUE_NAME));
     }
   }
+
+  @Test
+  public void testFixedSizeBinaryFirstInitialization() {
+    try (MapVector mapVector = MapVector.empty("map_vector", allocator, false)) {
+      UnionMapWriter writer = mapVector.getWriter();
+      writer.allocate();
+
+      // populate input vector with the following records
+      // {[11, 22] -> [32, 21]}
+      FixedSizeBinaryHolder holder1 = getFixedSizeBinaryHolder(new byte[] {11, 22});
+      FixedSizeBinaryHolder holder2 = getFixedSizeBinaryHolder(new byte[] {32, 21});
+
+      writer.setPosition(0); // optional
+      writer.startMap();
+      writer.startEntry();
+      // require byteWidth parameter for first-time initialization of `key` or `value` writers
+      assertThrows(NullPointerException.class, () -> writer.key().fixedSizeBinary().write(holder1));
+      assertThrows(
+          NullPointerException.class, () -> writer.value().fixedSizeBinary().write(holder2));
+      writer.key().fixedSizeBinary(holder1.byteWidth).write(holder1);
+      writer.value().fixedSizeBinary(holder2.byteWidth).write(holder2);
+      writer.endEntry();
+      holder1.buffer.close();
+      holder2.buffer.close();
+      writer.endMap();
+
+      writer.setValueCount(1);
+
+      // assert the output vector is correct
+      FieldReader reader = mapVector.getReader();
+      assertTrue(reader.isSet(), "shouldn't be null");
+
+      /* index 0 */
+      Object result = mapVector.getObject(0);
+      ArrayList<?> resultSet = (ArrayList<?>) result;
+      assertEquals(1, resultSet.size());
+      Map<?, ?> resultStruct = (Map<?, ?>) resultSet.get(0);
+      assertTrue(resultStruct.containsKey(MapVector.KEY_NAME));
+      assertTrue(resultStruct.containsKey(MapVector.VALUE_NAME));
+      assertArrayEquals(new byte[] {11, 22}, (byte[]) resultStruct.get(MapVector.KEY_NAME));
+      assertArrayEquals(new byte[] {32, 21}, (byte[]) resultStruct.get(MapVector.VALUE_NAME));
+    }
+  }
 }