ARROW-308: UnionListWriter.setPosition() should not call startList()

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

@@ -84,7 +84,6 @@ public void close() throws Exception {
   @Override
   public void setPosition(int index) {
     super.setPosition(index);
-    startList();
   }
 
   <#list vv.types as type><#list type.minor as minor><#assign name = minor.class?cap_first />

java/vector/src/test/java/org/apache/arrow/vector/complex/writer/TestComplexWriter.java

@@ -52,7 +52,7 @@
 
 public class TestComplexWriter {
 
-  static final BufferAllocator allocator = new RootAllocator(Integer.MAX_VALUE);
+  private static final BufferAllocator allocator = new RootAllocator(Integer.MAX_VALUE);
 
   private static final int COUNT = 100;
 
@@ -115,6 +115,36 @@ public void nullableMap() {
     parent.close();
   }
 
+  @Test
+  public void listOfLists() {
+    MapVector parent = new MapVector("parent", allocator, null);
+    ComplexWriter writer = new ComplexWriterImpl("root", parent);
+    MapWriter rootWriter = writer.rootAsMap();
+
+    rootWriter.start();
+    rootWriter.bigInt("int").writeBigInt(0);
+    rootWriter.list("list").startList();
+    rootWriter.list("list").bigInt().writeBigInt(0);
+    rootWriter.list("list").endList();
+    rootWriter.end();
+
+    rootWriter.setPosition(1);
+    rootWriter.start();
+    rootWriter.bigInt("int").writeBigInt(1);
+    rootWriter.end();
+
+    writer.setValueCount(2);
+
+    MapReader rootReader = new SingleMapReaderImpl(parent).reader("root");
+
+    rootReader.setPosition(0);
+    assertTrue("row 0 list is not set", rootReader.reader("list").isSet());
+    assertEquals(Long.valueOf(0), rootReader.reader("list").reader().readLong());
+
+    rootReader.setPosition(1);
+    assertFalse("row 1 list is set", rootReader.reader("list").isSet());
+  }
+
   @Test
   public void listScalarType() {
     ListVector listVector = new ListVector("list", allocator, null);