[ARROW-7073][Java] Support dense union vector

Author: liyafan82

java/vector/src/main/java/org/apache/arrow/vector/util/VectorAppender.java

@@ -27,6 +27,7 @@
 import org.apache.arrow.vector.ValueVector;
 import org.apache.arrow.vector.compare.TypeEqualsVisitor;
 import org.apache.arrow.vector.compare.VectorVisitor;
+import org.apache.arrow.vector.complex.DenseUnionVector;
 import org.apache.arrow.vector.complex.FixedSizeListVector;
 import org.apache.arrow.vector.complex.ListVector;
 import org.apache.arrow.vector.complex.NonNullableStructVector;
@@ -311,6 +312,11 @@ public ValueVector visit(UnionVector deltaVector, Void value) {
     return targetVector;
   }
 
+  @Override
+  public ValueVector visit(DenseUnionVector left, Void value) {
+    throw new UnsupportedOperationException();
+  }
+
   @Override
   public ValueVector visit(NullVector deltaVector, Void value) {
     Preconditions.checkArgument(targetVector.getField().getType().equals(deltaVector.getField().getType()),