@Override
    public String toString() {
      return tag + ":" + object;
    }
StandardUnionObjectInspector works on union data that is stored as
UnionObject.
It holds the list of the object inspectors corresponding to each type of the
object the Union can hold. The UniobObject has tag followed by the object
it is holding.

Always use the {@link ObjectInspectorFactory} to create new ObjectInspector
objects, instead of directly creating an instance of this class.
serde/src/java/org/apache/hadoop/hive/serde2/objectinspector/StandardUnionObjectInspector.java