@Override
  public boolean equals(Object obj) {
    return null != obj && obj instanceof WritableVoidObjectInspector;
  }
A WritableVoidObjectInspector inspects a NullWritable Object.
Note that this is also a constant object inspector.
serde/src/java/org/apache/hadoop/hive/serde2/objectinspector/primitive/WritableVoidObjectInspector.java