@Override
  public boolean equals(Object obj) {
    if (this == obj) return true;
    if (obj == null) return false;
    if (getClass() != obj.getClass()) return false;
    StubEntity other = (StubEntity) obj;
    if (id == null) {
      if (other.id != null) return false;
    } else if (!id.equals(other.id)) return false;
    return true;
  }