this.targets = ((targets == null) ? Collections.EMPTY_SET : new HashSet(targets));
  public HbReq(UID uid,               MessageAddress source,               Set targets,                     Object content,               Object response) {    this.uid = uid;    this.source = source;    this.targets = ((targets == null) ? Collections.EMPTY_SET : new HashSet(targets));    this.content = content;    this.response = response;  }