一个聊天程序的代码，利用了线程实现,可以切换群聊或者是私聊，服务器端用线程处理用户登录，且用了集合来存放用户名称，用XML处理聊天数据-A chat program code, the realization of thread, can switch group chat or private chat and the server use thread processing user login, and use the set to deposit the user names, with