Java聊天程序，服务器端可显示用户登录及退出信息。客户端可先显示登录界面，登录后再显示聊天界面。由于不想花太多时间，所以只实现这么点功能。-Java Chat room,the server shows the users,logging in and logging out message.The Client shows the log in interface first,and shows the chatting interface when logged in succed.