说明：  这是一个原创的基于多平台同步绘图的程序。用户可以实现的功能如下： 1.创建Host端口，如localhost， port：54321 2.连接Host，如填上localhost，若在其他电脑上，须填写相应的IP地址 3.用户可以选择不同颜色和尺寸的画笔进行绘画 4.画出的图形会同步，类似于Google doc的效果
(/*This is a collaborative drawing program * * Program Flow: * 1.CoPainter--> Server--> CoWindow--> CoCanvas--> CoLine * 2.CoPainter--> Client--> CoWindow--> CoCanvas--> CoLine * 3.Server <--> ClientHandler <--> Client * * For the Part 1: * 1.When the program is executed, the startup prompt appears. * 2.Upon clicking either of the buttons, the GUI will be replaced by the painter interface * 3.To see the painter interface, please refer to the class CoWindow * 4.the startup interface cannot be resized, while the painter interface can. * * For the Part 2: * 1.The message from Server to Client contains two part: * ExitValue (if the server is terminated, it is true) * The ArrayList of all the lines * 2.The message from Client to Server only contains one part: * The ArrayList of all the lines * * For multi-threading: * Thread: ClientHandler * Runnable: ServerThread, InReader)