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