一个简单的绘图程序，实现类的继承。可以画直线、圆、矩形。并实现简单的图形保存。直线画法为DDA，圆的画法为中点画圆法，矩形在直线基础上画。程序比较适合初学者。-A simple drawing program to achieve class inheritance. Can draw a straight line, round, rectangular. And to achieve a simple, save the drawing. Straight-line painting for