定义4个类，MyShape、MyLine、MyRectangle和MyOval，其中MyShape是其他三个类的父类。MyShape为抽象类，包括图形位置的四个坐标；一个无参的构造方法，将所有的坐标设置为0；一个带参的构造函数，将所有的坐标设置为相应值；每个坐标的设置和读取方法；abstract void draw(Graphics g)方法。MyLine类负责画直线，实现父类的draw方法；MyRectangle负责画矩形，实现父类的draw方法；MyOval负责画椭圆，实现父类的draw方法。编写一个应用程序，使用上面定义的类，随机选取位置和形状，绘制20个图形。-Defines four classes, MyShape, MyLine, MyRectangle and MyOval, which MyShape the other three parent class. MyShape is an abstract class, including graphics position four coordinates　a no-argument constructor, all the coordinates are set to 0　a parameterized constructor, all the coordinates is set to the corresponding values　each coordinate set and reading method　abstract void draw (Graphics g) method. MyLine class is responsible for drawing straight lines, to achieve the draw method of the parent class　MyRectangle responsible for drawing rectangles, to achieve the parent class draw method　MyOval responsible painted oval, draw method to achieve the parent class. Write an application that uses the class defined above, randomly selected location and shape, draw 20 graphics.