public class Main {
    public static void main(String[] args) {
        Shape circle = new Circle(5);
        Shape rect = new Rectangle(4, 6);
        Shape tri = new Triangle(3, 8);

        ShapeUtil.printArea(circle);  // 面积为：78.54
        ShapeUtil.printArea(rect);    // 面积为：24.00
        ShapeUtil.printArea(tri);     // 面积为：12.00
    }
}