public abstract class Shape {
    public abstract double getArea();
}
class Circle extends Shape {
    private double radius;
    public Circle(double radius) {
        this.radius = radius;
    }
    public double getArea() {
        return Math.PI * radius * radius;
    }
}
class Rectangle extends Shape {
    private double width, height;
    public Rectangle(double width, double height) {
        this.width = width;
        this.height = height;
    }
    public double getArea() {
        return width * height;
    }
}
class Triangle extends Shape {
    private double base, height;
    public Triangle(double base, double height) {
        this.base = base;
        this.height = height;
    }
    public double getArea() {
        return 0.5 * base * height;
    }
}
class ShapeUtil {
    public static void printArea(Shape shape) {
        System.out.println("面积：" + shape.getArea());
    }
}
class Main {
    public static void main(String[] args) {
        Shape circle = new Circle(5);
        Shape rect = new Rectangle(4, 6);
        Shape tri = new Triangle(3, 4);
        ShapeUtil.printArea(circle);
        ShapeUtil.printArea(rect);
        ShapeUtil.printArea(tri);
    }
}