// 抽象图形类
abstract class Shape {
    public abstract double getArea();
}

// 圆形类
class Circle extends Shape {
    private double radius;

    public Circle(double radius) {
        this.radius = radius;
    }

    @Override
    public double getArea() {
        return Math.PI * radius * radius;
    }
}

// 矩形类
class Rectangle extends Shape {
    private double width;
    private double height;

    public Rectangle(double width, double height) {
        this.width = width;
        this.height = height;
    }

    @Override
    public double getArea() {
        return width * height;
    }
}

// 三角形类
class Triangle extends Shape {
    private double base;
    private double height;

    public Triangle(double base, double height) {
        this.base = base;
        this.height = height;
    }

    @Override
    public double getArea() {
        return 0.5 * base * height;
    }
}

// 工具类
class ShapeUtil {
    public void printArea(Shape shape) {
        System.out.printf("面积：%.2f%n", shape.getArea());
    }
}

// 主类（程序入口）
public class Main {
    public static void main(String[] args) {
        ShapeUtil util = new ShapeUtil();
        util.printArea(new Circle(5));
        util.printArea(new Rectangle(4, 6));
        util.printArea(new Triangle(3, 4));
    }
}