// Main.java
public class Main {
    public static void main(String[] args) {
        ShapeUtil util = new ShapeUtil();

        // 测试圆形
        Shape circle = new Circle(5);
        System.out.print("圆形（半径5）：");
        util.printArea(circle);

        // 测试矩形
        Shape rectangle = new Rectangle(4, 6);
        System.out.print("矩形（长4，宽6）：");
        util.printArea(rectangle);

        // 测试三角形
        Shape triangle = new Triangle(3, 4);
        System.out.print("三角形（底3，高4）：");
        util.printArea(triangle);
    }
}

// 抽象类 Shape（不加 public）
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 length;
    private double width;

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

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

// 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 (base * height) / 2;
    }
}

// 工具类
class ShapeUtil {
    public void printArea(Shape shape) {
        double area = shape.getArea();
        System.out.printf("该图形的面积为：%.2f%n", area);
    }
}