abstract class Shape {

    public abstract double getArea();
}
class Circle extends Shape {
    private double radius;
    public Circle(double radius) {
        if (radius <= 0) {
            throw new IllegalArgumentException("半径必须为正数");
        }
        this.radius = radius;
    }
    @Override
    public double getArea() {
        return Math.PI * radius * radius;
    }
}
class Rectangle extends Shape {
    private double width;
    private double height;
    public Rectangle(double width, double height) {
        if (width <= 0 || height <= 0) {
            throw new IllegalArgumentException("宽和高必须为正数");
        }
        this.width = width;
        this.height = height;
    }

    @Override
    public double getArea() {
        return width * height;
    }
}
class Triangle extends Shape {
    private double a;
    private double b;
    private double c;
    public Triangle(double a, double b, double c) {
        if (a <= 0 || b <= 0 || c <= 0) {
            throw new IllegalArgumentException("边长必须为正数");
        }
        if (a + b <= c || a + c <= b || b + c <= a) {
            throw new IllegalArgumentException("三边不满足三角形构成条件");
        }
        this.a = a;
        this.b = b;
        this.c = c;
    }
    @Override
    public double getArea() {
        double s = (a + b + c) / 2;
        return Math.sqrt(s * (s - a) * (s - b) * (s - c));
    }
}
class ShapeUtil {
    public static void printArea(Shape shape) {
        if (shape == null) {
            System.out.println("图形对象不能为空");
            return;
        }
        System.out.printf("图形面积：%.2f%n", shape.getArea());
    }
}
public class ShapeAreaCalculator {
    public static void main(String[] args) {
        try {
            Shape circle = new Circle(3);
            System.out.print("圆形：");
            ShapeUtil.printArea(circle);
            Shape rectangle = new Rectangle(4, 5);
            System.out.print("矩形：");
            ShapeUtil.printArea(rectangle);
            Shape triangle = new Triangle(3, 4, 5);
            System.out.print("三角形：");
            ShapeUtil.printArea(triangle);
        } catch (IllegalArgumentException e) {
            System.out.println("错误：" + e.getMessage());
        }
    }
}