/**
 * 圆：面积 = π * r²
 */
public class Circle extends Shape {

    private final double radius;

    public Circle(double radius) {
        if (radius <= 0) {
            throw new IllegalArgumentException("半径必须为正数");
        }
        this.radius = radius;
    }

    public double getRadius() {
        return radius;
    }

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