public class Rectangle extends Shape {
    // 私有属性：长、宽
    private double length;
    private double width;

    // 构造方法
    public Rectangle(double length, double width) {
        this.length = length;
        this.width = width;
    }

    // 重写父类方法
    @Override
    public double getArea() {
        return length * width;
    }
}