创建一个类的两个实例，分别计算其面积和周长

创建一个类的实例并计算其面积和周长通常涉及到二维图形，比如矩形或圆形。这里我们假设有一个名为`Shape`的抽象基类，以及两个派生类`Rectangle`和`Circle`，每个都有自己的`area`和`perimeter`方法。

首先，定义`Shape`类：

abstract class Shape {
    abstract double area();
    abstract double perimeter();
}

接着，定义`Rectangle`和`Circle`类：

class Rectangle extends Shape {
    private double length;
    private double width;

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

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

    @Override
    double perimeter() {
        return 2 * (length + width);
    }
}

class Circle extends Shape {
    private double radius;

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

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

    @Override
    double perimeter() {
        return 2 * Math.PI * radius;
    }
}

现在，我们可以创建这两个类的实例并计算它们的面积和周长：

public class Main {
    public static void main(String[] args) {
        Rectangle rectangle = new Rectangle(5, 7);
        System.out.println("Rectangle area: " + rectangle.area());
        System.out.println("Rectangle perimeter: " + rectangle.perimeter());

        Circle circle = new Circle(3.5);
        System.out.println("Circle area: " + circle.area());
        System.out.println("Circle perimeter: " + circle.perimeter());
    }
}