如何在以下情况下正确使用抽象类

abstract class Shape {
    abstract double calculateArea();
    void display() {
        System.out.println("This is a shape.");
    }
}

class Circle extends Shape {
    double radius;
    
    Circle(double radius) {
        this.radius = radius;
    }
    
    double calculateArea() {
        return Math.PI * radius * radius;
    }
}

class Rectangle extends Shape {
    double length;
    double width;
    
    Rectangle(double length, double width) {
        this.length = length;
        this.width = width;
    }
    
    double calculateArea() {
        return length * width;
    }
}

public class Main {
    public static void main(String[] args) {
        Circle circle = new Circle(5);
        System.out.println("Circle area: " + circle.calculateArea());
        circle.display();
        
        Rectangle rectangle = new Rectangle(4, 6);
        System.out.println("Rectangle area: " + rectangle.calculateArea());
        rectangle.display();
    }
}