面向对象案例 - 多态，继承

package randi;
import static java.lang.Math.PI;
public class ShapeDemo {

    public static void main(String[] args) {

        Shape sha = new Shape();
        
        Circle s1 = new Circle(10);
        Rect s2 = new Rect(5, 6);
        Squr s3 = new Squr(10);
        
        sha.print(s1);
        sha.print(s2);
        sha.print(s3);
    }
}

class Shape {
    
    public void print(Shape sha) {
        System.out.println(sha.getName() + "面积: " + sha.getArea());
        System.out.println(sha.getName() + "周长: " + sha.getGirth());
    }

    public double getArea() {
        return 0;
    }

    public double getGirth() {
        return 0;
    }
    
    public String getName() {
        return null;
    }
}


class Circle extends Shape {
    private double radius;
    
    public Circle() {}
    public Circle(double radius) {
        this.radius = radius;
    }

    public double getArea() {
        return PI * radius * radius;
    }

    public double getGirth() {
        return PI * radius * 2;
    }

    public String getName() {
        return "Cir";
    }
}

class Rect extends Shape {
    private double length;
    private double width;
    
    public Rect() {}
    public Rect(double length, double width) {
        this.length = length;
        this.width = width;
    }

    public double getArea() {
        return width * length;
    }

    public double getGirth() {
        return (width + length) * 2;
    }

    public String getName() {
        return "Rect";
    }
}

class Squr extends Rect {
    
    public Squr() {}
    public Squr(double length) {
        super(length, length);

    }

    public String getName() {
        return "Squr";
    }
}