建立一个抽象父类Ishape,包括了两个抽象方法，一个是求周长的方法length(),一个是求面积的方法area(); （2）新建一个子类圆Circle继承Ishape （3）新建一个子类矩形Rect继承Ishape （4）新建一个三角形Triangle类继承Ishape （5）新建一个测试类Test,在里面通过上转型对象展示多态性

下面是代码实现：

abstract class Ishape {
    public abstract double length();
    public abstract double area();
}

class Circle extends Ishape {
    private double radius;
    public Circle(double radius) {
        this.radius = radius;
    }
    public double length() {
        return 2 * Math.PI * radius;
    }
    public double area() {
        return Math.PI * radius * radius;
    }
}

class Rect extends Ishape {
    private double width;
    private double height;
    public Rect(double width, double height) {
        this.width = width;
        this.height = height;
    }
    public double length() {
        return 2 * (width + height);
    }
    public double area() {
        return width * height;
    }
}

class Triangle extends Ishape {
    private double a;
    private double b;
    private double c;
    public Triangle(double a, double b, double c) {
        this.a = a;
        this.b = b;
        this.c = c;
    }
    public double length() {
        return a + b + c;
    }
    public double area() {
        double p = (a + b + c) / 2.0;
        return Math.sqrt(p * (p - a) * (p - b) * (p - c));
    }
}

public class Test {
    public static void main(String[] args) {
        Ishape shape1 = new Circle(5.0);
        Ishape shape2 = new Rect(4.0, 6.0);
        Ishape shape3 = new Triangle(3.0, 4.0, 5.0);

        System.out.println("圆的周长：" + shape1.length() + "，面积：" + shape1.area());
        System.out.println("矩形的周长：" + shape2.length() + "，面积：" + shape2.area());
        System.out.println("三角形的周长：" + shape3.length() + "，面积：" + shape3.area());
    }
}

输出结果为：

圆的周长：31.41592653589793，面积：78.53981633974483
矩形的周长：20.0，面积：24.0
三角形的周长：12.0，面积：6.0

可以看到，通过上转型对象，我们实现了对三个不同形状的计算，展示了多态性。