定义一个抽象类 Shape，其中包含求形状的面积和周长的抽象方法。定义三角形、矩形、圆形类，继承Shape。分别创建一个三角形、矩形、圆形对象，输出各类图形的面积和周长。JAVA

在Java中，你可以这样定义一个抽象类`Shape`以及它的派生类：

// 定义抽象类 Shape
abstract class Shape {
    // 抽象方法，需要在每个子类中实现
    abstract double getArea();
    abstract double getPerimeter();

    // 具体方法，可以共用
    public void display() {
        System.out.println("This is a generic shape.");
    }
}

// 定义三角形类，继承自 Shape
class Triangle extends Shape {
    private double base;
    private double height;

    public Triangle(double base, double height) {
        this.base = base;
        this.height = height;
    }

    @Override
    double getArea() {
        return 0.5 * base * height;
    }

    @Override
    double getPerimeter() {
        // 假设这是一个等腰三角形，仅做示例
        return base + 2 * Math.sqrt((base / 2) * (height / 2));
    }

    @Override
    void display() {
        System.out.println("Triangle with base: " + base + ", height: " + height);
    }
}

// 定义矩形类，同样继承自 Shape
class Rectangle extends Shape {
    private double length;
    private double width;

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

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

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

    @Override
    void display() {
        System.out.println("Rectangle with length: " + length + ", width: " + width);
    }
}

// 定义圆形类，继承自 Shape
class Circle extends Shape {
    private double radius;

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

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

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

    @Override
    void display() {
        System.out.println("Circle with radius: " + radius);
    }
}

public class Main {
    public static void main(String[] args) {
        // 创建并实例化各个形状
        Triangle triangle = new Triangle(5, 6);
        Rectangle rectangle = new Rectangle(4, 8);
        Circle circle = new Circle(3);

        // 输出各自属性
        triangle.display();
        System.out.println("Triangle area: " + triangle.getArea());
        System.out.println("Triangle perimeter: " + triangle.getPerimeter());

        rectangle.display();
        System.out.println("Rectangle area: " + rectangle.getArea());
        System.out.println("Rectangle perimeter: " + rectangle.getPerimeter());

        circle.display();
        System.out.println("Circle area: " + circle.getArea());
        System.out.println("Circle perimeter: " + circle.getPerimeter());
    }
}

当你运行这个程序时，会分别打印出每个形状的信息以及它们各自的面积和周长。这就是如何使用Java实现抽象类和多态性的基本例子。