JAVA 使用抽象方法和抽象类，编程实现图5.3所示的类层次图，并给出一个运行结果。

由于无法插入图片，以下是图5.3的类层次结构代码描述：

abstract class Shape {
    protected String color;

    public Shape(String color) {
        this.color = color;
    }

    public abstract double getArea();

    @Override
    public String toString() {
        return "颜色为" + color + "的";
    }
}

class Circle extends Shape {
    protected double radius;

    public Circle(String color, double radius) {
        super(color);
        this.radius = radius;
    }

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

    @Override
    public String toString() {
        return super.toString() + "圆形，半径为" + radius;
    }
}

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

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

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

    @Override
    public String toString() {
        return super.toString() + "矩形，长为" + length + "，宽为" + width;
    }
}

class Triangle extends Shape {
    protected double a;
    protected double b;
    protected double c;

    public Triangle(String color, double a, double b, double c) {
        super(color);
        this.a = a;
        this.b = b;
        this.c = c;
    }

    @Override
    public double getArea() {
        double p = (a + b + c) / 2;
        return Math.sqrt(p * (p - a) * (p - b) * (p - c));
    }

    @Override
    public String toString() {
        return super.toString() + "三角形，边长分别为" + a + "、" + b + "、" + c;
    }
}

下面是一个简单的测试程序：

public class ShapeTest {
    public static void main(String[] args) {
        Shape s1 = new Circle("红色", 2);
        Shape s2 = new Rectangle("黄色", 2, 3);
        Shape s3 = new Triangle("蓝色", 3, 4, 5);

        System.out.println(s1.toString() + "面积为：" + s1.getArea());
        System.out.println(s2.toString() + "面积为：" + s2.getArea());
        System.out.println(s3.toString() + "面积为：" + s3.getArea());
    }
}

运行结果如下：

颜色为红色的圆形，半径为2.0面积为：12.566370614359172
颜色为黄色的矩形，长为2.0，宽为3.0面积为：6.0
颜色为蓝色的三角形，边长分别为3.0、4.0、5.0面积为：6.0