设Shape是圆、矩形、三角形共享的类，含有求圆的周长和面积的方法getCircumference()与getArea()，以及属性DESCRIPTION以描述几何体的特征，试设计相关类圆、矩形、三角形类，并求圆、矩形、三角形的周长与面积。设计相应的测试类，求实际对象的面积与方法。（要求分别将Shape类用接口、抽象类实现）。

使用接口实现Shape类：

public interface Shape {
    double getCircumference();
    double getArea();
    String getDescription();
}

实现Circle类：

public class Circle implements Shape {
    private double radius;
    private final String DESCRIPTION = "一个圆形";

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

    public double getCircumference() {
        return 2 * Math.PI * radius;
    }

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

    public String getDescription() {
        return DESCRIPTION;
    }
}

实现Rectangle类：

public class Rectangle implements Shape {
    private double width;
    private double height;
    private final String DESCRIPTION = "一个矩形";

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

    public double getCircumference() {
        return 2 * (width + height);
    }

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

    public String getDescription() {
        return DESCRIPTION;
    }
}

实现Triangle类：

public class Triangle implements Shape {
    private double a;
    private double b;
    private double c;
    private final String DESCRIPTION = "一个三角形";

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

    public double getCircumference() {
        return a + b + c;
    }

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

    public String getDescription() {
        return DESCRIPTION;
    }
}

使用抽象类实现Shape类：

public abstract class Shape {
    abstract double getCircumference();
    abstract double getArea();
    abstract String getDescription();
}

实现Circle类：

public class Circle extends Shape {
    private double radius;
    private final String DESCRIPTION = "一个圆形";

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

    public double getCircumference() {
        return 2 * Math.PI * radius;
    }

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

    public String getDescription() {
        return DESCRIPTION;
    }
}

实现Rectangle类：

public class Rectangle extends Shape {
    private double width;
    private double height;
    private final String DESCRIPTION = "一个矩形";

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

    public double getCircumference() {
        return 2 * (width + height);
    }

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

    public String getDescription() {
        return DESCRIPTION;
    }
}

实现Triangle类：

public class Triangle extends Shape {
    private double a;
    private double b;
    private double c;
    private final String DESCRIPTION = "一个三角形";

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

    public double getCircumference() {
        return a + b + c;
    }

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

    public String getDescription() {
        return DESCRIPTION;
    }
}

测试类：

public class Test {
    public static void main(String[] args) {
        Shape circle = new Circle(5);
        Shape rectangle = new Rectangle(3, 4);
        Shape triangle = new Triangle(3, 4, 5);

        System.out.println(circle.getDescription() + "的周长为：" + circle.getCircumference() + "，面积为：" + circle.getArea());
        System.out.println(rectangle.getDescription() + "的周长为：" + rectangle.getCircumference() + "，面积为：" + rectangle.getArea());
        System.out.println(triangle.getDescription() + "的周长为：" + triangle.getCircumference() + "，面积为：" + triangle.getArea());
    }
}

输出结果：

一个圆形的周长为：31.41592653589793，面积为：78.53981633974483
一个矩形的周长为：14.0，面积为：12.0
一个三角形的周长为：12.0，面积为：6.0